SOIL ORGANIC MATTER DECOMPOSITION: EFFECTS OF ORGANIC MATTER ADDITION ON PHOSPHORUS DYNAMICS IN LATERITIC SOILS A thesis submitted by Fadly Hairannoor Yusran Ir. (Agriculture), Lambung Mangkurat University, Indonesia M.Sc. (Agriculture), McGill University, Canada for the degree of Doctor of Philosophy School of Earth and Geographical Sciences Faculty of Natural and Agricultural Sciences The University of Western Australia 2005
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SOIL ORGANIC MATTER DECOMPOSITION: EFFECTS OF ORGANIC MATTER ADDITION
ON PHOSPHORUS DYNAMICS IN LATERITIC SOILS
A thesis submitted
by
Fadly Hairannoor Yusran
Ir. (Agriculture), Lambung Mangkurat University, Indonesia
2.2.1 Some important properties of Ultisols .................................... 6 2.2.2 Some important properties of Oxisols .................................... 7
2.3.1 The important of soil organic matter....................................... 9 2.3.1.1 Organic amendments .................................................................. 9
2.3.1.2 Contribution of organic matter to soil nutrients for plants ......................................................................................... 10
Table of Contents vi
2.3.1.3 Peat as a source of organic matter and carbon......................... 11
2.3.1.4 Effect of organic matter addition on soil properties and crop responses ................................................................... 12
2.3.2 Organic matter decomposition and persistence................... 15 2.3.2.1 The age of soil organic matter ................................................... 16
2.3.2.2 Loss and persistence of newly added organic matter................ 18
3.4.1 The persistence of organic matter ....................................... 80 3.4.2 Phosphorus availability ........................................................ 81 3.4.3 Mineralisation of nitrogen and release of potassium............ 83
4.3.3 Concentrations of different forms of phosphorus ............... 109 4.3.3.1 Subsurface soil ........................................................................ 109
5.3.4.4 Correlations between all parameters in the leaching column ..................................................................................... 162
6.3.1 The effect of incubation time.............................................. 174
Table of Contents xi
6.3.2 The effect of organic matter sources.................................. 176 6.3.3 The effect of organic matter levels ..................................... 178 6.3.4 The effect of existing and new soil organic matter ............. 180 6.3.5 The phospphate adsorption capacity of the three
soils ................................................................................... 183 6.3.6 Correlation on phosphate adsorption parameters
with dissolved organic carbon, extractable aluminium, and extractable iron ......................................... 186
7.1 BRIEF SUMMARY OF THE MAJOR FINDINGS AND THEIR INTERPRETATIONS................................................. 193
7.2 LIMITATIONS AND FUTURE RESEARCH NEEDS.................. 201
REFERENCES
APPENDIX
Table of Contents xii
LIST OF FIGURES
Figure 2.3-1 Steps in carbon cycle and the formation of soil organic matter and humus in aerobic soils. Source: Modifiied from Coyne (1999) and Grunwald (2004)............................................................. 14
Figure 2.3-2 Soil organic matter pools according to the CENTURY model. The soil organic matter submodel is part of the overall model in which carbon cycling is affected by climate, soils, management, and topography. Source: (Brenner et al., 2001)............................... 24
Figure 2.4-1 Soil phosphorus cycle, its components and measurable fractions (adapted from Stewart and Sharpley (1987)). Arrows represent fluxes between reservoirs. ................................................ 27
Figure 3.3-1 Comparative effect of organic matter sources, organic matter levels, and incubation times on extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ...................................................................................... 52
Figure 3.3-2 Comparative effect of organic matter sources, organic matter levels, and incubation times on microbial biomass carbon in an Oxisol from York, Western Australia. = control, = peat,
= wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph............. 54
Figure 3.3-3 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil respiration in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ................................... 56
Figure 3.3-4 Cumulative respiration representing total carbon loss following the application of organic matter during the experiment in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay.................................................... 57
Figure 3.3-5 Interaction effect of organic matter sources, organic matter levels, and incubation times on ammonium (A) and nitrate (B) in an Oxisol from York, Western Australia. = control, = peat,
= wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph............. 60
List of Figures xiii
Figure 3.3-6 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil bicarbonate phosphorus in an Oxisol from York, Western Australia. = control, = peat,
= wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph............. 63
Figure 3.3-7 Interaction effect of organic matter sources, organic matter, and incubation times on extractable potassium in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ................................... 65
Figure 3.3-8 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil pH in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and
= lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. .............................................. 67
Figure 3.3-9 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water content in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ................................... 69
Figure 3.3-10 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water holding capacity in an Oxisol from York, Western Australia. = control, = peat,
= wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph............. 72
Figure 3.3-11 The effects of organic matter source on water content measured one week after watering in an Oxisol from York, Western Australia. C = control, P = peat, W = wheat straw, and L = lucerne hay. Solid line is mean of C1-C4. 1 = 40 ton ha-1, 2 = 80 ton ha-1, 3 = 120 ton ha-1, and 4 = 160 ton ha-1. Bar in each graph indicates LSD 5% for organic matter source in every graph................................................................................................. 73
Figure 3.3-12 The effects of the highest organic matter level (160 ton ha-1)
on water content in an Oxisol from York, Western Australia, measured one week after watering and maximum weekly temperature for Perth (Source: Aufdemkampe (2001)). = control, = peat, = wheat straw, and = lucerne hay. Solid line is maximum weekly temperature. Bar indicates LSD 5% for organic matter source. ...................................................................... 74
List of Figures xiv
Figure 4.3-1 Interaction effect of organic matter sources, type of organic matter, and incubation on dissolved organic carbon in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................... 96
Figure 4.3-2 Interaction effect of organic matter sources, type of organic matter, and incubation on bicarbonate phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph .......................................... 99
Figure 4.3-3 Interaction effect of organic matter sources, type of organic matter, and incubation on total phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................................... 101
Figure 4.3-4 Interaction effect of organic matter sources, type of organic matter, and incubation on non-extractable phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ....................................... 103
Figure 4.3-5 Interaction effect of organic matter sources, type of organic matter, and incubation on soil phosphatase in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................................... 106
Figure 4.3-6 Interaction effect of organic matter sources, type of organic matter, and incubation on soil microbial biomass phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ....................................... 108
Figure 4.3-7 Phosphorus status comparison between existing and new organic matter treatments in subsurface soil from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus....................................................................... 111
List of Figures xv
Figure 4.3-8 Phosphorus status comparison between existing and new organic matter treatments in an Ultisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay).
= total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus. .................................................................................... 112
Figure 4.3-9 Phosphorus status comparison between existing and new organic matter treatments in an Oxisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay).
= total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus. .................................................................................... 113
Figure 4.3-10 Interaction effect of organic matter sources, type of organic matter, and incubation on soil pH in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. .................................................................... 115
Figure 4.3-11 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable iron in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................................... 118
Figure 4.3-12 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable aluminium in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ....................................... 121
Figure 5.2-1 Leaching column with six layers of soil and one layer of soil + organic matter. Suction apparatus (right part of the graph) was set at 10 cm water........................................................................... 140
Figure 5.3-1 Cumulative dissolved organic carbon in leachate expressed per mass of soil after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors. ........................................................... 144
Figure 5.3-2 Cumulative pH on leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors. ............................................ 146
List of Figures xvi
Figure 5.3-3 Dissolved organic carbon concentration throughout the leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. .............................................. 147
Figure 5.3-4 Soil respiration throughout the experiment with tropical rainfall. = control, = peat, = wheat straw, and = lucerne hay.
Bar in each graph represent LSD 5% for organic matter. ............... 149
Figure 5.3-5 Soil respiration throughout the experiment with subtropical rainfall. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph represent LSD 5% for organic matter.............................................................................................. 149
Figure 5.3-6 Bicarbonate phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. .............................................................. 152
Figure 5.3-7 Cumulative dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors............................................................................................... 153
Figure 5.3-8 Total phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. ................................................................................ 155
Figure 5.3-9 Non-extractable phosphorus distribution in leaching column with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. ................................................................................ 156
Figure 5.3-10 Cumulative dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors............................................................................................... 158
Figure 5.3-11 Cumulative dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors. ......................... 160
List of Figures xvii
Figure 5.3-12 Extractable aluminium and iron distribution in leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. .............................................................. 161
Figure 5.4-1 Carbon losses (respiration and leaching) and deposition due to tropical rainfall (4000 mm year-1) in soil (Ultisol) column treated with wheat straw (80 ton ha-1). ........................................................ 166
Figure 6.3-1 The effect of incubation and organic matter sources on phosphate adsorption at 80 ton ha-1 organic matter in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Lines are a logarithmic trend of observations and a visual guide only. ............. 175
Figure 6.3-2 Linearised Langmuir (A) and Freundlich (B) plots of incubation effect on phosphate adsorption for peat treatment (80 ton ha-1) in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).................................. 176
Figure 6.3-3 The effect of organic matter sources of and their levels on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Lines are a logarithmic trend of observations and are a visual guide only........................................ 177
Figure 6.3-4 Linearised Langmuir (A) and Freundlich (B) plots of organic matter sources (120 ton ha-1) on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).................................. 178
Figure 6.3-5 Linearised Langmuir (A) and Freundlich (B) plots of peat levels at six months incubation time on phosphate adsorption. = 40, = 80, = 120, and = 160 ton ha-1 in “Balkuling” Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05)................... 179
Figure 6.3-6 The effect of soils, existing and new soil organic matter on phosphate adsorption in peat and lucerne hay treatments at 80 ton ha-1 and six-month incubation time. = subsurface soil, = Ultisol, and = Oxisol. Lines are polynomial trend for observations and are a visual guide only........................................ 182
List of Figures xviii
Figure 6.3-7 Linearised Brunauer-Emmett-Teller equation of soil type (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption in new soil organic matter + peat treatment. = subsurface soil,
= Ultisol, and = Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.01). .................................................................... 184
Figure 6.3-8 Linearised Brunauer-Emmett-Teller equation of soil organic matter type (existing and new) on phosphate adsorption in subsurface soil. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05)................... 185
Figure 6.3-9 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Ultisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05). .................................................... 185
Figure 6.3-10 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Oxisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05)................... 186
List of Figures xix
LIST OF TABLES
Table 2.3-1 Recent literature on extent to which addition of peat affects a range of physical, chemical and biological properties of soils .......... 12
Table 2.4-1 Half life comparison between three groups of organic phosphorus in soils ........................................................................... 32
Table 3.3-1 Some physical and chemical characteristics of the Oxisol (Balkuling soil) from York, Western Australia.................................... 50
Table 3.3-2 Some chemical characteristics of organic matter sources for the three organic matter treatments.................................................. 50
Table 3.3-3 Analysis of variance for extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. Only main factors and their interactions are shown ........................................... 51
Table 3.3-4 Analysis of variance for microbial biomass carbon in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 53
Table 3.3-5 Analysis of variance for soil respiration in an Oxisol from York, Western Australia. Only main factors and their interactions are shown................................................................................................ 55
Table 3.3-6 Total carbon loss (%) during the experiment (48 weeks) in an Oxisol from York, Western Australia, calculated from the initial total carbon in soils ........................................................................... 58
Table 3.3-7 Analysis of variance for ammonium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown................................................................................................ 59
Table 3.3-8 Analysis of variance for nitrate in an Oxisol from York, Western Australia. Only main factors and their interactions are shown ......... 59
Table 3.3-9 Analysis of variance for bicarbonate phosphorus in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 62
Table 3.3-10 Analysis of variance for extractable potassium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 64
List of Tables xx
Table 3.3-11 Analysis of variance for soil pH in an Oxisol from York, Western Australia. Only main factors and their interactions are shown................................................................................................ 66
Table 3.3-12 Analysis of variance for soil water content in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 68
Table 3.3-13 Analysis of variance for water holding capacity in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 71
Table 3.3-14 Correlation matrices on all soil parameters, presented according to incubation times. Only significant correlations are presented. * represents significant (p ≤ 0.05), ** represents highly significant (p ≤ 0.01) ............................................................... 75
Table 3.3-15 Correlation matrices on all soil parameters overall incubation times. Only significant correlations are presented. * represents significant (p ≤ 0.05), ** represents highly significant (p ≤ 0.01) ....... 79
Table 4.2-1 Description of soil used and locations in Western Australia where soils were collected ................................................................ 88
Table 4.2-2 Organic matter sources (peat, wheat straw, and lucerne hay) used as treatments in the experiment............................................... 89
Table 4.2-3 Treatment combinations and abbreviations used from three factors of the experiment. Soil (subsurface soil, Ultisol, and Oxisol), source of organic matter (control, peat, wheat straw, and lucerne hay), and type of soil organic matter (existing and new) .................................................................................................. 89
Table 4.2-4 Characteristics of three soils (subsurface soil, Ultisol, and Oxisol) used in the glasshouse incubation....................................... .90
Table 4.2-5 Some characteristics of soil (subsurface soil, Ultisol, and Oxisol) samples before and after combustion. Values after ± are standard errors. .......................................................................... 91
Table 4.3-1 Analysis of variance for dissolved organic carbon in three soils from Western Australia. Only main factors and their interactions are shown ......................................................................................... 95
Table 4.3-2 Analysis of variance for bicarbonate phosphorus in three soils from Western Australia. Only main factors and their interactions are shown ......................................................................................... 98
Table 4.3-3 Analysis of variance for total phosphorus in three soils from Western Australia. Only main factors and their interactions are shown.............................................................................................. 100
List of Tables xxi
Table 4.3-4 Analysis of variance for non-extractable phosphorus in three soils from Western Australia. Only main factors and their interactions are shown.................................................................... 102
Table 4.3-5 Analysis of variance for phosphatase activity in three soils from Western Australia. Only main factors and their interactions are shown.............................................................................................. 105
Table 4.3-6 Analysis of variance for microbial biomass phosphorus in three soils from Western Australia. Only main factors and their interactions are shown.................................................................... 107
Table 4.3-7 Analysis of variance for pH in three soils from Western Australia. Only main factors and their interactions are shown ....... 114
Table 4.3-8 Analysis of variance for extractable iron three soils from Western Australia. Only main factors and their interactions are shown.............................................................................................. 117
Table 4.3-9 Analysis of variance for extractable aluminium in three soils from Western Australia. Only main factors and their interactions are shown ....................................................................................... 120
Table 4.3-10 Correlation matrices on all soil parameters. Presented according to incubation times. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation...................................................... 123
Table 4.3-11 Correlation matrices between phosphorus, carbon, iron and soil pH for different incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation...................................................... 126
Table 4.3-12 Correlation matrices between phosphorus, carbon, iron and soil pH for different incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation...................................................... 127
Table 4.3-13 Correlation matrices between phosphorus, carbon, iron and soil pH, separated into soil types and into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation .............................................................................. 128
Table 5.2-1 Treatment combinations and their abbreviation from two types of soils (Ultisol and Oxisol), two rainfall simulations (tropical and subtropical), and four sources of organic matter (control, peat, wheat straw, and lucerne hay) ........................................................ 138
List of Tables xxii
Table 5.3-1 Analysis of variance for dissolved organic carbon in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Only main factors and their interactions are shown ... 143
Table 5.3-2 Cumulative carbon loss (%) from leaching following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical) ..................................................................................... 144
Table 5.3-3 Analysis of variance for leachate pH after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment................... 145
Table 5.3-4 Analysis of variance for dissolved organic carbon throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 147
Table 5.3-5 Analysis of variance for soil respiration after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment................... 148
Table 5.3-6 Respiration estimates (mg CO2) during the experiment for every source of organic matter (control, peat, wheat straw, and lucerne) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical).............................................. 149
Table 5.3-7 Cumulative carbon loss (%) from soil respiration following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical) .............................................................................. 150
Table 5.3-8 Balance between respiration and leaching on every treatment, calculated from loss (%) due to respiration divided by loss (%) due to leaching................................................................................ 150
Table 5.3-9 Analysis of variance for bicarbonate phosphorus throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ................................ 151
Table 5.3-10 Analysis of variance for dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ..................................................................... 152
List of Tables xxiii
Table 5.3-11 Cumulative dissolved reactive phosphorus (mg kg-1) from leaching following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical).............................................. 154
Table 5.3-12 Analysis of variance for total phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 154
Table 5.3-13 Analysis of variance for total phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ...................................................................................... 155
Table 5.3-14 Analysis of variance for non-extractable phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown.............................................................................................. 157
Table 5.3-15 Analysis of variance for extractable aluminium throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 157
Table 5.3-16 Analysis of variance for dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ...................................................................................... 159
Table 5.3-17 Analysis of variance for extractable iron throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 159
Table 5.3-18 Analysis of variance for dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ...................................................................................... 161
Table 5.3-19 Analysis of variance for soil pH throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ................................ 162
List of Tables xxiv
Table 6.3-1 The effect of organic matter sources (80 ton ha-1) and incubation time on phosphate adsorption parameters based on Langmuir and Freundlich equations................................................ 179
Table 6.3-2 The effect of organic matter sources and levels on phosphate adsorption parameters at six months incubation time based on Langmuir and Freundlich equations................................................ 180
Table 6.3-3 The effect of soil organic matter types (existing and new) and soil types (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption parameters based on fitting data to the Brunauer-Emmett-Teller equation................................................................... 183
Table 6.3-4 Correlation between inorganic phosphorus, dissolved organic carbon, extractable aluminium, and extractable iron in uncombusted soils treated with peat and lucerne hay treatments. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correalation ............................................................................ 187
Table 6.3-5 Correlation between phosphorus adsorption parameters and soil chemical properties in “Balkuling” Oxisol. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation.............................. 187
Table 7.1-1 Summary of findings in all experiments in this thesis .................... 195
List of Tables xxv
LIST OF APPENDICES
Appendix A Linearisation of Brunauer-Emmett-Teller equation ....................... 244
Appendix B Article for 17th World Congress of Soil Science, Bangkok 14-21 August 2002.................................................................................... 246
Appendix C Pictures from some experiments .................................................. 257
List of Appendices xxvi
C h a p t e r 1
INTRODUCTION
1.1 GENERAL INTRODUCTION
Marginal soils such as lateritic soils often become the most readily available
alternative to increase the area of agricultural land to maintain food security. This
situation occurs in developing countries of the tropics due to population pressure.
Food shortages occur not only because of the limited production and yield of the
crops, but also due to shrinkage of the production area when productive lands
are used for housing or industrial purposes. One way to address this problem is
to expand agricultural production into areas with lower soil quality, such as
lateritic soils.
Lateritic soils can be low in phosphorus (P) availability for plant growth due to
their high content of aluminium (Al) and iron (Fe)-oxides which are able to adsorb
phosphate from added fertilisers (Buol and Eswaran, 2000; West et al., 1998).
Soil organic matter can decrease the affinity of Al and Fe-oxides for phosphate
and provide biochemical conditions suited for making P more soluble (Dubus and
Becquer, 2001; Haynes and Mokolobate, 2001; Maguire and Sims, 2002).
However, the persistence of organic matter in soil is an important issue as
artificial sources of organic matter of agricultural origin often decompose rapidly,
especially in tropical areas (Chuyong et al., 2000; da Silva and Cook, 2003).
Furthermore, the effectiveness of newly applied organic matter in alleviating P
deficiency is limited, as early decomposition processes are not necessarily
favourable for P to be mineralised or transformed from organic-P to inorganic-P
(Iyamuremye and Dick, 1996).
Peat is an abundant source of organic matter which might be suitable for
reducing the affinity of sesquioxides for phosphate in the longer term because it
is composed of more recalcitrant substances than agricultural organic matter,
allowing it to withstand microbial attack and delay microbial decomposition.
However, the low nutrient content in peat, including P, is a limiting factor for peat
to be applied as organic amendment. Therefore, organic matter sources with
higher nutrient content applied with peat may be a good combination in the soil in
I. General Introduction 1
order to release phosphate by mineralisation or competitive adsorption in the
longer term.
Processes involved in interactions between soil solutes and the soil solution such
as ligand exchange, sorption, and desorption, may be considered as being
directly involved in P transformation in association with organic matter. Some soil
micro-organisms play an important role in mediating P dynamics in soils,
especially where P input from fertilisers is limited (Beck and Sanchez, 1994; Yao
et al., 2002). Immobilisation of soluble phosphate and the promotion of P
mineralisation by production of phosphatase are among the biochemical
processes involving soil organic matter which affect P transformation.
Several studies have concluded that the loss of carbon (C) as dissolved organic-
C via leaching could reach 50 % of the total C loss from soil (Cronan, 1985;
Magill and Aber, 2000). However, these studies mainly concern C loss at the soil
surface. Investigation of C leaching within the soil profile is crucial, especially in
areas with very high annual rainfall. Carbon transfer due to leaching through the
soil profile is important because soluble organic-C may affect soil chemical
properties such as sesquioxide concentrations. For lateritic soils in tropical
areas, heavy rainfall not only causes nutrient leaching (Duwig et al., 2000; Lessa
and Anderson, 1996) but also removes organic substances (Haberhauer et al.,
2002) which may affect sorption and desorption of nutrients such as phosphate.
Soil organic matter can change the phosphate fixing capacity of some soils (Erich
et al., 2002; Iyamuremye and Dick, 1996; Kwabiah et al., 2003; Ohno and
Crannel, 1996). Several mechanisms have been proposed to explain how soil
organic matter affects phosphate adsorption, either due to biotic or abiotic
processes (Iyamuremye and Dick, 1996). In biotic processes, soil organic matter
affects P mineralisation and transformation (Frossard et al., 2000; Magid et al.,
1996), and abiotic processes affect P dynamics via mechanisms such as organic
ligand exchange (Hinsinger, 2001; Violante and Gianfreda, 1993), dissolution
(MacKay et al., 1986; Traina et al., 1986), and desorption (Burkitt et al., 2002;
Rhue and Harris, 1999).
Organic-C leaching may lead to loss of applied organic matter and, at the same
time, may affect P dynamics throughout the soil profile. Interactions between soil
solutes and the soil solution where the process of P sorption and desorption
occur need to be investigated, especially when lateritic soils are used being used.
I. General Introduction 2
This is not only because of higher rainfall in the tropics where lateritic soils are
common, but also due to high phosphate sorbing capacity of the soils. Any
novel information within the frame of limiting factors of these soils is needed to be
repeatedly investigated for the final results in the applied knowledge according to
the specific areas.
1.2 AIMS OF RESEARCH
The main aims of this study were:
1. To measure the persistence of soil organic matter with different C:N ratio over
time in lateritic soils, especially after addition organic matter. At the same
time contributions of organic matter addition to the availability of nutrients N,
P, and potassium (K) were also measured, as well as C dynamics were
measured.
2. To study the contribution of recently applied organic matter to the P status of
lateritic soils. In this regard, the question of whether microbial biomass
phosphorus and soil organic-C will generate more inorganic-P in a low P
environment was addressed, as well as whether phosphatase becomes more
active indicating a transformation from organic-P to inorganic-P.
3. To study the effect of simulated rainfall regimes relevant to tropical and
subtropical environments, on the loss of C by respiration and leaching from
the lateritic soil profile, and to measure the effect of leaching on the change in
soil organic-C, phosphate, and extractable-Al and Fe in the soil.
4. To study the long-term effect of added organic matter with contrasting C:N
ratios on phosphate adsorption across soil type, sources, and level of organic
matter applied. In addition, the effect of phosphate adsorption and P
mineralisation after organic matter addition on the process of bicarbonate-P
release and the alteration of dissolved organic-C after organic matter
treatment on closely related factors of phosphate adsorption in soil, e.g. Al,
Fe, and pH was investigated.
1.3 ORGANISATION OF THE THESIS
Chapter 2 reviewed the role of soil organic matter in highly weathered soils, its
rapid decomposition process, and its persistence. It also considered soil organic
I. General Introduction 3
matter in relation to tropical conditions as well as the main properties of tropical
lateritics such as Ultisols and Oxisols. Peat was evaluated as a source of applied
organic matter in relation to its effects on soil properties. Chapter 3 investigated
the effect of organic matter sources (control, peat, wheat straw, and lucerne hay),
organic matter levels (40, 80, 120, and 160 ton ha-1), and incubation times (3, 6,
9, 12 months) on organic matter persistence and soil nutrient status. Chapter 4
covered in more detail the effectiveness of freshly added organic matter as a
nutrient source and/or as an agent of transformation in lateritic soil. In order to
study the effect (i.e. separated from intrinsic soil organic matter) of recently
applied organic matter (peat, wheat straw, and lucerne hay), soil samples were
first combusted (450° C) to remove the inherent soil organic matter. Subsurface
regolith material with low intrinsic organic matter, an Ultisol, and an Oxisol were
included in this experiment and organic matter was incubated in the soils for 3, 6,
or 9 months. Phosphorus transformation and dynamics were examined in detail
as the lateritic soils have low capacity to supply P in a form that plants can use.
As soil organic-C can be associated with P transformation (Chapters 3 and 4),
the experiment in Chapter 5 investigated how the process of leaching dissolved
organic-C also might affect P dynamics for different simulated rainfall regimes
(tropical and subtropical). Soil type (Ultisol and Oxisol) and organic matter
sources (peat, wheat straw, and lucerne hay) were used to study C dynamics and
its effects on P solubility in leaching columns. Organic forms of P were
considered as non-extractable-P in this study. The most important mechanism
controlling P availability in lateritic soils, i.e. phosphate adsorption, was discussed
in Chapter 6. This experiment used soil samples from the first and second
experiment to determine the effects of added organic matter on phosphate
adsorption. Factors related to the adsorption of P were investigated in three soils
(Oxisol, Ultisol, and subsurface soil from mining site), different sources of organic
matter (peat, wheat straw, and lucerne hay), and four application rates (40, 80,
120, and 180 ton ha-1). The thesis concludes in Chapter 7 with a General
Conclusion. In this chapter, general findings in the experimental scenarios are
discussed, as well as future research.
I. General Introduction 4
C h a p t e r 2
LITERATURE REVIEW
2.1 INTRODUCTION
The loss of soil organic matter in agricultural lands, usually by erosion and/or
rapid mineralisation, is often considered to be the most serious factor in causing
soil degradation (Craswell and Lefroy, 2001; Gregorich et al., 1994; Katyal et al.,
2001; Paustian et al., 1997). This loss can have detrimental effects on soil
physical, chemical, and biological properties. Indeed, maintaining and improving
soil organic matter content is generally accepted as being an important objective
of any sustainable system of agriculture (Gregorich et al., 1994; Kobayashi et al.,
1999; Rodolfi and Zanchi, 2002).
In most developing countries, extending the area of agricultural land is a major
problem because agricultural areas are under the pressure of high rates of
population growth and the expansion of urban areas into productive agricultural
soils. Consequently, soils with low fertility tend to be the most likely alternative
for expanding agricultural development. However, in many developing countries,
the knowledge required for exploiting these soils is far behind that of existing
agricultural soils.
Lateritic soils such as Ultisols and Oxisols, which are commonly low in soil
organic matter, are acidic, have limited cation exchange capacity, and have low
nutrient status could become more suitable for food crops if levels of soil organic
matter are raised. This could be achieved by amending soil with various sources
of organic matter. More specifically, the transformation of soil organic carbon
during the decomposition of organic matter allows soil to provide nutrients for
plants, especially P. In this respect, soil organic carbon (C) has been known to
influence phosphate adsorption (Brennan et al., 1994; Erich et al., 2002; Leytem
et al., 2002) and to be positively correlated with phosphatase activity (Baligar et
al., 1999).
Peat as a source of organic matter may be useful as an organic amendment in
managing unfertile lateritic soils. Broad scale utilisation of peat, i.e. as a growth
medium in forestry nurseries (Gad, 2003; Prasad and Maher, 2004) and as a soil
II. Literature Review 5
conditioner (Pietola and Tanni, 2003), might also resolve the limited content of
soil organic matter in lateritic soils, especially in a tropical country such as
Indonesia. There are abundant peat deposits in Sumatra, Borneo, and
Indonesian Papua (Abdurachman and Suriadikarta, 2000; Jauhiainen and
Vasander, 2002; Page et al., 2002; Whinam et al., 2003). There are also vast but
ignored areas of lateritic soil which might become the next alternatives for
agricultural production to secure food supplies in these areas.
2.2 LATERITIC SOILS
Lateritic soils are very common in tropical regions (Eswaran, 1993). Those in
high rainfall areas are highly weathered with inorganic colloids being mainly
kaolinitic with significant amounts of (hydr)oxides, especially those of iron (Fe)
and aluminium (Al) (Buol and Eswaran, 2000; Eswaran, 1993; West et al., 1998).
Because of the high ambient temperatures throughout the year and the
abundance of water, the turnover of soil organic matter is rapid (Rezende et al.,
1999; da Silva and Cook, 2003; Silver, 1998), with a half life of 9-33 days
(Rezende et al., 1999). However, soil organic matter associated with inorganic
colloids in these soil environments appears to have a considerable turnover time
in soil environments, ranging from 14-275 years (Monreal et al., 1997). This kind
of soil organic matter may have an important role in the reactivities of the soil
colloidal component and in soil fertility, especially in lateritic soils in the tropics.
2.2.1 Some important properties of Ultisols Ultisols are part of a group of soils with an argillic and/or kandic horizon that have
developed in a humid climate (West et al., 1998). Important features of these
soils are: (1) the parent material contains minerals which weather to form silicate
clays, and (2) the climate during soil development characteristically has more
precipitation than evapotranspiration. Ultisols are common in tropical and
subtropical areas between 40° N and 40° S (Eswaran, 1993).
The main characteristic of Ultisols that differentiate them from other soils is that
they must have 35 % or less base saturation in the lower part of the subsoil (Soil
Survey Staff, 1975). This defining characteristic is related to other properties
such as low pH, low cation exchange capacity, and high Al saturation, resulting in
negative effects of the ability of these soils to sustain agricultural plant growth.
II. Literature Review 6
Therefore, chemical properties are the most commonly discussed limiting factors
in managing Ultisols for plant production.
With regards to cation exchange capacity, Ultisols have permanent negative
charges from isomorphic substitution of cations within the clay, but they also have
variable charges (Barnhisel and Bertsch, 1989; Wada and Wada, 1985), which
are positive at low pH. Consequently, most Ultisols are expected to have very
limited net negative charge and this influences on nutrient retention, especially for
cations.
2.2.2 Some important properties of Oxisols Oxisols are characterised by the existence of oxic horizons which usually have a
minimum of 15% clay (Buol and Eswaran, 2000; El Swaify, 1980). Physical
properties of Oxisols are determined by their sesquioxides and kaolinite
mineralogy. The fine and very fine granular structure is very porous and leads to
low bulk density which is generally between 1.0-1.3 Mg m-3 (El Swaify, 1980).
Oxic material with high oxide content is generally not sticky, and can be
hydrophobic to some extent. Water moves rapidly through the large pores
between aggregates. The combination of high porosity (Tejedor et al., 2003) and
low wettability (Scott, 2000) can make these soils susceptible to erosion (El
Swaify, 1980; Scott, 2000; Tejedor et al., 2003), leading to loss of organic matter.
Oxisols have a low capacity to retain cations (Buol and Eswaran, 2000;
Krishnaswamy and Richter, 2002). Cation exchange capacity for these soils
arises from kaolinitic clays and organic matter, it is pH dependent, and effective
cation exchange capacity values are less than cation exchange capacity at pH 7.
Oxisols with substantial content of Fe-oxides have a high fixation capacity for
phosphate applied from fertilizers (Haynes and Mokolobate, 2001; Leytem et al.,
2002). Oxisols also have low quantities of essential elements for plant growth
(Melgar et al., 1992; Moraghan and Mascagni, 1991). As a consequence,
Oxisols and Ultisols are usually the next options in the reclamation program for
agricultural development for many countries in tropical regions. Managing these
soils requires comprehensive knowledge in order to gain profits from their
management, not only for agricultural products but also for sustainability of the
soil resource.
II. Literature Review 7
2.3 SOIL ORGANIC MATTER
The term ‘soil organic matter’ has been widely used to describe the organic
components in soils. The initial concept of soil organic matter refers to the whole
of the organic material in soils, including litter, light fraction, microbial biomass,
water soluble organics, and stabilised organic matter (humus) (Baldock and
Nelson, 1998; Stevenson, 1994). Soil organic matter is defined as the total of all
organic materials contained within and on soils (Baldock and Nelson, 1998), as
well as non-decayed plant and animal tissues, their partial decomposition
products and the living soil biomass (MacCarthy et al., 1990).
Soil organic matter investigations, have been established for decades. The
importance of soil organic matter as a source and sink of C has also been known
for some time (Lal, 1999; Lal, 2001a). Considerable recent research on soil
organic matter has been focused on the dynamics of dissolved organic-C,
ranging from description and chemical composition (Kaiser et al., 2001; Strobel et
al., 2001), quantification and its role in soil chemistry and pedogenesis (Jansen et
al., 2003; Kaiser and Zech, 1998), and the availability of dissolved organic-C to
soil microflora (Kalbitz et al., 2003; Yano et al., 2000). Moreover, knowledge of
dissolved organic matter, as well as dissolved organic nitrogen (N), is well
documented (McDowell, 2003). However, the relationship between these
processes and soil phosphorus (P) is less well understood, especially dissolved
organic-P. In agricultural soils where fertiliser input was regular, dissolved
organic-P was about 70% of total-P in solution (Chardon et al., 1997), and
dissolved organic matter was responsible for redistribution and loss of P in forest
soils (Donald et al., 1993). Dissolved organic-C is in close contact with soil
particles that adsorb phosphate so it is expected that this pool is important in
controlling P dynamics in soil solid and soil solution interactions.
In lateritic soils, soil organic matter is a major contributor to the soil exchange
capacity (Buol and Eswaran, 2000; Pushparajah, 1998; West et al., 1998). Since
the clays in lateritic soils are mainly kaolinitic (Allen and Fanning, 1983; West et
al., 1998), soil organic matter is a major contributor to the negatively charged
colloids and plays an important role in soil chemical (Pushparajah, 1998;
Schnitzer, 2000) and biological properties (Marinari et al., 2000).
II. Literature Review 8
2.3.1 The importance of soil organic matter Organic matter content in highly weathered soils is generally low (Buol and
Eswaran, 2000; Pushparajah, 1998; West et al., 1998). Critical factors include:
climate, where warmer temperature and high rainfall create the fast breakdown of
organic residues, and erosion where soil organic matter is lost (West et al.,
1998). Erosion may also lead to leaching of some forms of organic matter. In
this case, there must be an excess of precipitation in relation to the capacity of
the soil to retain water, so that water will also percolates through the solum
(Miller, 1983). As a consequence, highly weathered soils commonly occur in
warm areas from the humid tropics to humid warm temperate climates.
2.3.1.1 Organic amendments The application of organic matter to soil to improve soil physical, chemical, and
biological properties (Anikwe and Nwobodo, 2002; Khalilian et al., 2002; Larbi et
al., 2002) has been a practice since prehistoric time (Kleber et al., 2003). In the
past, the types of organic matter applied were generally manure, green manure,
compost, crop residues, and to some extent sewage sludge and biosolids.
Today, the term organic amendment has become broader in meaning and
includes materials from various terrestrial and marine sources such as fish bone
meal and crab meal.
The use of organic amendments is increasing with the development of organic
farming and the increase is even higher among conventional farmers (Hartz et
al., 2000). This is not only because of social pressure for healthy food under
conditions that protect the environment, but also as a result of pressure for
recycling organic resources (Thuries et al., 2001).
Among physical properties, organic matter enhances soil particle aggregation for
better water permeability and gas exchange (Poch et al., 2000). Organic matter
increases water retention by preventing shrinking and drying (Hajnos et al., 2003;
Stehouwer, 2003). The black colour of organic matter may facilitate soil warming
in temperate regions by balancing the radiative heat (Schmidt and Noack, 2000).
In relation to chemical characteristics, organic matter increases cation exchange
capacity and buffering capacity to minimise changes in solute concentrations and
pH. Soil organic matter can also adsorb and buffer trace soil components
(Barancikova et al., 2004; Burt et al., 2003; Minkina et al., 2000). In addition, an
improved biological environment in soil results from organic matter addition
II. Literature Review 9
increasing microbial activity, leading to mineralisation and enhanced availability
of nutrients such us N, P, potassium (K), and sulphur (S) for plant growth
(Fortuna et al., 2003; Krishna, 2002; Williamson and Wardle, 2003). These
interactive effects of soil organic matter will be discussed in the following
sections.
Overall, organic amendments are considered the most beneficial for soils which
have low organic matter contents since the small amount of application can
improve soil characteristics of importance to agricultural production.
2.3.1.2 Contribution of organic matter to soil nutrients for plants Organic matter affects nutrient availability for plants directly and indirectly
(Stevenson, 1994). Organic matter is a source of N for plants when mineralised
(Parfitt et al., 1998; Russell and Fillery, 1999), a process which also supplies P
(Parfitt et al., 1998) and S (Blair et al., 1994; Eriksen et al., 1995). The amounts
of each element released during mineralisation, and the rate of release, depend
on the content of the element and elemental ratios in the biomass, which reflects
the origin of the organic matter. Indirectly, organic matter contributes to the
mineral nutrition of plants in soils through incorporation of N and S into humic
substances during decomposition, or by complexation of calcium (Ca), Al, and Fe
from their respective phosphates by humic substances to increase concentrations
of soluble phosphate (Stevenson, 1994).
Incorporation of N (Kelley and Stevenson, 1995) and S (Brown, 1986; Xia et al.,
1998) into humic substances, as well as P (Cooper et al., 1998; Singh and
Amberger, 1990), keeps the nutrients from volatilising (except P) and leaching
and also provides those nutrients to plants for longer periods of time.
Furthermore, there is a relationship between those nutrients within organic matter
which has been described as a definite ratio. The C:N ratio of organic matter has
been used as an indicator for the maturity of compost (Contreras-Ramos et al.,
2004; Priya and Garg, 2004) and the stage of C sequestration in soils (Tan et al.,
2004). The average C/N/P/S ratio of 140:10:1.3:1.3 was claimed to be an
optimum for those nutrients to sustain plant growth (Stevenson, 1994).
Another key reason why organic matter can participate in a wide range of
chemical reactions in soils is due to the presence of oxygen-containing functional
groups (-CO2H, -OH, C=O). These functional groups are capable of enhancing
dissolution of soil minerals by complexing and dissolving metals, transporting
II. Literature Review 10
them throughout the soil solution, and making them available for plants and
microbes (Schnitzer, 2000). Interactions between soil organic matter and metal
ions include ion exchange, surface adsorption, chelation, peptisation, and
coagulation reactions (Schnitzer, 1978).
2.3.1.3 Peat as a source of organic matter and carbon Peat contains 0.551 x 1014 kg C globally (Brake et al., 1999), which is equal to
one quarter of the total organic-C reserve of world soils (Bohn, 1976). Peat
deposits are located mainly in cold and humid regions of Europe and North
America (Charman, 2002; Weiss et al., 2002), but they also exist over thousands
of km2 in equatorial zones, including Indonesia (Nichol, 1999; Page et al., 2002).
Compared with peat in Europe and America, tropical peat is less well studied
(Sieffermann et al., 1988).
Peat is widely used for a variety of purposes such as industrial or domestic fuel
(Korpilahti, 1998; McGovern et al., 2000). In agriculture, peat is useful as an
ingredient for compost mixtures. This mixture is able to retain not only water but
also oil up to seven times its weight (Viraraghavan et al., 1987). Peat can also be
used as an effluent treatment in industrial and municipal landfills to decrease the
content of pollutants and eutrifiers (Lyons and Reidy, 1997). For the purpose of
water treatment, peat can consume 35-50% biological oxygen demand (BOD)
and chemical oxygen demand (COD) after two-hour contact in septic tanks
(Viraraghavan et al., 1987).
Interest in the function of peat soils in global C turnover has also increased due to
the threat of global climate changes (Lal, 1999; Lal, 2001a; Reicosky, 2001;
Rosenzweig and Hillel, 2000), especially in term of the sources and sinks for CO2
and other gases (e.g. CH4)produced by peat land (Karjalainen et al., 1998;
Moore, 2002; Schilstra, 2001). Recently, different peat forming conditions such
as concave wetland and waterlogged organic deposit have been found to be
potential sources of CO2, CH4, and N2O to the atmosphere (Martikainen, 1996;
Weber et al., 2003).
The application of peat as an organic amendment has been studied to assess the
impact on soil physical (Cook and Baker, 1998; McCoy, 1998; Paul and Lee,
1976), chemical (Cook and Baker, 1998; Laverdiere and Kimpe, 1984), and
biological properties (Fernandez et al., 1999). Peat has also been used as a raw
material for some agricultural products. For example, organo-mineral fertilizers
II. Literature Review 11
containing humic substances such as ammoniated peat could reduce the
application of fertilizers. A slow-release fertilizer from the ammoniation of peat
can produce yields of field crops similar to those using mineral fertilizer (Abbes et
al., 1996; Abbes et al., 1995; Coca et al., 1984). Overall, peat could benefit soils
through effects related to (1) surface reaction with clay-size particles, (2)
chelation of polyvalent cations, and (3) biostimulation of micro-organisms, by
similar mechanisms to intrinsic soil organic matter (Johnston et al., 1997; Scagel,
2003; Torun et al., 2003). Table 2.3-1 summarises examples of individual effects
of peat on soil properties.
Table 2.3-1 Recent literature on extent to which addition of peat affects a range of physical, chemical and biological properties of soils
Soil property Effect Author Aggregate stability +
+ (Almendros, 1994) (Muggler et al., 1999)
Hydraulic conductivity + +
(Baker et al., 2000) (Nkongolo et al., 2000)
Physical
Porosity + +
(Baker et al., 2000) (Nkongolo et al., 2000)
Cation exchange capacity + (Almendros, 1994) (Balasoiu et al., 2001)
pH buffering capacity + (Balasoiu et al., 2001) Metal retention (Cu and Cr) + (Balasoiu et al., 2001) Water repellency + (Waniek et al., 2000) As adsorption - (Grafe et al., 2001)
Chemical
Al toxicity - (Pushparajah, 1998) 59Fe uptake + (Cesco et al., 2002) P uptake + (Cooper et al., 1998) Plant yield +
- (Almendros, 1994) (Nkongolo et al., 2000)
Plant growth + (Pushparajah, 1998) Dry weight + (Clapp et al., 1998) Microbial biomass carbon + (Willson et al., 2001)
(Manjaiah and Dhyan, 2001)
Biological
Bacteria and algae colonisation
+ (Beyer et al., 2001)
+ = increase - = decrease
2.3.1.4 Effect of organic matter addition on soil properties and crop responses
The usual dark colour of organic matter helps soil to retain the heat in temperate
regions. This colour can be used as an indirect factor for modelling organic
matter mineralisation in relation to soil moisture and soil temperature (Leiros et
al., 1999). In lateritic soils, organic matter increases aggregation of soil
associated with its Al and Fe content (Barthes et al., 1996; Muggler et al., 1999).
II. Literature Review 12
Macro-aggregates were formed within five months in an Oxisol having a high
content of exchangeable-Al (Barthes et al., 1996). Moreover, addition of organic
matter can increase water holding capacity of the soils (Das et al., 2002; Gispert
et al., 2000; Isaac and Nair, 2002) more than the weight of organic matter itself.
As shown in Figure 2.3-1, these physical effects of organic matter mainly occur at
the soil surface where litter, dead organisms, and their excretions are present.
The stable pool of organic matter which can reach the lower horizons is highly
humified materials, physically protected, and usually exists in micro-aggregates
(Monreal et al., 1997).
Functional groups in soil organic matter can participate in chelation reactions to
form stable complexes with trace element cations, resulting in more or less
availability of micronutrients for plant growth (Greenland, 1986; Pushparajah,
1998).
II. Literature Review 13
Litter, roots, dead organisms, excretion by organisms
Figure 2.3-1 Steps in carbon cycle and the formation of soil organic matter and humus in aerobic soils. Source: Modified from Coyne (1999) and Grunwald (2004).
In terms of chemical properties of soils, the most significant effects of soil organic
matter are cation exchange capacity, nutrient mineralisation, and pH buffering
capacity. These characteristics, together with pH, are the main factors in
capturing, holding, and providing nutrients for plants. In soils low in inherent
negative charge (lateritic soils), soil organic matter plays a considerable role in
increasing cation exchange capacity (Greenland, 1986; Pushparajah, 1998) since
II. Literature Review 14
soil organic matter has between eight to ten times the negative charge of
kaolinitic clays on a mass basis. Figure 2.3-1 describes all chemical reactions
which not only occur at the soil surface, but can also reach the lower part of soil
profile.
2.3.2 Organic matter decomposition and persistence In the first stage of decomposition, litter reaching the soil surface decreases in
volume due to the activity of earthworms and other soil animals (Lee, 1974;
Striganova and Bienkowski, 2000; Topoliantz et al., 2002). Following this stage,
microbial decomposition and intense leaching of solubilized organic substances
are mostly responsible for litter breakdown (Lee, 1974). The next stage is
decomposition of organic substances by microbial cells producing CO2, NH3, and
organic acids (Chen et al., 2001b; Haynes and Mokolobate, 2001). A wide range
of organic compounds are produced by micro-organisms. They include low
molecular organic acids or ligands such as malonic, fumaric, succinic acids
(Chen et al., 2001c; Cieslinski et al., 1997), and soluble humic molecules (Conte
and Piccolo, 2002; Negre et al., 2002; Piccolo et al., 2003). These breakdown
products can complex with phytotoxic monomeric Al to reduce its toxicity and
become adsorbed to Al and Fe oxides and aluminosilicates to block phosphate
from adsorption sites (Haynes and Mokolobate, 2001). The last stage is gradual
decomposition of more resistant substances such as lignin. Lignin decomposition
is a key factor influencing the rate of formation of humic substances. This is
partly due to the retarding effect of macronutrients in soil to the lignin
decomposition (Berg, 2000), even though it is not the case with N (Hobbie, 2000).
Either by the help of soil micro-organism, or by the formation of polyphenol, or by
condensation of sugar and amino acids, the end product of lignin decomposition
is humic substances. Lignin decomposition is one of four major pathways in the
formation of humic substances in soils (Stevenson, 1994). In this pathway, lignin
will be broken into products with low molecular weight. It can also be modified to
lose metoxyl (OCH3) groups in order to generate o-hydroxyphenol and to oxidise
aliphatic chains to form COOH groups. Finally, the modified material is converted
to humic acids and fulvic acids. The half life of lignin can reach more than 10
years (Ganjegunte et al., 2004). It can be assumed that the more lignin content
in organic materials the more resistant the materials from decomposition.
II. Literature Review 15
Accordingly, it can be assumed that modified lignin is major part of peat and
humus from poorly drained soils or from lake sediments.
Soil organic matter persistence is usually related to the age of the soil organic
matter itself, especially resistant soil organic matter which commonly occurs
within micro-aggregates (Monreal et al., 1997). Other persistent forms of soil
organic matter include ‘black carbon’ in Terra Preta soil in Amazon, characterized
by a large amount and stable soil organic matter pools and have high stocks of
nutrient N, P, and Ca (Glaser et al., 2001a; Sombroek et al., 1993; Zech et al.,
1990) or a product of incomplete combustion of vegetation (Schmidt et al., 2001).
Due to the polyaromatic component of the structure of black carbon, it is
chemically and microbiologically resistant in the environment over thousands
years (Glaser et al., 2001a; Glaser et al., 2001b). Therefore, charring of soil
organic matter may transform labile pools of soil organic matter to become stable.
Most studies of soil organic matter persistence have investigated the degradation
of organic amendments such as crop residues (Krauss and Deacon, 1994; Muhr
et al., 1999), composts (Jhorar et al., 1991), manures (Esse et al., 2001), and to
some extent, peat (Murayama and Bakar, 1996). The dominant factor
responsible for decomposition rates of such material is the C:N ratio (Jhorar et
al., 1991; Mishra et al., 2001). The higher the C:N ratio, the slower the
decomposition of the materials (Rahn et al., 2003) because the material contain
higher amounts of lignin, wax, and chitin which are more resistant to
decomposition.
2.3.2.1 The age of soil organic matter Soil organic matter is composed of materials ranging in age from days for plant
residues to greater than 1000 years for humic substances (Collins et al., 2000;
Trumbore et al., 1996; Wang and Chang, 2001) or charcoal (Schmidt et al.,
2001). Measurement of the age of soil organic matter is based on the mean
residence time which can be detected using radiocarbon (14C) dating (Desjardins
et al., 1996; Kalbitz et al., 2003). This procedure measures how long C in a
particular substance decreases to half its initial concentration. Carbon dating
utilises the naturally occurring 14C in soil organic matter to estimate mean
residence time. However, these measurements are not precise because it is very
difficult to monitor small changes in soil organic matter in the short term due to
the large background C concentrations and the natural variability of soils
II. Literature Review 16
(Sparling, 1992), including continued decomposition of old soil organic matter and
resynthesis of new organic matter by micro-organisms (Stevenson, 1994).
To compensate for difficulties in determining the age of soil organic matter, some
methods have been developed to study the age of soil organic matter more
accurately. Radiocarbon dating of inert organic matter in the interlayer space of
swelling clays has been used to determine the age of soil organic matter (Theng
et al., 1992). The combination of radiocarbon dating and aliphatic hydrocarbons
(biomarker analyses) in soil lipids was applied to access the age of peat soil (Bol
et al., 1996). For the young and active soil organic matter usually less than 10
years old, modelling based on decomposition rate (Janssen, 1984) or organic
matter compartment and separation can be used to predict the fate of organic
matter in soils (Andriulo et al., 1999; Balesdent, 1996). However, whilst the age
of soil organic matter has mostly been studied over longer time frames, shorter-
term decomposition of newly added organic matter could be more important in
regard to soil fertility because of a closer link to nutrient dynamics in soils.
Organic matter decomposition rates depend on C:N ratio of the source materials
(Jhorar et al., 1991; Mishra et al., 2001). The higher the C:N ratio, the slower the
decomposition of the materials (Rahn et al., 2003). In soils with very low organic
matter content such as lateritic soils, it is not known for how long recently added
organic matter with different C:N ratios can persist in the soil, and which C:N ratio
has an advantage in maintaining soil C storage in the long term. Many
experiments have been carried out on the mineralisation of organic matter in
soils, especially focusing on organic-C and N, to determined the appropriate rate
of organic matter application as materials such as manure or green manure
(Dunn and Beecher, 1994; Lupwayi and Haque, 1998), straw (Mueller et al.,
1998; Strong et al., 1987), and sewage sludge (Boucher et al., 1999; Iakimenko
et al., 1996) on various soils. However, the particular C:N ratio and the level of
application of biomass for Oxisols in Western Australia in relation to the
persistence of soil organic-C, and the availability of N and P has not been
studied. In order to determine what happens when different types of organic
matter are applied in these soil, it is important to know how and why various
sources of organic matter differ in C and N mineralisation, and their effect on
other soil properties.
II. Literature Review 17
2.3.2.2 Loss and persistence of newly added organic matter Soils play an effective role as storage for C, and as a source or loss of C to pools
outside the soil system. The balance between storage and loss depends on soil
management, as well as climate and soil properties. The losses of C from soil
are mainly from soil organic matter decomposition (respiration), erosion, and
leaching (Akala and Lal, 2000).
One of the problems in managing soil organic matter is its substantial loss due to
temperature and cultivation (Dalal and Chan, 2001). The first factor includes
rainfall and temperature of certain areas. Rainfall, especially mean annual
precipitation, affected initial organic-C content of undisturbed soil as well as
organic-C content of agricultural soil after 20 years of cultivation (Dalal and
Mayer, 1986). In general, temperature differences were associated with different
rates of loss of soil organic matter which were faster in tropical areas than in
subtropical or temperate regions (Bridge and Bell, 1994; Cogle et al., 1995). The
second factor is cultivation; loss of soil organic-C is enhanced by agricultural
activities or pasture establishment compared to losses under native vegetation.
This loss has been reported to be between 10-60% over 10-80 years of
cultivation (Chan, 1997; Russell and Williams, 1982; Standley et al., 1990).
Carbon losses from soil are mainly from soil organic matter decomposition
(respiration), erosion, and leaching (Akala and Lal, 2000). Leaching of C as
dissolved organic-C contributes between 6-46% of total-C loss in forest soils
(Cronan, 1985; Magill and Aber, 2000). Investigations of C balance have focused
on the effect of temperature on C mineralisation (Liechty et al., 1995; Tate et al.,
1993; Zogg et al., 1997), which is only important in surface soil (MacDonald et al.,
1999). For soils in the tropics, the balance between upward (respiration) and
downward (dissolved organic-C leaching) loss could be important, especially for
lateritic soils if the effectiveness of organic matter application is to be understood.
Heavy rainfall may be an additional factor in increasing organic-C loss from soil,
not only by increasing erosion but also by increasing infiltration of water through
the soil profile.
Soils at high latitudes had mean residence time of C between 10-50 years (Bird
et al., 1996). Soil microbial biomass-C was negatively correlated with elevation
and positively correlated with mean annual temperature (Piao et al., 2001). Soil
texture has also been known to affect the persistence of soil organic matter (Bird
II. Literature Review 18
et al., 2000). This could be because of the ability of organic matter to associate
with clays within aggregates or micropores, which protects it from microbial
decomposition. Furthermore, the sizes of aggregates are related to the age of
to simulate N and C flow in soils (Molina, 1996; Nicolardot et al., 1994). In its
development, NCSOIL also simulates 14C and 15N as well as 12C and 14N and
incorporated into a deterministic model (NCSWAP) which simulates interaction of
C and N dynamics with crop growth and soil water (Lengnick and Fox, 1994).
Simplification was made to this model by collapsing microbial succession into one
microbial component and its dynamics determined by the rate of C flow through
populations (Molina, 1996).
The DNDC (Denitrification and Decomposition) model uses the denitrification and
decomposition which are influenced by the soil environment to predict CO2, N2O,
and N2 from agricultural soils (Li, 1996). DNDC has four submodels which
interact with one another, i.e. soil climate, decomposition, denitrification, and
plant growth. The plant growth submodel consists of fertilisation, irrigation,
tillage, crop rotation and manure addition to simulate soil organic matter turnover
(Li et al., 1994).
II. Literature Review 22
The most widely used model, CENTURY, was developed for grassland (Parton et
al., 1988), but has later been updated and enhanced to simulate organic matter
and nutrient dynamics for agricultural crops (Carter et al., 2003; Santen et al.,
2002). This model simulates the long-term dynamics of C, N, P, and S in the top
20 cm of soils on a monthly basis. The pools of soil organic matter included are:
active, slow, and passive; these have different turnover times. Figure 2.3-1
describes the overall conceptual model used in CENTURY, which consists of
several submodels including a soil organic matter submodel.
The active pool of soil organic matter represents soil micro-organisms and
microbial products. It has a turnover time of several months to a few years
depending on the environment. Soil texture influences the turnover rate of this
pool. Sandy soils have higher rates of turnover, whilst clay soils have slower
turnover due to higher stabilisation rate to convert active soil organic matter into
slow soil organic matter. The slow pool includes resistant plant material from the
structural pool and microbial products from active pools. It has a turnover time
from 20 to 50 years. The passive pool is the most resistant to decomposition and
includes physically and chemically stabilised soil organic matter and has a
turnover time of 400 to 2000 years (Brenner et al., 2001; Gijsman et al., 2002). In
this regards, peat is considered as a passive pool of soil organic matter as well
as inert soil organic matter that is present in soil micro- and macro-aggregates.
II. Literature Review 23
Climate Soil Management Topography
Residues
ActiveSOM
SlowSOM
PassiveSOM
SOMSubmodel
PlantGrowth
Submodel
WaterBalance
Submodel
CO2CO2
CO2
CO2
CO2
CO2
Figure 2.3-2 Soil organic matter pools according to the CENTURY model. The soil organic matter submodel is part of the overall model in which carbon cycling is affected by climate, soils, management, and topography. Source: (Brenner et al., 2001).
Further development of the CENTURY model has been made in order to meet
the requirements for farmers in developing countries (Gijsman et al., 2002).
Instead of relying on chemical fertilisers as in developed countries, low input
agricultural systems have been integrated into CENTURY using DSSAT
(Decision Support System for Agrotechnology Transfer) crop models. This new
development will make CENTURY more popular and useful for land reclamation
projects in developing countries.
Among the models discussed, CENTURY is known to have low errors for dataset
applied, to be low in bias, and able to simulate both low and high-N treatments
(Gijsman et al., 2002; Kelly et al., 1997; Smith et al., 1997), and is therefore
considered to be the superior model. However, CENTURY still does not include
II. Literature Review 24
soil pH as a factor affecting soil organic matter turnover (Smith et al., 1997) and
may overestimate soil organic-C in intensively managed agricultural soils (van
Santen et al., 2002). It has also resulted in failure to simulate final measured
data points in a deciduous woodland (Smith et al., 1997).
The C pool plays the main role in all models. For models like NCSOIL and
CENTURY, the C pool is even divided into several sub-pools according to C
stability in corporation with other organic compartments (plant residues, human
related-C, and stable soil organic matter), or in reactivity to other substances.
These pools are taken into account in order to follow their effects on the
dynamics of other elements. In most cases, soil C and its isotopes are at the
centre of the models indicating the dependency of other nutrient pools. Thus,
pools of C are followed by pools and sub-pools of N and P. The ability of C to
form many complex compounds is central to most nutrient cycles (Coyne, 1999)
making them dependent on C for their dynamics. However, in applying any
model, the behaviour of C may vary with type, source, and origin of organic
materials. Therefore, before use, each model needs to be precisely according to
the characteristics of the local area.
2.4 PHOSPHORUS CYCLING IN RELATION TO SOIL ORGANIC MATTER
Phosphorus is an essential nutrient for living organisms due to its vital role in life
processes including photosynthesis in green plants and transformation of energy
in all forms of life (Sanyal and De Datta, 1991). Compared with other essential
nutrients, P is by far the least mobile and least available to plants under most soil
conditions (Hinsinger, 2001). Therefore, P often becomes a major limiting factor
for plant growth.
In the soil solution, P usually occurs at fairly low concentration as orthophosphate
or organic phosphate, while a large proportion is more or less strongly held by
soil minerals (Frossard et al., 2000). Some phosphate ions can be adsorbed to
aluminosilicate clays and/or Fe and Al oxides. Phosphate ions can also form a
range of minerals in combination with metal cations such as Ca2+, Fe3+, and Al3+.
These sorption-desorption and precipitation-dissolution equilibria control
phosphate concentration and the same time both the chemical mobility and
bioavailability. According to Hinsinger (2001) the major factors that determine
II. Literature Review 25
those equilibria are: (1) soil pH, (2) the concentration of anions that compete with
phosphate ions for ligand exchange reactions. Including in these anions is
phosphate ion itself, and; (3) the concentration of metals (Ca, Fe, and Al) that can
precipitate with phosphate ions.
Other factors affecting phosphate availability are the amount and type of
adsorbing phases, such as dominant clay mineral and various oxides (Hue, 1991;
Wahba et al., 2002). Therefore, by considering these factors, the effect of
organic matter on phosphate ions must relate to the second factor due to organic
ligands such as carboxyl.
In general, the effect of soil organic matter on the P cycle is related to the effect
of biotic processes that control P release to soil solution (Frossard et al., 2000).
In this process, P turnover from organic matter plays an important role despite
the fact that organic matter also contributes via abiotic processes such as
adsorption-desorption and dissolution-precipitation. In biotic processes, soil
organic matter plays the central role in mineralisation and immobilisation.
Considering phosphate rocks as a non-renewable resource and the availability of
P is relatively low in soils, P supply to plant growth must be rationalised. This is
true especially for Oxisols and Ultisols that have Fe and Al-oxides which strongly
adsorb soluble phosphate from fertilisers. To improve the efficiency of P supply
in soils, it is imperative to maximise P recycling from crop residues or even from
organic and mineral fertilisers.
The P cycle is very dynamic and involves both geochemical and biochemical
reactions (Figure 2.4-1). The cycle of P is different from that of C, N, and S. This
is because P has no pathways to atmospheric pools. The overall cycle ranges
from solubilisation and fixation at clays and oxide surfaces in the soil solution to
mineralisation-immobilisation processes mediated by micro-organisms. The roles
of soil organic matter and soil micro-organisms are very significant in the P cycle.
Microbial activity is an agent that functions as a reversible sink for P, continuously
consuming and releasing P to the soil solution (Stewart, 1981).
II. Literature Review 26
Organicfertiliser
Mineralfertiliser
Plantaerial parts
Plant roots
Inorganic-P in primary minerals
Other forms of inorganic-P
Inorganic-P in the soil solution Microbial-
P
Labileorganic-P
Stableorganic-
P
Losses by erosion, run-off, and leaching
Soil/plant system
Exportation in plant and animal products
Figure 2.4-1 Soil phosphorus cycle, its components and measurable fractions (adapted from Stewart and Sharpley (1987)). Arrows represent fluxes between reservoirs.
In highly weathered soils, which are commonly acidic (Eswaran, 1993; West et
al., 1998), phosphate is usually adsorbed to Al and Fe oxides or precipitated as
insoluble Al- and Fe-phosphates (Iyamuremye and Dick, 1996; Lindsay et al.,
1989; Stevenson and Cole, 1999). Both forms are poor sources of P for plants
and P deficiency is common in soils rich in Fe and Al, such as Oxisols and
Ultisols of the tropics and sub tropics. The application of organic matter to those
soils may complex Al and Fe, in either ionic form or as oxides. Furthermore, in
many soils, the availability of P may depend more on the turnover of easily
decomposable soil organic matter than on the release of adsorbed phosphate.
2.4.1 Dissolution and precipitation The beginning of the P cycle involves parent materials, climate, and time as
factors affecting the existence and amount of P in soils. Dissolution of P from its
origin can usually be explained as dissolution from apatite [Ca10X2(PO4)6, where
27II. Literature Review
X = OH- or F-; Ca may also be substituted with Na or Mg, and PO4 with CO3]
which are the most common primary phosphate minerals.
Precipitation of phosphate with Ca carbonates and its adsorption on Al and Fe
hydrous oxides has been known since the mid-nineteenth century. Calcium
phosphate is formed following phosphate adsorption to calcite (Syers and Curtin,
1989). As phosphate is adsorbed to the surface of calcite, monocalcium
phosphate [Ca(H2PO4)2] precipitates and transforms to become dicalcium
phosphate dihydrate (CaHPO4.2H2O), to octocalcium [Ca8H2(PO4)6.5H2O] and
finally to hydroxyapatite [Ca10(PO4)6(OH)2]. Adsorption of phosphate with Al and
Fe oxides resulted in the formation of amorphous Al-phosphate and Fe-
phosphate which may later transform to variscite (AlPO4.2H2O) and strengite
(FePO4.2H2O) after prolonged aging (Lindsay et al., 1989). Phosphate
adsorption is not only attributed to the hydrous oxides of Al and Fe, but also to
1:1 lattice clay such as kaolinite, especially in acid tropical soils (Dubus and
Becquer, 2001; Sanyal et al., 1993; Uehara and Gillman, 1981).
Apatite dissolution requires a source of H+, which originates from plant roots,
micro-organisms, or from the soil itself (MacKay et al., 1986; Smillie et al., 1987).
Dissolution from precipitated P (with Ca, Al, and Fe) is also possible
(Iyamuremye and Dick, 1996), and can make P more available when organic
residues are added to soils. Dissolution of both groups of solid phases relies on
organic acids such as citrate and formic acid (Traina et al., 1986) as the source of
H+. Organic ligands such as citrate can form stable complexes with Al3+ or Fe 3+,
promoting dissolution of solid phases containing Fe or Al and phosphate.
2.4.2 Sorption and desorption The term sorption is used to describe the surface accumulation of phosphate on
soil components which can be accompanied by penetration of adsorbed P by
diffusion into the adsorbent body, resulting in further adsorption of the adsorbed
species (Sanyal and De Datta, 1991). These two processes take place
simultaneously. Desorption is defined as the adsorbed phosphate ions being
released as a reciprocal action of sorption.
The sorption of phosphate ions has been interpreted as a biphasic reaction
(Rhue and Harris, 1999), i.e. the initial and rapid sorption which is believed to last
in the order of minutes or hours. The second sorption is slow reaction lasting
weeks or months. There are two mechanisms responsible for the process:
II. Literature Review 28
1. Ion exchange, mechanism from the electrostatic attraction of phosphate
anions to positively charged sites exist on variable-charge surfaces below
the zero point charge, and
2. Ligand exchange, mechanism by which a phosphate anion replaces a
surface hydroxyl that is coordinated with a metal cation in a solid phase
(Rhue and Harris, 1999).
The last mechanism, ligand exchange, is also referred as specific adsorption and
is characterised by:
• Adsorption is accompanied by the release of OH-,
• Ligand exchange shows a high degree of specificity,
• The adsorption step often occurs much more rapidly than the
desorption step, leading to apparent hysteresis in the isotherm,
and
• Adsorption is accompanied by an increase in surface negative
charge (McBride, 1994).
The second sorption or the slow phase is thought to have two mechanisms, i.e.
diffusion (either into soil particles or to surface sites of limited accessibility), and
precipitation (either by direct heterogenous nucleation or following the dissolution
of the host solid, following an initial adsorption reaction) (Rhue and Harris, 1999).
Phosphorus sorption and desorption are important mechanisms controlling soil
phosphate partitioning between the sorbed and solution phases (Burkitt et al.,
2002) and have crucial implication for P management. This mechanism is
commonly referred to phosphate buffering capacity, which describes a soil’s
capacity to moderate changes in phosphate solution concentration when P is
added or removed from the soil (Ozanne, 1980).
In relation to soil organic matter, the release of phosphate by mineralisation may
be difficult to separate from the sorption mechanisms, especially in soils with high
sorbing capacity such as lateritic soils. This is not only because of the high
content of sesquioxides and 1:1 clay content in these soils, but also due to
negligible amounts of soil organic matter. Phosphate adsorbed to metal oxy-
hydroxides may be desorbed by ligand exchange with organic anions (Fox et al.,
1990). However, by observing the net release of extractable-P and determining
II. Literature Review 29
phosphate adsorption isotherms, the two processes can be separated as more
and less important in releasing phosphates to the soil solution. Furthermore, as
Afif et al. (1995) found that the effect of soluble organic matter on phosphate
release from Oxisols to be transient, this leads to the question of whether peat
would have a longer-term effect on P adsorption due to its resistance to
decomposition. At the same time peat can possibly slowly release soluble
organic ligands which compete for adsorption sites with phosphate. This also
could be determined by analysing phosphate adsorption isotherms.
2.4.3 Organic phosphorus In intensively-managed agricultural soils, most organic-P is considered a poor
nutrient source for agricultural production (Gressel and McColl, 1997). In
countries with fertiliser-based agriculture, research on organic-P has therefore
been limited (Magid et al., 1996). However, countries with low-input agricultural
systems rely more heavily on mineralisation of organic-P to supply P to crops.
Organic-P can also be important in P transfer, because organic-P constitutes a
large amount of total-P in the soil solution (Shand and Smith, 1997) or in the soil
leachate (Turner and Haygarth, 2000). Even though the process of transforming
of organic-P to inorganic-P remains poorly understood (Frossard et al., 2000)
research on this subject is challenging, especially in areas where P supply is very
limited due to fixation and lack of P natural resources.
Organic-P represents 15-80% of total-P in soils (Dalal, 1977; Stevenson and
Cole, 1999). The higher content would be in peat or Histosols and undisturbed
forest soils. Tropical soils, such as highly weathered lateritics, are known to have
higher proportions of organic-P than soils of temperate regions, especially in soils
in which P fertilisers are not utilized (Beck and Sanchez, 1994).
The relationship between soil organic matter decomposition and organic-P
mineralisation can be approached from different points of view. First, P can be
considered as part of soil organic matter quality, and therefore P content of
organic matter is a factor determining decomposition rate (Sinsabaugh and
Moorhead, 1994; Stevenson and Cole, 1999). Second, more inorganic-P release
to the soil can be considered as an independent function of decomposition of
organic residues (Gressel and McColl, 1997). Despite the fact that both
approaches only differ in the role of P, decomposition of soil organic matter itself
II. Literature Review 30
is strongly related to soil organic-C in the organic residues. Hence, the organic-P
mineralisation is expected to be strongly dependent on organic-C.
2.4.3.1 Organic phosphorus classification Three major groups of compounds have been identified so far in soil organic-P.
1. Inositol phosphates represent 50% of total organic-P in soils. The most
common inositol found in nature is myo-inositol (Stevenson and Cole, 1999).
Other isomers of this group also known to exist in soils are scyllo-inositol,
chiro-inositol, and neo-inositol (Cosgrove, 1980). Inositol phosphates in soil
exist in complex forms and are sometimes present in a complex containing
carbohydrate or protein. However, organic-P additions increased plant
growth in soil limited with available inorganic-P. In a pot experiment, the
application of inositol phosphate produced a greater amount of dry matter of
lupin (Lupinus albus and Lupinus angustifolius) (Adams and Pate, 1992).
The half life of this fraction of organic-P can exceed one year which suggests
that it is broken down slowly (Mueller-Harvey and Wild, 1986) (Table 2.4-1).
2. Phospholipids represent 0.5-7.0% of total organic-P in soils (Stevenson and
Cole, 1999). Phosphoglycerides contribute a major part of phospholipids,
although phosphoglycolipids, phosphodiollipids, phosphosphingolipids, and
phosphonolipids have also been found, the concentrations were much
smaller. Similar to inositol phosphate, phospholipids are derived from plant
debris, animal waste, and microbial biomass. This is in agreement with
phospholipids being the major fraction of total organic-P in plant tissue
(Anderson and Malcolm, 1974). The half life of phospholipids during
decomposition by microorganisms such as Streptomyces sp., thermophillic
Bacillus spp., and Mycobacterium spp. in soil was predicted to be about four
to six months, i.e. easier to decompose than inositol phosphate (Soliman and
Radwan, 1981) (Table 2.4-1).
3. Nucleic acids or their derivatives represent up to 3% of total organic-P in soils
(Stevenson and Cole, 1999). This form of organic-P can be rapidly
mineralised in soil and incorporated into micro-organisms (Dalal, 1977). This
fraction of organic-P has a short half life (Table 2.4-1), ranging from 4-40
hours (Brodl and Ho, 1992; DeRocher et al., 1991; Mason, 1986; Mueller-
Harvey and Wild, 1986).
II. Literature Review 31
The form of the rest of the organic-P in soil, about 40% of total organic-P, is
unknown. This fraction may play an important role in transformation of organic-P
to inorganic-P, especially where the amount of organic-P is high such as in
lateritic soils of the tropics.
Table 2.4-1 Half life comparison between three groups of organic phosphorus in soils
Organic phosphorus Item Half life Author
Inositol phosphates IP5+6 > 22 months (Mueller-Harvey and Wild, 1986)
Phospholipids - 4-6 months (Soliman and Radwan, 1981) Phosphatidylcho
pH (1:2.5) 5.46 2Total-C (%) 3.12 2Total-N (%) 0.03 3Bicarbonate-P (mg kg-1) 1.07 4Exch. cation (cmol+ kg-1) CEC 6.5 Ca 0.9 Mg 2.5 K 0.3 Na 0.6 1Pipette method, 2Leco C and N analyser, 3Bicarbonate extraction, 4Silver thiourea.
Table 3.3-2 Some chemical characteristics of organic matter sources for the three organic matter treatments
Source of carbon 1Total-N
(%) 1Total-C
(%) C/N ratio
2Total-P (%)
Peat 0.56 31.5 56 0.16 Wheat straw 0.76 43.4 57 0.20 Lucerne hay 2.69 41.3 15 0.25 1Leco C and N analyser, 2HNO3 and HClO4 digestion.
3.3.1 Carbon
3.3.1.1 Extractable organic carbon Extractable organic-C (0.5 M K2SO4) content of the soil decreased (p ≤ 0.01,
Table 3.3-3) with incubation time at all levels of organic matter. The decrease
ranged about 16-25% from the initial extractable organic-C. Incubation of organic
matter for three and six months led to the highest extractable organic-C content
in the soil. For the first two incubation times, the level of 120 and 160 ton ha-1 for
lucerne hay treatment showed decreasing extractable organic-C contents and
after that showed little change until the end of the experiment (Figure 3.3-1). The
trend was similar to wheat straw application with lower extractable organic-C
content. In contrast to lucerne hay and wheat straw, peat application showed no
effect on soil extractable organic-C with incubation time.
III. Transformation of Soil Organic Matter in An Oxisol... 50
Organic matter application increased the extractable organic-C content in the soil
and the increase followed the order of control=peat<wheat straw<lucerne hay
(Figure 3.3-1). Among three different sources of organic matter, only wheat straw
and lucerne hay significantly increased (LSD 5%) extractable organic-C content.
The higher the level of organic matter applied, the more extractable organic-C
formed in soils, especially at three and six-month incubation. Peat treatment did
not affect extractable organic-C content and were not significant compared with
controls. The order of effect on extractable organic-C in all levels of organic
matter was control<peat<wheat straw<lucerne hay (LSD 5%).
Table 3.3-3 Analysis of variance for extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 1026371 342124 8.97 < 0.001 Organic matter 3 29535481 9845160 258.13 < 0.001 Level of organic matter 3 10590147 3530049 92.55 < 0.001 Inc x OM 9 855714 95079 2.49 0.012 Inc x Level 9 389145 43238 1.13 0.344 OM x Level 9 17685078 1965009 51.52 < 0.001 Inc x OM x Level 27 639967 23702 0.62 0.924 Residual 128 4881989 38141 Total 191 65603893 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 51
Figure 3.3-1 Comparative effect of organic matter sources, organic matter levels, and incubation times on extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
0
1000
2000
3000
0 3 6 9 12
0
1000
2000
3000
0 3 6 9 12
0
1000
2000
3000
0 3 6 9 12
0
1000
2000
3000
0 3 6 9 12
month
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
Ext
ract
able
org
anic
car
bon
(mg
kg-1
)
III. Transformation of Soil Organic Matter in An Oxisol... 52
3.3.1.2 Microbial biomass carbon The main effect of organic matter application increased (p ≤ 0.01, Table 3.3-4)
soil microbial biomass-C over time. However, for wheat straw and lucerne hay,
the highest microbial biomass-C content was at six or nine months after
treatment (Figure 3.3-2). At 12 months after treatment, the level of microbial
biomass-C dropped and this occurred for all rates of wheat straw and lucerne hay
applications. Peat application, on the contrary, showed a steady increase in soil
microbial biomass-C over time as did controls. Peat even displayed higher
content of microbial biomass-C in soil at the level of 40 ton ha-1 compared with
wheat straw and lucerne hay application after 12 months’ incubation.
The organic matter content was related to soil microbial biomass-C (Figure
3.3-2). The higher the amount of organic matter applied the more microbial
biomass-C was present in the soils. Wheat straw and lucerne hay mostly
increased microbial biomass-C content more than peat applications. Lucerne
hay showed the greatest overall effect (LSD 5%) on soil microbial biomass-C
followed by wheat straw, then peat.
Table 3.3-4 Analysis of variance for microbial biomass carbon in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 1732398 577466 21.21 < 0.001 Organic matter 3 9680147 3226716 118.49 < 0.001 Level of organic matter 3 3960820 1320273 48.48 < 0.001 Inc x OM 9 2338144 259794 9.54 < 0.001 Inc x Level 9 552106 61345 2.25 0.023 OM x Level 9 3551483 394609 14.49 < 0.001 Inc x OM x Level 27 1728032 64001 2.35 < 0.001 Residual 128 3485715 27232 Total 191 27028844 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 53
Figure 3.3-2 Comparative effect of organic matter sources, organic matter levels, and incubation times on microbial biomass carbon in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
0
500
1000
1500
2000
0 3 6 9 12
0
500
1000
1500
2000
0 3 6 9 12
0
500
1000
1500
2000
0 3 6 9 12
0
500
1000
1500
2000
0 3 6 9 12
month
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
Mic
robi
al b
iom
ass
carb
on (m
g kg
-1)
III. Transformation of Soil Organic Matter in An Oxisol... 54
3.3.1.3 Soil respiration The incubation of soil with organic matter, in general, increased CO2 emission
from the soil at the beginning and this decreased as incubation time increased
(Figure 3.3-3). At the lower organic matter applications (40 and 80 ton ha-1), CO2
emissions were steady up to nine months after treatments. But at higher organic
matter additions (120 and 160 ton ha-1), CO2 emissions dropped after six months
incubation. The amount of CO2 emitted at twelve-month incubation time was
about half of the amount after three months incubation, especially for wheat straw
and lucerne hay at 120 and 160 ton ha-1.
Meanwhile, organic matter levels effected CO2 emission at regular pattern
(Figure 3.3-3). Although not significant at 40 ton ha-1 level, the higher the level of
OM applied the larger the amount of CO2 emission. Organic matter type also
affected CO2 emission from soil. At the level of 120 and 160 ton ha-1, lucerne hay
and wheat straw hay had significant differences (LSD 5%) from peat treatments,
especially at three, six, and nine-month incubation time, and the order was
control<peat<wheat straw<lucerne hay.
Table 3.3-5 Analysis of variance for soil respiration in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 91642.6 30547.5 43.09 < 0.001 Organic matter 3 668505.6 222835.2 314.34 < 0.001 Level of organic matter 3 346589.6 115529.9 162.97 < 0.001 Inc x OM 9 122749.7 13638.9 19.24 < 0.001 Inc x Level 9 48163.0 5351.4 7.55 < 0.001 OM x Level 9 310527.0 34503.0 48.67 < 0.001 Inc x OM x Level 27 73602.3 2726.0 3.85 < 0.001 Residual 128 90739.3 708.9 Total 191 1752519.1 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 55
Figure 3.3-3 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil respiration in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
month
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
0
100
200
300
400
0 3 6 9 12
0
100
200
300
400
0 3 6 9 12
0
100
200
300
400
0 3 6 9 12
0
100
200
300
400
0 3 6 9 12
Soi
l res
pira
tion
(mg
CO
2 kg-1
wee
k-1 )
56III. Transformation of Soil Organic Matter in An Oxisol...
3.3.1.4 The loss of carbon as carbon dioxide Cumulative respiration describing the amount of CO2 loss from all treatments is
presented in Figure 3.3-4.
Figure 3.3-4 Cumulative respiration representing total carbon loss following the application of organic matter during the experiment in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay.
The higher the levels of organic matter the more CO2 emission from soil, but the
trends decreased after three, six, and nine-month incubation. Carbon loss from
wheat straw and lucerne hay as CO2 was higher than peat.
Table 3.3-6 shows the percentage of C-loss as CO2 calculated from the initial
total-C in soil and each treatment. Carbon loss ranged from 0.4-5.5% from the
initial total-C in soil after treatment. Peat was the lowest and lucerne hay was the
highest in C loss. In addition, the higher the level of organic matter, the larger
proportional amount of C loss from corresponding organic matter treatment,
except controls.
40 ton ha-1
0
600
1200
0 12 24 36 48
80 ton ha-1
0
600
1200
0 12 24 36 48
120 ton ha-1
0
600
1200
0 12 24 36 48
160 ton ha-1
0
600
1200
0 12 24 36 48
mg
CO 2 k
g -1
wee
k-1
w eeks w eeks
mg
CO 2 k
g -1
wee
k-1
III. Transformation of Soil Organic Matter in An Oxisol... 57
Table 3.3-6 Total carbon loss (%) during the experiment (48 weeks) in an Oxisol from York, Western Australia, calculated from the initial total carbon in soils
Level of organic matter (ton ha-1) Organic matter source 40 80 120 160 Control 0.5a 0.5a 0.5a 0.4a
Peat 0.6a 0.7a 0.7a 0.9a
Wheat straw 1.0ab 1.8abc 2.8c 3.1cd
Lucerne hay 1.1ab 2.5bc 4.4de 5.5e
Numbers followed by the same letters in superscript indicate differences according to LSD 5% (1.5).
3.3.2 Nitrogen
3.3.2.1 Ammonium Ammonium concentration decreased with increasing incubation time (Figure
3.3-5). The concentrations of ammonium were at the highest values at the first
two incubation times. For the first two incubation times, lucerne hay showed
significant differences (LSD 5%) in ammonium content compared with controls at
every level of organic matter addition, but these differences gradually declined as
the experiment progressed. The lower values of ammonium concentrations at
nine and twelve months incubation were still higher (LSD 5%) than the control.
Among the three different sources of organic matter, only lucerne hay had
the rate of lucerne hay applied, the more ammonium formed in soils, especially at
three and six-months of incubation. The order of effect on ammonium was wheat
straw≤control≤peat<lucerne hay. However, there was no difference (LSD 5%) in
ammonium concentration in control soil samples or soil amended with peat or
wheat straw.
III. Transformation of Soil Organic Matter in An Oxisol... 58
Table 3.3-7 Analysis of variance for ammonium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 879943.5 293314.5 336.26 < 0.001 Organic matter 3 5669619.4 1889873.1 2166.57 < 0.001 Level of organic matter 3 266073.9 88691.3 101.68 < 0.001 Inc x OM 9 2812928.6 312547.6 358.31 < 0.001 Inc x Level 9 232900.4 25877.8 29.67 < 0.001 OM x Level 9 788233.8 87581.5 100.40 < 0.001 Inc x OM x Level 27 683774.2 25325.0 29.03 < 0.001 Residual 128 111652.7 872.3 Total 191 11445126.5 Inc = incubation, OM = organic matter.
Table 3.3-8 Analysis of variance for nitrate in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 16226.77 5408.92 114.20 < 0.001 Organic matter 3 32882.43 10960.81 231.41 < 0.001 Level of organic matter 3 717.43 239.14 5.05 0.002 Inc x OM 9 34766.46 3862.94 81.56 < 0.001 Inc x Level 9 992.46 110.27 2.33 0.018 OM x Level 9 2843.80 315.98 6.67 < 0.001 Inc x OM x Level 27 2575.22 95.38 2.01 0.005 Residual 128 6062.67 47.36 Total 191 97067.24 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 59
Figure 3.3-5 Interaction effect of organic matter sources, organic matter levels, and incubation times on ammonium (A) and nitrate (B) in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
0
500
1000
1500
0 3 6 9 12
0
500
1000
1500
0 3 6 9 12
0
500
1000
1500
0 3 6 9 12
80 ton ha-1
0
500
1000
1500
0 3 6 9 12month
0
40
80
120
0 3 6 9 12
month
Am
mon
ium
(mg
kg-1
)
Nitr
ate
(mg
kg-1
)
0
40
80
120
0 3 6 9 12
0
40
80
120
0 3 6 9 12
0
40
80
120
0 3 6 9 1
month
40 ton ha-1
120 ton ha-1
160 ton ha-1
A B
2
III. Transformation of Soil Organic Matter in An Oxisol... 60
3.3.2.2 Nitrate Incubation time showed a significant effect (p ≤ 0.01, Table 3.3-8) on nitrate
concentration in soil. At three and twelve-month incubation, nitrate concentrations
were higher than that of six and nine-month incubation over the whole dataset.
This effect (p ≤ 0.01) occurred at all levels of organic matter, i.e. 40, 80, 120, and
180 ton ha-1 (Figure 3.3-5).
Among organic matter applications, peat application recorded a higher (LSD 5%)
nitrate content in soil compared with wheat straw, but lucerne hay was the
highest (LSD 5%), especially at the end of the experiment (twelve-month
incubation).
3.3.3 Phosphorus Incubation time had a decreasing effect (p ≤ 0.01, Table 3.3-9) on bicarbonate-P,
especially at 80, 120, and 160 ton ha-1 (Figure 3.3-6). This was despite the fact
that the increasing level of all organic matter treatments increased bicarbonate-P
contents in soil. The three-month incubation had the highest bicarbonate-P
concentration for peat, wheat straw, or lucerne hay, at 40, 80, 120, or 160 ton
ha-1. The exception was for the lucerne hay treatment (120 ton ha-1) where
maximum bicarbonate-P was at six months incubation.
on bicarbonate-P concentration in soil. Among all organic matter sources,
lucerne hay had the highest (LSD 5%) bicarbonate-P content in the soil. The
higher the level of lucerne hay applied the more bicarbonate-P content. The
overall order of bicarbonate-P contents among organic matter source was
control<wheat straw<peat<lucerne hay.
III. Transformation of Soil Organic Matter in An Oxisol... 61
Table 3.3-9 Analysis of variance for bicarbonate phosphorus in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 1175.40 391.80 12.13 < 0.001 Organic matter 3 16127.90 5375.97 166.45 < 0.001 Level of organic matter 3 8695.19 2898.40 89.74 0.002 Inc x OM 9 982.19 109.13 3.38 < 0.001 Inc x Level 9 612.73 68.08 2.11 0.033 OM x Level 9 5803.90 644.88 19.97 < 0.001 Inc x OM x Level 27 1400.69 51.88 1.61 0.043 Residual 128 4134.00 32.30 Total 191 38931.98 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 62
Figure 3.3-6 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil bicarbonate phosphorus in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
0
20
40
60
80
0 3 6 9 12
0
20
40
60
80
0 3 6 9 12
0
20
40
60
80
0 3 6 9 12
0
20
40
60
80
0 3 6 9 12
month
Bic
arbo
nate
pho
spho
rus
(mg
kg-1
)
III. Transformation of Soil Organic Matter in An Oxisol... 63
3.3.4 Potassium As for bicarbonate-P, extractable-K decreased (p ≤ 0.01, Table 3.3-10) with
incubation time (Figure 3.3-7). At six and nine-month incubation, K contents
were at the highest points for wheat straw application at all levels and lucerne
hay at 40, 80, and 120 ton ha-1. The rest of the treatments had highest K content
after three months incubation.
Extractable-K in the soil increased as organic matter application rate increased
(p ≤ 0.01). The order of effect for organic matter source was control<peat<wheat
straw<lucerne hay.
Table 3.3-10 Analysis of variance for extractable potassium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 319945 106648 11.92 < 0.001 Organic matter 3 28853900 9617967 1075.27 < 0.001 Level of organic matter 3 5222754 1740918 194.63 < 0.001 Inc x OM 9 593368 65930 7.37 < 0.001 Inc x Level 9 214673 23853. 2.67 0.007 OM x Level 9 5950499 661167 73.92 < 0.001 Inc x OM x Level 27 491309 18197 2.03 0.005 Residual 128 1144917 8945 Total 191 42791366 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 64
Figure 3.3-7 Interaction effect of organic matter sources, organic matter, and incubation times on extractable potassium in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5 % for organic matter source in every graph.
0
500
1000
1500
2000
0 3 6 9 12
0
500
1000
1500
2000
0 3 6 9 12
0
500
1000
1500
2000
0 3 6 9 12
0
500
1000
1500
2000
0 3 6 9 12
month
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
Pot
assi
um (m
g kg
-1)
III. Transformation of Soil Organic Matter in An Oxisol... 65
3.3.5 Other chemical properties
3.3.5.1 Soil pH Incubation with no organic matter treatment increased soil pH to 5.76 (total mean,
ANOVA). Peat tended to increase (LSD 5%) soil pH over time relative to
controls, as did wheat straw. Lucerne hay treatment decreased (LSD 5%) soil pH
over time (Figure 3.3-8).
The increasing level of organic matter demonstrated an increasing effect on soil
pH, especially for lucerne hay treatment, but a decreasing effect from peat. Both
were significant (LSD 5%).
Table 3.3-11 Analysis of variance for soil pH in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 0.18501 0.06167 3.70 0.014 Organic matter 3 35.94693 11.98231 718.38 < 0.001 Level of organic matter 3 0.32141 0.10714 6.42 < 0.001 Inc x OM 9 1.60576 0.17842 10.70 < 0.001 Inc x Level 9 0.24160 0.02684 1.61 0.119 OM x Level 9 5.31778 0.59086 35.42 < 0.001 Inc x OM x Level 27 0.41181 0.01525 0.91 0.591 Residual 128 2.13500 0.01668 Total 191 46.16529 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 66
Figure 3.3-8 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil pH in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
5
6
7
8
0 3 6 9 12
5
6
7
8
0 3 6 9 12
5
6
7
8
0 3 6 9 12
5
6
7
8
0 3 6 9 12
month
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
Soil
pH
III. Transformation of Soil Organic Matter in An Oxisol... 67
3.3.5.2 Water content Water content of the soils (one week after watering to 60% water holding
capacity) increased (p ≤ 0.01, Table 3.3-12) with incubation time (Figure 3.3-9).
At the longest period (12 months), however, the water content decreased to a
point similar to that of the beginning of the experiment. This effect was significant
(p ≤ 0.01). At nine-month incubation, peat increased (p ≤ 0.01) water content
about 132% from the initial water content; whilst wheat straw increased (p ≤ 0.01)
water content about 77% and lucerne hay about 153%. Wheat straw and lucerne
hay showed their significant differences from control and peat on the last three
highest levels (80, 120, and 160 ton ha-1) on every incubation time.
The increased level of organic matter source raised the one-week water content
of the soil, especially wheat straw and lucerne hay at 80 and 120 ton ha-1. At the
highest level of organic matter (160 ton ha-1), the effect on water content was
about the same as 120 ton ha-1. The effect was about the same as of 40 ton ha-1,
especially at nine-month incubation. Peat levels had no significant effect on
water content and were not different from the control. The order of organic
matter sources in increasing soil water content was control<peat<wheat
straw<lucerne hay.
Table 3.3-12 Analysis of variance for soil water content in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 3054.946 1018.315 146.37 < 0.001 Organic matter 3 1883.195 627.732 90.23 < 0.001 Level of organic matter 3 556.260 185.420 26.65 < 0.001 Inc x OM 9 44.195 4.911 0.71 0.703 Inc x Level 9 41.503 4.611 0.66 0.741 OM x Level 9 380.362 42.262 6.07 < 0.001 Inc x OM x Level 27 105.812 3.919 0.56 0.958 Residual 128 890.538 6.957 Total 191 6956.812 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 68
Figure 3.3-9 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water content in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
0
10
20
30
0 3 6 9 12
0
10
20
30
0 3 6 9 12
0
10
20
30
0 3 6 9 12
0
10
20
30
0 3 6 9 12
month
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
Wat
er c
onte
nt (%
w/w
)
III. Transformation of Soil Organic Matter in An Oxisol... 69
3.3.5.3 Water holding capacity As for soil moisture content, soil water holding capacity increased (p ≤ 0.01,
Table 3.3-13) with incubation time until nine months of incubation (Figure 3.3-10).
At twelve-month incubation, water holding capacity was returned to the value
near that of six-month incubation.
At the three, six, and nine-month incubation, wheat straw and lucerne hay
showed a significant effect to give more water holding capacity in soil compared
with peat application. But for longer times (twelve-month), peat addition
generated more water holding capacity in soils compared with wheat straw and
lucerne hay. This effect was not revealed by moisture content.
With regard to water content and water holding capacity, Figure 3.3-11 shows the
weekly records on the water remaining in pots after a week watering to 60%
water holding capacity in soils. Wheat straw and lucerne hay, as expected,
recorded more variation in creating the ability of soil to retain water. The higher
the level of organic matter applied, the more water can be held in soils. Between
organic matter sources, lucerne hay showed the strongest effect in increasing the
ability of soils in retaining water (Figure 3.3-12).
Daily temperatures affected water retention in the soil. The lower daily
temperature, the more water remained in the soil. Comparison between weekly
temperature and moisture content in Figure 3.3-12 illustrates the relationship. At
the end of experiment (twelve-month incubation), water content in soils tended to
return to the same as the beginning of the experiment, especially for control and
organic matter level of 40 ton ha-1. But for the higher levels (120 and 160
ton ha-1), the water content after a week watering was still higher than the initial
values after treatments.
III. Transformation of Soil Organic Matter in An Oxisol... 70
Table 3.3-13 Analysis of variance for water holding capacity in an Oxisol from York, Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation 3 3908.89 1302.96 98.58 < 0.001 Organic matter 3 512.58 170.86 12.93 < 0.001 Level of organic matter 3 1241.29 413.76 31.31 < 0.001 Inc x OM 9 728.54 80.95 6.12 < 0.001 Inc x Level 9 172.99 19.22 1.45 0.172 OM x Level 9 655.37 72.82 5.51 < 0.001 Inc x OM x Level 27 449.62 16.65 1.26 0.197 Residual 128 1691.73 13.22 Total 191 9361.02 Inc = incubation, OM = organic matter.
III. Transformation of Soil Organic Matter in An Oxisol... 71
Figure 3.3-10 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water holding capacity in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.
20
40
60
80
0 3 6 9 12
20
40
60
80
0 3 6 9 12
20
40
60
80
0 3 6 9 12
20
40
60
80
0 3 6 9 12
month
40 ton ha-1
80 ton ha-1
120 ton ha-1
160 ton ha-1
Wat
er h
oldi
ng c
apac
ity (%
w/w
)
III. Transformation of Soil Organic Matter in An Oxisol... 72
Figure 3.3-11 The effects of organic matter source on water content measured one week after watering in an Oxisol from York, Western Australia. C = control, P = peat, W = wheat straw, and L = lucerne hay. Solid line is mean of C1-C4. 1 = 40 ton ha-1, 2 = 80 ton ha-1, 3 = 120 ton ha-1, and 4 = 160 ton ha-1. Bar in each graph indicates LSD 5% for organic matter source in every graph.
0
10
20
30
ec-99 Mar-00 Jun-00 Sep-00 Dec-00D
C1 C2 C3 C4 Series5
0
10
20
30
Dec-99 Mar-00 Jun-00 Sep-00 Dec-00
W1 W2 W3 W4 Mean C
0
10
20
30
Dec-99 Mar-00 Jun-00 Sep-00 Dec-00
P1 P2 P3 P4 Mean C
0
10
20
30
Dec-99 Mar-00 Jun-00 Sep-00 Dec-00
L1 L2 L3 L4 Mean C
Wat
er c
onte
nt (%
w/w
)
III. Transformation of Soil Organic Matter in An Oxisol... 73
III. Transformation of Soil Organic Matter in An Oxisol... 74
Figure 3.3-12 The effects of the highest organic matter level (160 ton ha-1) on water content in an Oxisol from York, Western Australia, measured one week after watering and maximum weekly temperature for Perth (Source: Aufdemkampe (2001)). = control, = peat, = wheat straw, and = lucerne hay. Solid line is maximum weekly temperature. Bar indicates LSD 5% for organic matter source.
-12 The effects of the highest organic matter level (160 ton ha-1) on water content in an Oxisol from York, Western Australia, measured one week after watering and maximum weekly temperature for Perth (Source: Aufdemkampe (2001)). = control, = peat, = wheat straw, and = lucerne hay. Solid line is maximum weekly temperature. Bar indicates LSD 5% for organic matter source.
0
10
20
30
Dec-99 Mar-00 Jun-00 Sep-00 Dec-0015
28
40
3.3.6 Correlation matrices on all parameters
Table 3.3-14 Correlation matrices on all soil parameters, presented according to incubation times. Only significant correlations are presented. * represents significant (p ≤ 0.05), ** represents highly significant (p ≤ 0.01)
A. Three-month incubation Parameter
Parameter TEC EOC CO2 MBC NH4-N NO3-N BPa Ka WC WHC
Over all incubation times (3-12 months), bicarbonate-P was dependent on almost
all soil parameters measured, as the following:
III. Transformation of Soil Organic Matter in An Oxisol... 82
BP = 42.26 + 0.54 TEC + 0.01 EOC + 0.03 NH4-N + 0.04 NO3-N + 0.01 K - 0.63
WC + 0.37 WHC - 9.93 pH
R2 = 0.73
where BP = bicarbonate-P, TEC = total extractable-C, EOC = extractable
organic-C, NH4-N = ammonium, NO3-N = nitrate, WC = water content, and WHC
= water holding capacity.
Relationships in these regression equations are confirming the correlations as of
Table 3.3-14 and Table 3.3-15. The relationship between bicarbonate-P with
total extractable-C and extractable organic-C might be related to organic ligand
exchange (Bhadoria et al., 2002; Hinsinger, 2001; Iyamuremye and Dick, 1996).
The relationships between P and organic-C were also observed by Brennan et al.
in a range of soils of Western Australia (1994) and Condron et al. in cultivated
and uncultivated soils (1990). Moreover, as in this work, soil organic-C has been
shown previously to be positively correlated to the activity of phosphatase,
especially in acid soils (Baligar et al., 1999; Barrett et al., 1998; Canarutto et al.,
1995; Dick et al., 1988).
The relationship between ammonium, nitrate, and to some extent, potassium with
bicarbonate-P occurred perhaps due to residual effect and the increase of
electrical conductivity and soil pH from organic matter application (Eghball et al.,
2004), to make their solubility increased. Moreover, interaction effect between N
and P availability in soils (Abreu et al., 2002; Jha et al., 1993; Teng and Timmer,
1994) might enhance the relationship, especially in this type of soil where
nitrification developed without hindrance (Jha et al., 1993) to make N more
available.
3.4.3 Mineralisation of nitrogen and release of potassium Ammonium was formed in large amounts within the first three months after
organic matter addition, especially with lucerne hay application (Figure 3.3-5).
The higher N content of lucerne hay (Table 3.3-2) undoubtedly contributed to
higher concentration of ammonium in soil. The greatest ammonification rate, for
lucerne hay organic matter with low C:N ratio, may have been initiated as soon
as organic matter was applied to the soil and continued until six months. This is
in accordance to Azam et al. (1993) who found that from 13 to 41% of the applied
N from N-labelled plant residues were mineralised during a four-week incubation
III. Transformation of Soil Organic Matter in An Oxisol... 83
(25° C) in Mollisols, the proportions decreasing according to their total N content.
The ammonification of organic-N may have contributed to the pH increases
observed for lucerne hay addition (Figure 3.3-8). This is despite the substantial
nitrification which occurred, a transformation known to produce acidity (Bolan and
Hedley, 2003; Sakala et al., 2004).
As OM decomposition enhances nutrient availability (Hartemink and O'Sullivan,
2001; Lupwayi and Haque, 1999; Maharudrappa et al., 2000; Tian, 1998;
Villegas-Pangga et al., 2000; Zaharah and Bah, 1999), this process is beneficial
for marginal soils such as Oxisols. Mineralisation of P and K showed the same
trend for all organic matter applications. Extractable amounts of both P and K
were generally greatest at the three-month sampling time (Figure 3.3-6 and
Figure 3.3-7), and subsequently declined. Thus, the first three months of
incubation was the most active decomposition period for P and K as well as for
ammonium, especially at the higher levels of organic matter applied. There was
little evidence of net nitrification until 12 months, indicating likely immobilisation of
N into the microbial biomass.
The order of mineralisation was control<peat<wheat straw<lucerne hay for nitrate
and K, whilst for ammonium and P the order was control<wheat
straw<peat<lucerne hay. Peat was more effective than wheat straw at releasing
ammonium, perhaps because peat contains more amine functional groups
(Landgraf et al., 1998). In addition, the C:N ratio of peat was slightly lower than
that of wheat straw (Table 3.3-2). Overall, a legume green manure, in this case
lucerne hay, produced greater net mineralisation for C, N, P, and K as
hypothesised, even though this effect was only observed within the first three
months after organic matter application.
3.5 CONCLUSION
Organic matter persistence in an Oxisol of Western Australia was dependent on
its C:N ratio. The smaller C:N ratio (i.e. lucerne hay), the faster the
decomposition processes.
The P-availability, which is expressed as net release of bicarbonate-P, increased
due to organic matter application, especially from lucerne hay and peat. Peat
persisted longer and can be expected to sustain bicarbonate-P availability over a
longer term than the other amendments. Bicarbonate-P related to total
III. Transformation of Soil Organic Matter in An Oxisol... 84
extractable-C, extractable organic-C, NH4-N, and soil pH through out the
experiment. Thus, if organic amendments were the option to resolve limited
supply of P in lateritic soils, peat application is a reasonable choice for longer
term effect.
Organic matter mineralisation enhanced nutrient availability, especially
ammonium, nitrate, bicarbonate-P, and extractable-K. This process occurred
soon after organic matter applied to the soil, especially for lucerne hay. The
maximum mineralisation took place within the first three months.
III. Transformation of Soil Organic Matter in An Oxisol... 85
C h a p t e r 4
EXISTING VERSUS ADDED SOIL ORGANIC MATTER IN RELATION TO PHOSPHORUS
AVAILABILITY ON LATERITIC SOILS
4.1 INTRODUCTION
Organic matter is involved indirectly in many reactions and processes in soils,
creating favourable conditions for the availability of nutrients due to its high
capacity for cation adsorption, water retention, and pH buffering capacity
(Schnitzer, 2000; Stevenson and Cole, 1999). Organic matter also promotes soil
aggregation and improves soil structure (Marinari et al., 2000; Monreal et al.,
1997) and influences soil biological characteristics of soils such as microbial
population and activities as well as soil enzymes also improve (Abbott and
Murphy, 2003; Beyer et al., 2001; Manjaiah and Dhyan, 2001). The
decomposition of soil organic matter will produce a pool of organic carbon (C)
which is easily mineralised and another pool which is slowly degradable
(Eusterhues et al., 2003). This pool is decomposed slowly due to: (i) chemical
recalcitrance, i.e. stabilisation attributable to the structural properties of the
organic matter, (ii) inclusion of organic matter into aggregates or micropores,
leading to physical protection from microbial attack (Strong et al., 1999), and (iii)
interaction of carbon compounds with soil minerals (Baldock and Skjemstad,
2000; Sollins et al., 1996).
Plant available phosphate (measured by bicarbonate extraction at pH 8.5) is
usually present only in very small amount in Ultisols and Oxisols, partly because
of its strong adsorption by aluminium (Al) and iron (Fe)-oxides (Linquist et al.,
1996; Zoysa et al., 1999) or by its precipitation with Al and Fe (Iyamuremye and
Dick, 1996). However, the concentration of organic phosphorus (P) can reach
more that 50 % of total-P in lateritic or intensely weathered soils (Beck and
Sanchez, 1994; Chepkwony et al., 2001) as highly charged monoester-P allows
rapid adsorption on soil minerals and extensive interaction with sesquioxides
which protect inositol phosphate from degradation (Turrion et al., 2001). Hence,
any mechanism that can ensure the continuity of organic-P to inorganic-P
IV. Existing versus Added Organic Matter … 86
transformation after fresh organic matter addition may become important in
lateritic soils.
The effect of recently added organic matter needs to be distinguished from the
effect of inherent soil organic matter. The stable and inert pools of organic matter
in soil can be very old, ranging from hundred to thousand years (Eusterhues et
al., 2003; Falloon and Smith, 1998; Hassink, 1997; Monreal et al., 1997; Oades,
1995) and their identity, as well as their physical and chemical properties, are not
well understood (Eusterhues et al., 2003; Hsieh, 1992; Ruhlmann, 1999; Theng
et al., 1992). In order to study the effect of adding organic matter to soil in terms
of phosphate adsorption, it would be useful to remove the stable pools of soil
organic matter from soil samples so that the effect of newly added organic matter
does not interact with that of stable pools.
Soil organic matter plays an important role in phosphate adsorption (Brennan et
al., 1994; Erich et al., 2002; Leytem et al., 2002; Sinaj et al., 2002). Newly added
organic matter has abundance of organic-C. With the assumption that at least
some organic compounds added in organic amendments are those competitive
for phosphate adsorption, any effects of freshly added organic matter on
phosphate adsorption must be greater than for inert and stable C pools. This
could be related to the formation of organic ligands adsorbing to sesquioxides,
and consequent blocking of phosphate adsorption sites (Haynes and Mokolobate,
2001; Syers et al., 1971). As stable and inert pools of soil organic matter usually
interact or are associated with clay minerals and/or Fe-oxides (Baldock and
Skjemstad, 2000; Eusterhues et al., 2003), removing this pool from soil may
disturb phosphate desorption in soil with newly added organic matter. Moreover,
soil organic-C has been shown to be positively correlated to the activity of
phosphatase, especially in acid soils (Baligar et al., 1999; Barrett et al., 1998;
Canarutto et al., 1995; Dick et al., 1988). All of these processes need to be
assessed in order to decide the most crucial mechanism in affecting P dynamics
if organic matter were added to lateritic soils.
The hypotheses drawn from this brief review are:
• at the same soil organic-C concentration, newly added soil organic matter will
be more reactive compared with pre-existing soil organic matter in minimising
phosphate adsorption indicated by more release of bicarbonate-P to the soil,
IV. Existing versus Added Organic Matter … 87
• with addition of fresh organic matter, soil micro-organisms will become more
active in mediating the transformation of organic-P to inorganic-P, indicated
by an increase in phosphatase activity.
The objectives of this study were therefore:
• to study the contribution of newly applied organic matter on different forms of
P in soil, such as bicarbonate-P, total-P, non-extractable-P, and microbial
biomass-P.
• to measure if microbial biomass-P and/or soil soluble organic-C generate
more P in low P environment indicated by their positive relationship with
bicarbonate-P, and
• to measure whether phosphatase becomes more active in mediating the
transformation of organic-P to inorganic-P in totally newly added organic
matter indicated by positive correlation between bicarbonate-P and
phosphatase activity.
4.2 MATERIALS AND METHODS
4.2.1 Design of the experiment This incubation experiment was set up in a factorial design using completely
randomised block design. The treatments are presented in Table 4.2-2 with
treatment combination in Table 4.2-3.
Table 4.2-1 Description of soil used and locations in Western Australia where soils were collected
Soil Location Description Subsurface regolith ‘soil’ Boddington Subsurface soil from mining site; no soil organic
matter, high in extractable-Al and Fe Ultisol (Typic kandiudult) Jarrahdale Lower in P and higher in extractable-Al and Fe
Oxisol (Plinthic eutrudox) Bunbury Lower in P and higher in extractable-Al and Fe
IV. Existing versus Added Organic Matter … 88
Table 4.2-2 Organic matter sources (peat, wheat straw, and lucerne hay) used as treatments in the experiment
Type of soil organic matter Source
• Peat (organic-C in 80 ton ha-1 peat + organic-C in existing soil organic matter)
• Wheat straw (organic-C in 80 ton ha-1 wheat straw + organic-C in existing soil organic matter) Existing
• Lucerne hay (organic-C in 80 ton ha-1 lucerne hay + organic-C in existing soil organic matter)
• Peat (organic-C in 80 ton ha-1 peat + organic-C in additional peat equal to that of pre-existing soil organic matter)
• Wheat straw (organic-C in 80 ton ha-1 wheat straw + organic-C in additional wheat straw equal to that of pre-existing soil organic matter)
New
• Lucerne hay (organic-C in 80 ton ha-1 lucerne hay + organic-C in additional lucerne hay equal to that of pre-existing soil organic matter)
Table 4.2-3 Treatment combinations and abbreviations used from three factors of the experiment. Soil (subsurface soil, Ultisol, and Oxisol), source of organic matter (control, peat, wheat straw, and lucerne hay), and type of soil organic matter (existing and new)
Peat, wheat straw, and lucerne hay were oven-dried (60 °C) for two days, and
chopped with a grinding mill (RetschMühle). They were then finely ground with
another mill (C and N Junior, ≤1 mm sieve) for more rapid decomposition.
Soil samples from the field were air-dried and sieved with a ≥2 mm sieve. For the
newly added organic matter treatments, soils were combusted at 450° C for 12
hours using a pottery kiln (Kiln West, model 6191Z). Soil samples were put into
three separate steel containers having dimension of 35x35x15 cm each. Soil
samples were poured into the container to about 3.5 cm depth to ensure a
homogenous heat distribution. Each container had a loosely fitting lid, allowing
fumes from combustion of samples to be released easily. Soil was combusted
until the colours become lighter than the original and the organic-C content about
0.00%. A temperature of 450° C is hot enough to deplete organic-C content, but
has been found in some previous works to have a minimal effect on other
physical and chemical properties of soils such as porosity, textural class, and soil
acidity (Giovannini et al., 1988; Kang and Sajjapongse, 1980).
Table 4.2-5 Some characteristics of soil (subsurface soil, Ultisol, and Oxisol) samples before and after combustion. Values after ± are standard errors.
Soil Subsurface soil
‘regolith’ soil Ultisol
(Typic kandiudult) Oxisol
(Plinthic eutrodox) Characteristic
Before After Before After Before After Org-C (%) 0.04 0.01 5.81 0.02 2.85 0.00
increased dissolved organic-C contents in all soils at three months following
addition compared with existing organic matter. In the Oxisol and Ultisol, the
IV. Existing versus Added Organic Matter … 94
increases in dissolved organic-C contents were more than 100%. In subsurface
soil the increase was only about 11%.
The source of organic matter addition also increased dissolved organic-C in all
three soils. Lucerne hay had the highest effect followed by wheat straw and peat.
Table 4.3-1 Analysis of variance for dissolved organic carbon in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 85020.1 42510.1 44.00 < 0.001 Soil 2 143665.1 71832.5 74.35 < 0.001 Organic matter 3 1747817.3 582605.8 603.05 < 0.001 Type of SOM 1 116236.6 116236.6 120.32 < 0.001 Inc x soil 4 27812.4 6953.1 7.20 < 0.001 Inc x TSOM 2 47807.3 23903.7 24.74 < 0.001 Soil x TSOM 2 51276.1 25638.0 26.54 < 0.001 Inc x OM 6 141104.4 23517.4 24.34 < 0.001 Soil x OM 6 229846.9 38307.8 39.65 < 0.001 TSOM x OM 3 173780.3 57926.8 59.96 < 0.001 Inc x soil x TSOM 4 3879.1 969.8 1.00 0.408 Inc x soil x OM 12 80466.5 6705.5 6.94 < 0.001 Inc x TSOM x OM 6 68470.9 11411.8 11.81 < 0.001 Soil x TSOM x OM 6 163640.0 27273.3 28.23 < 0.001 Inc x soil x TSOM x OM 12 52706.5 4392.2 4.55 < 0.001 Residual 144 139118.0 966.1 Total 215 3272647.8 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 95
Existing New
0
350
700
0 3 6 90
350
700
0 3 6 9
0
350
700
0 3 6 9
0
350
700
0 3 6 9
0
350
700
0 3 6 90
350
700
0 3 6 9
monthmonth
Sub
Ult
Oxi
Dis
solv
ed o
rgan
ic c
arbo
n (m
g kg
-1)
Sub
Ult
Oxi
Figure 4.3-1 Interaction effect of organic matter sources, type of organic matter, and incubation on dissolved organic carbon in three soils from Western Australia.
= control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 96
4.3.2 Phosphorus
4.3.2.1 Bicarbonate extractable phosphate
There was no main effect of incubation time on bicarbonate-P (Table 4.3-2). However, interaction effect between incubation and soil type was significant (p ≤ 0.01). Bicarbonate-P in the Oxisol and subsurface soil decreased over time, but increased in the Ultisol (
Figure 4.3-2).
Bicarbonate-extractable P differed between soil types at the first sampling with
the existing soil organic matter. The highest concentration on bicarbonate-P was
observed in the subsurface soil, followed by the Oxisol and then the Ultisol.
Treatments with new organic matter had more bicarbonate-P than those with
existing soil organic matter (p ≤ 0.01, Table 4.3-2). This effect was more
pronounced in the Oxisol and Ultisol than in the subsurface soil. The addition of
new organic matter increased the bicarbonate-P concentration by about 130%
above the existing soil organic matter in the Oxisol and 148% in the Ultisol. In
contrast, in the subsurface soil, the new soil organic matter decreased (p ≤ 0.01)
bicarbonate-P by about 20%.
As in the first experiment, organic matter addition, especially lucerne hay,
increased bicarbonate-P in soils, followed by peat and wheat straw. Only lucerne
hay showed a trend for bicarbonate-P to increase throughout the incubation,
especially in the Ultisol.
IV. Existing versus Added Organic Matter … 97
Table 4.3-2 Analysis of variance for bicarbonate phosphorus in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 81.86 40.93 1.77 0.174 Soil 2 12495.53 6247.76 270.07 < 0.001 Organic matter 3 71344.98 23781.04 1027.98 < 0.001 Type of SOM 1 7526.04 7526.04 325.32 < 0.001 Inc x soil 4 655.44 163.86 7.08 < 0.001 Inc x TSOM 2 232.69 116.35 5.03 0.008 Soil x TSOM 2 12454.19 6227.10 269.17 < 0.001 Inc x OM 6 3077.73 512.96 22.17 < 0.001 Soil x OM 6 2597.40 432.90 18.71 < 0.001 TSOM x OM 3 5305.98 1768.66 76.45 < 0.001 Inc x soil x TSOM 4 470.78 117.69 5.09 < 0.001 Inc x soil x OM 12 1589.52 132.46 5.73 < 0.001 Inc x TSOM x OM 6 902.90 150.48 6.50 < 0.001 Soil x TSOM x OM 6 7165.40 1194.23 51.62 < 0.001 Inc x soil x TSOM x OM 12 520.19 43.35 1.87 0.042 Residual 144 3331.33 23.13 Total 215 129751.96 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 98
New
0
40
80
120
0 3 6 90
40
80
120
0 3 6 9
0
40
80
120
0 3 6 90
40
80
120
0 3 6 9
0
40
80
120
0 3 6 90
40
80
120
0 3 6 9monthmonth
Sub
Ult
Oxi
Bic
arbo
nate
pho
spho
rus
(mg
kg-1
)
Existing
Sub
Ult
Oxi
Figure 4.3-2 Interaction effect of organic matter sources, type of organic matter, and incubation on bicarbonate phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 99
4.3.2.2 Total phosphorus Incubation time had a significant main effect (p ≤ 0.01) on total-P (Table 4.3-3).
Total-P concentrations increased at the second sampling (six-month incubation)
and decreased again at the end of the experiment (Figure 4.3-3).
The type of soil organic matter had a very significant effect (p ≤ 0.01, Table 4.3-3)
on total-P concentrations. New organic matter treatments had about 27% higher
total-P than the existing soil organic matter treatments for all soil types. For
Ultisol, the increase was even more pronounced (64%) than in the Oxisol (50%).
Total-P in soil was affected by the source of organic matter applied. Soil treated
with lucerne hay had the highest total-P contents followed by peat and wheat
straw treatments. Lucerne hay raised total-P concentration about 100% from the
control, while the increase for peat was about 46% and for wheat straw about
32%.
Table 4.3-3 Analysis of variance for total phosphorus in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 6980.0 3490.0 10.46 < 0.001 Soil 2 968976.6 484488.3 1451.55 < 0.001 Organic matter 3 548346.9 182782.3 547.62 < 0.001 Type of SOM 1 106577.8 106577.8 319.31 < 0.001 Inc x soil 4 2164.4 541.1 1.62 0.172 Inc x TSOM 2 515.1 257.6 0.77 0.464 Soil x TSOM 2 36838.2 18419.1 55.18 < 0.001 Inc x OM 6 1597.6 266.3 0.80 0.573 Soil x OM 6 15989.2 2664.9 7.98 < 0.001 TSOM x OM 3 35546.5 11848.8 35.50 < 0.001 Inc x soil x TSOM 4 644.1 161.0 0.48 0.749 Inc x soil x OM 12 2230.2 185.9 0.56 0.873 Inc x TSOM x OM 6 1440.5 240.1 0.72 0.635 Soil x TSOM x OM 6 21755.3 3625.9 10.86 < 0.001 Inc x soil x TSOM x OM 12 1698.0 141.5 0.42 0.952 Residual 144 48063.3 333.8 Total 215 1799363.9 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 100
New
0
200
400
0 3 6 90
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 90
200
400
0 3 6
monthmonth
Sub
Ult
Oxi
Tota
l pho
spho
rus
(mg
kg-1
)
Existing
Sub
Ult
Oxi
9
Figure 4.3-3 Interaction effect of organic matter sources, type of organic matter, and incubation on total phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 101
4.3.2.3 Non-extractable phosphorus Incubation time also significantly (p ≤ 0.01) decreased non-extractable-P content
in soil samples, though non-extractable-P also slightly increased at the second
sampling (Figure 4.3-4).
The main effect of soil types was the same as for in bicarbonate-P and total-P.
Subsurface soil had higher non-extractable-P concentration, followed by the
Oxisol and then the Ultisol.
The new organic matter significantly (p ≤ 0.01, Table 4.3-4) increased non-
extractable-P content in all soil samples, with the mean increased of about 23%.
This differed according to the type of soil; the increase was about 7% for
subsurface soil, 38% for the Oxisol, and 48% for the Ultisol.
Non-extractable-P in soil was also affected by the type of organic matter applied.
Lucerne hay increased non-extractable-P about 76% from control, while with peat
the increase was about 33% and with wheat straw about 27%.
Table 4.3-4 Analysis of variance for non-extractable phosphorus in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 5569.8 2784.9 7.69 < 0.001 Soil 2 769915.1 384957.6 1063.54 < 0.001 Organic matter 3 227023.0 75674.3 209.07 < 0.001 Type of SOM 1 57428.2 57428.2 158.66 < 0.001 Inc x soil 4 1031.7 257.9 0.71 0.585 Inc x TSOM 2 113.2 56.6 0.16 0.855 Soil x TSOM 2 6404.1 3202.1 8.85 < 0.001 Inc x OM 6 1502.1 250.4 0.69 0.657 Soil x OM 6 20395.7 3399.3 9.39 < 0.001 TSOM x OM 3 13498.0 4499.3 12.43 < 0.001 Inc x soil x TSOM 4 994.0 248.5 0.69 0.602 Inc x soil x OM 12 1580.2 131.7 0.36 0.974 Inc x TSOM x OM 6 627.2 104.5 0.29 0.941 Soil x TSOM x OM 6 5227.0 871.2 2.41 0.030 Inc x soil x TSOM x OM 12 2250.3 187.5 0.52 0.900 Residual 144 52122.0 362.0 Total 215 1165681.7 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 102
New
0
100
200
300
0 3 6 90
100
200
300
0 3 6 9
0
100
200
300
0 3 6 90
100
200
300
0 3 6 9
0
100
200
300
0 3 6 90
100
200
300
0 3 6 9
monthmonth
Sub
Ult
Oxi
Non
-ext
ract
able
pho
spho
rus
(mg
kg-1
)
Existing
Sub
Ult
Oxi
Figure 4.3-4 Interaction effect of organic matter sources, type of organic matter, and incubation on non-extractable phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 103
4.3.2.4 Phosphatase Phosphatase activities increased with increasing incubation time in all soils,
especially in the Ultisol and the Oxisol (Figure 4.3-5). By the end of experiment,
the mean activities had almost doubled compared with the initial phosphatase
activities in all soils.
Soil types were different in terms of changes in phosphatase activity. Subsurface
soil recorded lower phosphatase activities than those in the Ultisol and the
Oxisol. The Ultisol and Oxisol showed higher activities, especially the Oxisol,
which had about four times (main effect of soil type) higher phosphatase activity
than subsurface soil, and about 135% higher than the Ultisol.
The new soil organic matter treatments had significantly lower (p ≤ 0.01, Table
4.3-5) initial phosphatase activity in soils than existing soil organic matter
treatments.
The source of organic matter applied had different effects. Lucerne hay gave the
highest phosphatase activities, especially for the new soil organic matter
application. Peat showed a greater effect than wheat straw in the Oxisol. In the
existing soil organic matter treatments, the phosphatase activities were almost at
the same magnitude regardless of the type of organic matter applied, especially
on the Ultisol and Oxisol.
IV. Existing versus Added Organic Matter … 104
Table 4.3-5 Analysis of variance for phosphatase activity in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 1318.911 659.456 599.10 < 0.001 Soil 2 2210.482 1105.241 1004.09 < 0.001 Organic matter 3 627.752 209.251 190.10 < 0.001 Type of SOM 1 950.461 950.461 863.47 < 0.001 Inc x soil 4 723.057 187.764 164.22 < 0.001 Inc x TSOM 2 101.641 50.821 46.17 < 0.001 Soil x TSOM 2 393.373 196.687 178.69 < 0.001 Inc x OM 6 222.014 37.002 33.62 < 0.001 Soil x OM 6 52.426 8.738 7.94 < 0.001 TSOM x OM 3 332.842 110.947 100.79 < 0.001 Inc x soil x TSOM 4 156.398 39.099 35.52 < 0.001 Inc x soil x OM 12 79.187 6.598 5.99 < 0.001 Inc x TSOM x OM 6 79.214 13.202 11.99 < 0.001 Soil x TSOM x OM 6 264.462 44.077 40.04 < 0.001 Inc x soil x TSOM x OM 12 79.867 6.656 6.05 < 0.001 Residual 144 158.507 1.101 Total 215 7750.584 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 105
0
10
20
0 3 6 9
0
10
20
0 3 6 9
0
10
20
0 3 6 9
New
0
10
20
0 3 6 9
0
10
20
0 3 6 9
0
10
20
0 3 6
monthmonth
Sub
Ult
Oxi
Pho
spha
tase
act
ivity
(mg
kg-1
)
Existing
Sub
Ult
Oxi
9
Figure 4.3-5 Interaction effect of organic matter sources, type of organic matter, and incubation on soil phosphatase activity in three soils from Western Australia.
= control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 106
4.3.2.5 Microbial biomass phosphorus Microbial biomass-P concentration in soils tended to decrease with increasing
incubation time, especially for soil with new organic matter treatments (Figure
4.3-6). For these treatments, very high microbial biomass-P values were
observed at the beginning of incubations, but at the end microbial biomass-P
dropped to near zero.
Soil types had no significant effect (p = 0.11, Table 4.3-6) on microbial biomass-P
concentration.
In the same way as for phosphatase activity (Figure 4.3-5), new organic matter
had significantly lower microbial biomass-P in all soils. The lower microbial
biomass-P content for these treatments was observed until the end of the
experiment.
The type of organic matter applied created various effects on microbial biomass-
P, with lucerne hay causing the largest initial increase in microbial biomass-P (at
three months incubation) followed by wheat straw and peat (Figure 4.3-6).
Table 4.3-6 Analysis of variance for microbial biomass phosphorus in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 3841.78 1920.89 64.57 < 0.001 Soil 2 133.33 66.67 2.24 0.110 Organic matter 3 3802.53 1267.51 42.61 < 0.001 Type of SOM 1 5.04 5.04 0.17 0.681 Inc x soil 4 151.97 37.99 1.28 0.282 Inc x TSOM 2 536.33 268.17 9.01 < 0.001 Soil x TSOM 2 5.78 2.89 0.10 0.908 Inc x OM 6 2866.59 477.77 16.06 < 0.001 Soil x OM 6 256.37 42.73 1.44 0.205 TSOM x OM 3 761.13 253.71 8.53 < 0.001 Inc x soil x TSOM 4 62.81 15.70 0.53 0.715 Inc x soil x OM 12 999.55 83.30 2.80 0.002 Inc x TSOM x OM 6 712.56 118.76 3.99 < 0.001 Soil x TSOM x OM 6 214.00 35.67 1.20 0.310 Inc x soil x TSOM x OM 12 274.19 22.85 0.77 0.682 Residual 144 4284.00 29.75 Total 215 18907.96 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 107
New
-5
20
45
0 3 6 9 -5
20
45
0 3 6 9
-5
20
45
0 3 6 9 -5
20
45
0 3 6 9
-5
20
45
0 3 6 9 -5
20
45
0 3 6 9
monthmonth
Sub
Ult
Oxi
Mic
robi
al b
iom
ass
pho
spho
rus
(mg
kg-1
)
Existing
Sub
Ult
Oxi
Figure 4.3-6 Interaction effect of organic matter sources, type of organic matter, and incubation on soil microbial biomass phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 108
4.3.3 Concentrations of different forms of phosphorus
4.3.3.1 Subsurface soil Concentrations of different forms of P in subsurface soil showed a steady
concentration throughout the incubation time (Figure 4.3-7). Meanwhile, organic
matter application, either by peat, wheat straw, or lucerne hay, increased the
concentration of P (microbial biomass-P, bicarbonate-P, non-extractable-P, and
total-P) from control. Among three sources of organic matter, lucerne hay
increased concentrations of the different forms of P more than peat and wheat
straw.
At the same source of organic matter, the proportioning between microbial
biomass-P, bicarbonate-P, non-extractable-P, and total-P in subsurface soil
remained relatively constant from the existing to the new organic matter
treatments. Microbial biomass-P was about 1% (of total-P) higher in the existing
than in the new organic matter treatments. Bicarbonate-P was 1-6% (of total-P)
higher in the existing than in the new organic matter treatments. Non-
extractable-P was from 1-6% (of total-P) higher in the new than in the existing
organic matter treatments.
4.3.3.2 Ultisol As in the subsurface soil, the Ultisol showed relatively steady concentrations of
all forms of P throughout the incubation time (Figure 4.3-8). New soil organic
matter application, as either peat, wheat straw, or lucerne hay, increased the
concentration of P (microbial biomass-P, bicarbonate-P, non-extractable-P, and
total-P) more than the control or existing soil organic matter treatments. Lucerne
hay, as in subsurface soil, increased P more than did peat or wheat straw.
At the same source of organic matter, the proportioning between microbial
biomass-P, bicarbonate-P, non-extractable-P, and total-P in the Ultisol remained
approximately constant from the existing to the new organic matter treatments.
Microbial biomass-P was 3-7% (of total-P) higher in the existing than in the new
organic matter treatments. Bicarbonate-P was 6-12% (of total-P) higher in the
new than in the existing organic matter treatments. Non-extractable-P was 6-8%
(of total-P) higher in the existing than in the new organic matter treatments.
IV. Existing versus Added Organic Matter … 109
4.3.3.3 Oxisol As in the subsurface soil and the Ultisol, all forms of P in the Oxisol showed
steady concentrations throughout the incubation time (Figure 4.3-9). New soil
organic matter application, as either peat, wheat straw, or lucerne hay, increased
the concentration of P (microbial biomass-P, bicarbonate-P, non-extractable-P,
and total-P) more than the control or existing soil organic matter treatments.
Lucerne hay, as in the subsurface soil and the Ultisol, increased P more than
peat and wheat straw.
At the same source of organic matter, the proportioning between microbial
biomass-P, bicarbonate-P, non-extractable-P, and total-P in the Ultisol remained
approximately constant. Microbial biomass-P was 1-5% (of total-P) higher in the
existing than in the new organic matter treatments. Bicarbonate-P was 7-10% (of
total-P) higher in the new than in the existing organic matter treatments. Non-
extractable-P was from 7-9% (of total-P) higher in the existing than in the new
organic matter treatments.
To summarise, the application of new soil organic matter to all soils increased
non-extractable-P and decreased microbial biomass-P and bicarbonate-P,
proportion to total-P. Application of lucerne hay resulted in the largest increases
in the proportion of non-extractable-P followed by peat and wheat straw. In
addition, lucerne hay tended to reduce microbial biomass-P concentration in all
soils. Among three soils, subsurface soil had higher concentrations of various
forms of P followed by the Oxisol and then the Ultisol.
IV. Existing versus Added Organic Matter … 110
0
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 9
New
0
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 9
C
P
W
L
monthmonth
mg kg-1 mg kg-1
Existing
C
P
W
L
Figure 4.3-7 Phosphorus status comparison between existing and new organic matter treatments in subsurface soil from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus.
IV. Existing versus Added Organic Matter … 111
New
month
-100
150
400
0 3 6 9-100
150
400
0 3 6 9
-100
150
400
0 3 6 9-100
150
400
0 3 6 9
-100
150
400
0 3 6 9-100
150
400
0 3 6 9
-100
150
400
0 3 6 9 -100
150
400
0 3 6 9
month month
mg kg-1 mg kg-1
Existing
C
P
W
L
C
P
W
L
Figure 4.3-8 Phosphorus status comparison between existing and new organic matter treatments in an Ultisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus.
IV. Existing versus Added Organic Matter … 112
month month
New
-100
150
400
0 3 6 9-100
150
400
0 3 6 9
-100
150
400
0 3 6 9-100
150
400
0 3 6 9
-100
150
400
0 3 6 9-100
150
400
0 3 6 9
-100
150
400
0 3 6 9-100
150
400
0 3 6
mg kg-1 mg kg-1
month month
Existing
C
P
W
L
C
P
W
L
9
Figure 4.3-9 Phosphorus status comparison between existing and new organic matter treatments in an Oxisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus.
IV. Existing versus Added Organic Matter … 113
4.3.4 Other parameters
4.3.4.1 Soil pH Soil pH increased with increasing incubation time (p = 0.01, Table 4.3-7).
The main effect of soil types was different among three soils. Subsurface soil
gave higher pH followed by Ultisol and Oxisol (Figure 4.3-10).
Addition of new soil organic matter significantly changed pH of soils (p ≤ 0.01),
but the direction of this change differed according to the type of soil. The new
soil organic matter application increased the soil pH in subsurface soil and the
Ultisol. In the Oxisol, addition of new soil organic matter decreased the soil pH.
Soil pH was also affected by the source of organic matter applied. Whilst only
lucerne hay increased soil pH, peat and wheat straw made the soil pH lower than
control, regardless of the soil type. However, only peat consistently decreased
soil pH in every type of soil. Wheat straw had a similar trend to peat, except in
the Ultisol with the new soil organic matter (Figure 4.3-10).
Table 4.3-7 Analysis of variance for pH in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 0.10562 0.05281 4.74 0.010 Soil 2 10.98550 5.49275 492.48 < 0.001 Organic matter 3 34.15630 11.38543 1020.82 < 0.001 Type of SOM 1 3.52667 3.52667 316.20 < 0.001 Inc x soil 4 6.59131 1.64783 147.74 < 0.001 Inc x TSOM 2 0.42391 0.21195 19.00 < 0.001 Soil x TSOM 2 5.97785 2.98893 267.99 < 0.001 Inc x OM 6 19.88841 3.31474 297.20 < 0.001 Soil x OM 6 1.89261 0.31544 28.28 < 0.001 TSOM x OM 3 0.30416 0.10139 9.09 < 0.001 Inc x soil x TSOM 4 14.47416 3.61854 324.44 < 0.001 Inc x soil x OM 12 3.47214 0.28935 25.94 < 0.001 Inc x TSOM x OM 6 2.44707 0.40785 36.57 < 0.001 Soil x TSOM x OM 6 3.46983 0.57830 51.85 < 0.001 Inc x soil x TSOM x OM 12 3.19575 0.26631 28.33 < 0.001 Residual 144 1.60607 0.01115 Total 215 112.51737 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 114
New
3.5
5.0
6.5
8.0
0 3 6 93.5
5.0
6.5
8.0
0 3 6 9
3.5
5.0
6.5
8.0
0 3 6 93.5
5.0
6.5
8.0
0 3 6 9
3.5
5.0
6.5
8.0
0 3 6 93.5
5.0
6.5
8.0
0 3 6 9
monthmonth
Sub
Ult
Oxi
Soi
l pH
Existing
Sub
Ult
Oxi
Figure 4.3-10 Interaction effect of organic matter sources, type of organic matter, and incubation on soil pH in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 115
4.3.4.2 Extractable iron Incubation time had no effect on soil oxalate extractable-Fe (Figure 4.3-11).
The oxalate extractable-Fe content in the soil was significantly different for each
type of soil (p ≤ 0.01, Table 4.3-8). The Oxisol had the highest content of
extractable-Fe (six-fold higher that subsurface soil), followed by the Ultisol (three-
fold higher than subsurface).
The new organic matter treatments had significantly increased extractable-Fe
contents (p ≤ 0.01) compared with the existing organic matter treatments,
especially the peat treatment. When soil types were considered separately, only
in subsurface soil and Ultisol did the new soil organic matter increase oxalate
extractable-Fe, whilst in the Oxisol the new soil organic matter decreased oxalate
extractable-Fe content in the soil.
When different sources of organic matter were considered separately, peat
increased oxalate extractable-Fe significantly (p ≤ 0.01) with concentrations
about 36% higher than the control. Meanwhile, oxalate extractable-Fe content
was significantly lower (p ≤ 0.01) following wheat straw (9% lower than control)
and lucerne hay (6% lower than control) applications.
IV. Existing versus Added Organic Matter … 116
Table 4.3-8 Analysis of variance for extractable iron in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 1032.7 516.4 1.14 0.322 Soil 2 2091701.9 1045851.0 2312.65 < 0.001 Organic matter 3 140920.1 46973.4 103.87 < 0.001 Type of SOM 1 30364.4 30364.4 67.14 < 0.001 Inc x soil 4 3369.2 842.3 1.86 0.120 Inc x TSOM 2 1031.8 515.9 1.14 0.322 Soil x TSOM 2 203174.4 101587.2 224.64 < 0.001 Inc x OM 6 6073.0 1012.2 2.24 0.043 Soil x OM 6 26038.5 4339.7 9.60 < 0.001 TSOM x OM 3 36088.6 12029.5 26.60 < 0.001 Inc x soil x TSOM 4 1424.4 356.1 0.79 0.535 Inc x soil x OM 12 4403.4 366.9 0.81 0.638 Inc x TSOM x OM 6 2516.3 419.4 0.93 0.477 Soil x TSOM x OM 6 24893.8 4149.0 9.17 < 0.001 Inc x soil x TSOM x OM 12 11346.2 945.5 2.09 0.021 Residual 144 65121.2 452.2 Total 215 2649500.1 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 117
New
0
200
400
0 3 6 90
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 9
0
200
400
0 3 6 90
200
400
0 3 6
monthmonth
Sub
Ult
Oxi
Ext
ract
able
iron
(g k
g-1)
Existing
Sub
Ult
Oxi
9
Figure 4.3-11 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable iron in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 118
4.3.4.3 Extractable aluminium Oxalate extractable-Al in soil increased as incubation time increased (Figure
4.3-12). Although not significant, there was a decrease in oxalate extractable-Al
at the six-month incubation time. At the end of the experiment, oxalate
extractable-Al was almost 50 % higher compared with the initial content in soil.
The different soils had significantly different oxalate extractable-Al contents (p ≤
0.01). The Oxisol had the highest content of oxalate extractable-Al, followed by
Ultisol and subsurface soil. Compared with the subsurface soil, oxalate
extractable-Al contents were six times higher in the Oxisol and three times higher
in the Ultisol.
The new soil organic matter treatments had significantly higher oxalate
extractable-Al content (p ≤ 0.01, Table 4.3-9) than that of the existing organic
matter treatments. The average increase was about three-fold, and occurred for
all soil types and sources of organic matter.
Considering the main effect of source of organic matter, lucerne hay decreased
extractable-Al significantly (p ≤ 0.01) followed by wheat straw and peat. The
decreases from control were about 23% for lucerne hay, 18% for wheat straw,
and 7% for peat.
IV. Existing versus Added Organic Matter … 119
Table 4.3-9 Analysis of variance for extractable aluminium in three soils from Western Australia. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Incubation time 2 7269.069 3634.534 1271.10 < 0.001 Soil 2 14102.642 7051.321 2466.06 < 0.001 Organic matter 3 527.847 175.949 61.53 < 0.001 Type of SOM 1 13203.914 13203.914 4617.80 < 0.001 Inc x soil 4 1673.980 418.495 146.36 < 0.001 Inc x TSOM 2 1574.181 787.091 275.27 < 0.001 Soil x TSOM 2 3853.143 1926.572 673.78 < 0.001 Inc x OM 6 119.256 19.876 6.95 < 0.001 Soil x OM 6 178.955 29.826 10.43 < 0.001 TSOM x OM 3 418.940 139.647 48.84 < 0.001 Inc x soil x TSOM 4 227.600 56.900 19.90 < 0.001 Inc x soil x OM 12 73.824 6.152 2.15 0.017 Inc x TSOM x OM 6 89.724 14.954 5.23 < 0.001 Soil x TSOM x OM 6 187.080 31.180 10.90 < 0.001 Inc x soil x TSOM x OM 12 54.530 4.544 1.59 0.101 Residual 144 411.747 2.859 Total 215 43966.433 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter
IV. Existing versus Added Organic Matter … 120
New
0
35
70
0 3 6 90
35
70
0 3 6 9
0
35
70
0 3 6 90
35
70
0 3 6 9
0
35
70
0 3 6 9
0
35
70
0 3 6 9
monthmonth
Sub
Ult
Oxi
Ext
ract
able
alu
min
um (g
kg
-1)
Existing
Sub
Ult
Oxi
Figure 4.3-12 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable aluminium in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.
IV. Existing versus Added Organic Matter … 121
IV. Existing versus Added Organic Matter … 122
4.3.5 Correlations between all parameters Correlation analysis revealed that bicarbonate-P was positively correlated
(p ≤ 0.01) with dissolved organic-C throughout the experiment, and the coefficient
correlations (r) were increasing (0.47, 0.61, and 0.70) with increasing incubation
time, indicating stronger relationships. Bicarbonate-P was negatively correlated
(p ≤ 0.05) with oxalate extractable-Fe at the beginning of the experiment until six-
month incubation. Bicarbonate-P was also positively correlated with soil pH from
six-month incubation until the end of the experiment.
1
Table 4.3-10 Correlation matrices on all soil parameters. Presented according to incubation times. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation
Table 4.3-11 Correlation matrices between phosphorus, carbon, iron, and soil pH for different incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation
Organic matter Existing New Incubation
time Parameter NP BP DOC Fe NP BP DOC Fe
NP
BP 0.74** 0.48**DOC 0.39* 0.54** 0.35** 0.49**Fe -0.48** -0.44** -0.64**
NP = Non-extractable Phosphorus, BP = Bicarbonate Phosphorus, and DOC = Dissolved Organic Carbon. 2
IV. Existing versus Added Organic Matter … 126
1
Table 4.3-12 Correlation matrices between phosphorus, carbon, iron, and soil pH over all incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation
Organic matter Existing New Parameter
NP BP DOC Fe NP BP DOC Fe NP BP 0.78** 0.47**DOC 0.44** 0.64** 0.34** 0.47**Fe -0.48** -0.43** -0.35** -0.61** -0.26**pH 0.26** 0.30** 0.34** -0.27** -0.24*
NP = Non-extractable Phosphorus, BP = Bicarbonate Phosphorus, and DOC = Dissolved Organic Carbon. 2
IV. Existing versus Added Organic Matter … 127
ded Organic Matter … 128
1
Table 4.3-13 Correlation matrices between phosphorus, carbon, iron, and soil pH, separated into soil types and into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation
Organic matter Existing New Soil Parameter
NP BP DOC Fe NP BP DOC Fe NP
BP 0.73** 0.78**DOC 0.70** 0.74** 0.50**Fe -0.40* -0.46**
Subsurface soil
pH -0.73**NP BP 0.70** 0.73**DOC 0.84** 0.56** 0.67** 0.59**Fe -0.58**
NP = Non-extractable Phosphorus, BP = Bicarbonate Phosphorus, and DOC = Dissolved Organic Carbon. 2
IV. Existing versus Ad
4.4 DISCUSSION
The new organic matter treatment had significantly increased bicarbonate-P
concentrations compared with existing organic matter treatments, especially in
the Ultisol (ca. 150% increase) and the Oxisol (ca. 130% increase). In the case
of the subsurface soil, P immobilisation is likely to have been taking place after
lucerne hay addition, indicated by the higher microbial biomass-P in the first six
months of incubation (Figure 4.3-6). The higher bicarbonate-P content in
subsurface soil (Table 4.2-4) has probably allowed the population of soil micro-
organisms to increase faster than Ultisol and Oxisol, even in combusted soil
samples, causing immobilisation of P. Although P immobilisation can persist
longer due to green manure addition (da Costa, 2000), the results in this
experiment showed that at the end of the incubation, this P immobilisation had
declined indicated by smaller microbial biomass-P (Figure 4.3-6).
The increase of total-P (Figure 4.3-3) allows two interpretations. First, additional
organic matter to replace the existing soil organic matter contributed to the
increase. The increase in the Ultisol treated with lucerne hay, however, was too
high (90%) compared with the P content calculated from the P content of lucerne
hay (0.25% P; 41.3% C; Table 3.3-2, Chapter 3). Second, the possibility of
mineralisation or solubilisation of occluded P and stabilised source of P in the
existing soil organic matter or in inorganic forms pre-existing in the soil (Giardina
et al., 2000; Serrasolsas and Khanna, 1995) which can not be extracted with
total-P procedures by Nelson (1987). This occluded and stabilised source of P
might also be derived from monocalcium phosphate added one month before the
experimental treatments. Higher total-P in subsurface soil where organic-C
content was negligible (Table 4.2-5) and lower total-P in soils with higher organic-
C (Ultisol and Oxisol) were the indication of this pool of P. However, as soon as
the existing soil organic matter was removed by combustion, P was more readily
extracted. Alternately, the increase in soil pH (Figure 4.3-10), extractable-Al
(Figure 4.3-12), and extractable-Fe (Figure 4.3-11) in the Ultisol might have been
from the precipitation of Al and Fe-phosphate, or even Ca-phosphate (Giardina et
al., 2000; Olsen and Sommers, 1982) after combustion and/or new soil organic
matter treatment. In total-P extraction process, Al and Fe-phosphate are soluble
in alkaline extractants such as NaHCO3 and Ca-phosphate is soluble in acid
IV. Existing versus Added Organic Matter … 129
extractants such as H2SO4-K2S2O8 (Bhadoria et al., 2002) to increase the
concentration of extractable Al and Fe. Both acid extractants were used in total-
P analysis.
As total-P content increased with the new organic matter treatment,
consequently, non-extractable-P content in soils increased. The fact that peat
produced more non-extractable-P in soils than wheat straw treatment was
unexpected, since the total-P content in wheat straw biomass was higher than
that of peat (Table 3.3-2, Chapter 3). Different types of non-extractable-P as
products of decomposition of peat and wheat straw could be responsible for the
result, as peat has more phosphate diesters and other forms of P, including
inorganic-P (Bedrock et al., 1994).
Closer inspection of results obtained for subsurface soil indicated that the lower
concentration of bicarbonate-P in new soil organic matter treatment can be
assumed to be due to the heating process imposed on soil samples. The original
soil organic matter content in subsurface soil was negligible at 0.035% (Table
4.2-4) and the combustion of the soil increased extractable-Al and Fe content
(Table 4.2-1). The increased reactive Al and Fe would be expected to increase
phosphate adsorption capacity (Iyamuremye and Dick, 1996; Zoysa et al., 1999)
which may indicated by the lower bicarbonate-P in samples with new soil organic
matter treatment (Figure 4.3-2). However, for the other soils (the Ultisol and the
Oxisol), a decrease in bicarbonate-P was not observed, even though it might be
reasonable to assume a similar increase in phosphate adsorption capacity. The
increases in extractable-Al and Fe from the burning process occurred together
with increased bicarbonate-P concentration in the Ultisol and the Oxisol. This
could be due to process related to mineralisation of organic matter or desorption
process but not related to heating process as that of subsurface soil. Therefore,
the experiment could not distinguish mineralisation from desorption, but the effect
of combustion on phosphate adsorption can probably be neglected. In line with
this finding, increased concentration of extractable-P have also observed by
several authors, if soils were treated by combustion (Condron et al., 1990; Oniani
et al., 1973; Williams et al., 1970), especially in highly weathered soils. This
phenomenon was probably due to incomplete oxidation of organic-P during
combustion and the changes in acid solubility of soil inorganic-P as a result of
combustion (Anderson, 1960; Condron et al., 1990; Williams et al., 1970).
Condron et al. (1990) even concluded that these potential errors have to be
IV. Existing versus Added Organic Matter … 130
avoided if dealing with organic-P in strongly weathered soil samples such as
those used in this experiment. However, since subsurface soil did not show the
increase in either total-P or non-extractable-P (Figure 4.3-3 and Figure 4.3-4) due
to new organic matter applications (combustion procedure as well), it can be
assumed that two mechanisms might not have occurred. Unfortunately, there
was no data to support such claim in this experiment.
The Ultisol was the most responsive to organic matter applications (Figure 4.3-4)
by giving a greater percentage increase in non-extractable-P due to newly-added
organic matter, compared with increases in the subsurface soil and the Oxisol.
Higher O2 supply due to the sandy texture of the soil (Table 4.2-4) possibly
enhanced mineralisation processes in the Ultisol. However, the nature of the
increase in non-extractable-P content was not in the form of microbial biomass-P
as predicted beforehand. The positive correlation between non-extractable-P
and microbial biomass-P at three- and six-month incubation was not observed at
nine-month incubation (Table 4.3-10 and Table 4.3-11). Besides, new organic
matter application decreased microbial biomass-P in the soil. These
observations suggest that the increase of non-extractable-P may develop from
processes other than direct addition of organic-P from organic matter biomass
such as mineralisation or some other unknown process. All newly applied
organic matter decreased non-extractable-P relative to total-P, but increased the
proportion of bicarbonate-P relative to total-P. In other words, newly applied
organic matter not only increased non-extractable-P but might also have released
more bicarbonate-P from organic-P to inorganic-P transformations, despite the
observation that the treatment itself had created more bicarbonate-P.
In accordance with the increase in non-extractable-P content, a negative
correlation between non-extractable-P and sesquioxides of Al and Fe (oxalate-
extractable) was observed. The higher the non-extractable-P content, the lower
the Al and Fe concentration. As organic matter could increase non-extractable-P
due to direct effect of organic matter addition and mineralisation, the treatments
could precipitate Al- and Fe-oxides (Hue, 1992; Iyamuremye and Dick, 1996).
Moreover, the formation of complexes between Al and Fe and humic substances
(Chen et al., 1994; McCracken et al., 2002; Yang et al., 2001) may have taken
place during the experiment, making Al and Fe concentrations decline in soil
solution. Since acid ammonium oxalate also effectively extracts Al and Fe from
IV. Existing versus Added Organic Matter … 131
organic matter (McKeague and Schuppli, 1985; Rayment and Higginson, 1992),
therefore, the precipitation of Al and Fe must have occurred in the experiment.
If we follow the incubation process, bicarbonate-P content was positively
correlated with dissolved organic-C content (Table 4.3-10) and the relationship
was stronger at higher incubation times, indicated by increasing r values from
three- to nine-month incubation. At the same time, bicarbonate-P was negatively
correlated with extractable-Fe from three to six months of incubation. These
observations lead to the deduction that dissolved organic-C played a positive role
in releasing bicarbonate-P to the soil solution. Moreover, as the positive
correlation between dissolved organic-C and non-extractable-P was steady over
time and so was non-extractable-P and bicarbonate-P (Table 4.3-2), suggested
that the increase of bicarbonate-P content in soil solution was more likely to have
been a contribution from dissolved organic-C than from these other factors.
Three months after organic matter was applied into the soil, mineralisation
processes were occurring, indicated by the increased concentrations of dissolved
organic-C (derived either from soil organic matter and/or derived from added
organic materials), bicarbonate-P, and higher phosphatase activity compared
with control soils. At the same time, organic complexes were likely to have been
formed with Fe3+ and precipitated as insoluble Fe(OH)3 (Hue, 1992; Iyamuremye
and Dick, 1996; Schwertmann and Taylor, 1989). Another possible mechanism
is from soluble humic molecules and low molecular weight aliphatic organic acids
from added organic matter, which can adsorb to the surface of Al and Fe-oxides
and block phosphate adsorption sites, thus increasing soluble phosphate
concentration (Haynes and Mokolobate, 2001). A similar process of phosphate
adsorption sites blocking was also observed as higher C content organic matter
such as lucerne hay and wheat straw (Table 3.3-2, Chapter 3) significantly (p ≤
0.001) decreased extractable-Al (Table 4.3-9). The same applications, lucerne
hay and wheat straw treatments, significantly (p ≤ 0.001) increased bicarbonate-
P (Table 4.3-2). However, no correlation between extractable-Al with
bicarbonate-P (Table 4.3-10) might lead to estimation that the process occurred
and equilibrated before the first sampling due to lower initial content of soil
extractable-Al than extractable-Fe (Table 4.2-4).
The correlation of extractable phosphate with soil pH was explained by assuming
that a pH increase due to organic matter addition (data not presented) would also
IV. Existing versus Added Organic Matter … 132
favour dissolution of soil organic-C (Beck et al., 1999), and thus increase
dissolved organic-C in soils (Figure 4.3-1). The increase in pH was probably
caused by the oxidation of organic acid anions (e.g. oxalate, malate, and citrate)
from organic residues that consumed H+ and released OH- (Noble et al., 1996;
Tang et al., 1999), or from decarboxylation of those anions which consumed
protons, released CO2, and led to a liming effect (Haynes and Mokolobate, 2001;
Tang et al., 1999; Yan et al., 1996).
In the case of phosphatase, despite the application of organic matter, as either
peat, wheat straw, or lucerne hay increased phosphatase activity, there was no
correlation between phosphatase and bicarbonate-P during the experiment
(Table 4.3-10). Phosphatase may have been at steady-state, whilst bicarbonate-
P could have been continuously released or reach nearly steady-state
concentrations as well. Such a situation would not lead to a correlation even
though a causative effect existed. It is also possible that phosphatase may pre-
exist in soil and persist for a long time. In addition, phosphatase activity is
subject to end-product suppression by high levels of extractable-P (Allison and
Vitousek, 2005; Feng and Xiong, 2002) and phosphatase is not the only enzyme
involved in soil P transformation. At least in this experiment, phosphatase activity
was not a reliable parameter to estimate P extractability. Therefore, the last
hypothesis is only partly supported by the experiment. According to McCallister
et al. (2002) even though phosphatase was positively correlated with
bicarbonate-P, it could not be used to predict P supply to plants nor could it be
used as an estimate for the differences of P concentration in plant tissues (de
Andrade et al., 2001).
Despite the fresh supply of organic-C from new organic matter, micro-organisms
producing the phosphatase enzyme were not active enough to generate similar
phosphatase activity to that observed for the soils with the existing soil organic
matter. The trend of increasing phosphatase activity in samples treated with new
organic matter was similar to samples with existing soil organic matter. The
magnitude of increase for new organic matter was around half to that of samples
with existing soil organic matter only, especially in the Ultisol (Figure 4.3-6).
Therefore, it is not high enough to transform non-extractable-P to bicarbonate-P
as indicated by the lack of correlation between phosphatase activity and
bicarbonate-P (Table 4.3-11), or there was insufficient accessibility of organic-P
to phosphatases. Another explanation for this was the effect of soil combustion
IV. Existing versus Added Organic Matter … 133
on phosphatase. Although phosphatase is resistant to microwave irradiation
(Speir et al., 1986), its activity is heat-sensitive (Gosewinkel and Broadbent,
1986). It would be very unlikely if phosphatase or microbial biomass would have
survived the high temperature during the combustion period. In addition, the soil
starter may have had insufficient microbial population to produce significant
amounts of phosphatase.
4.5 CONCLUSIONS
Bicarbonate-extractable P increased in soil with applied organic matter (peat,
wheat straw, and lucerne hay) in which the existing soil organic matter had been
removed to a greater extent than in soils with existing soil organic matter. The
Ultisol had a larger increase in bicarbonate-P than the Oxisol. The addition of
organic matter increased non-extractable-P concentrations up to six-month
incubation. Transformation of non-extractable-P to extractable phosphate must
have been at its maximum around this time indicated by non-extractable-P
decreases as well as increases in bicarbonate extractable-phosphate.
Stronger correlation over time between dissolved soil organic-C and bicarbonate-
P indicated the importance of dissolved organic-C in generating bicarbonate-P
that could be from organic-P hydrolysis, transformation, or displacement of
adsorbed bicarbonate-P by soluble organic compounds. Contrary to the initial
hypothesis, there was only weak evidence that microbial biomass-P played a role
in providing more bicarbonate-P content, as well as phosphatase. However, data
showed that microbial biomass-P had greater effect in bicarbonate-P content
compared with phosphatase. Other mechanisms, either chemical or biological,
might have been involved in the process.
To sum, newly applied organic amendment was more effective in releasing
available P compared with the pre-existing soil organic matter. However, in order
to utilise other pools of P, the increased proportions of non-extractable-P to total-
P need to be resolved or analysed further, especially in lateritic soils.
IV. Existing versus Added Organic Matter … 134
C h a p t e r 5
SOIL ORGANIC CARBON LOSSES: THE BALANCE BETWEEN RESPIRATION AND
LEACHING, AND PHOSPHORUS MOBILITY IN LATERITIC SOILS
5.1 INTRODUCTION
Soils play an effective role as a sink or storage for carbon (C), and as a source or
loss of C to pools outside the soil system. The balance between storage and loss
depends on soil management, as well as climate and soil properties. The losses
of C from soil are mainly from soil organic matter decomposition (respiration),
erosion, and leaching (Akala and Lal, 2000). According to Cronan (1985) and
Magill and Aber (2000), C leaching contributes between 6-46% of total-C loss as
dissolved organic-C in forest soils. Investigations of C balance have focused on
the effect of temperature on C mineralisation (Liechty et al., 1995; Tate et al.,
1993; Zogg et al., 1997), which is only effective in surface soil (MacDonald et al.,
1999). The leaching of dissolved organic-C from soil may return to the
atmosphere as CO2 loss from streams, lakes, or oceans (Kling et al., 1991). For
soils in the tropics the balance between upward (respiration) and downward
(dissolved organic-C leaching) loss could be important, especially for lateritic
soils if the effectiveness of organic matter application is to be understood. Heavy
rainfall may be an additional factor in increasing organic-C loss from soil, not only
in erosion but also in infiltration of water through the soil profile.
Phosphorus adsorption, measured by phosphate buffering capacity and
phosphate retention index, was correlated with organic phosphorus (P) in soils
from Western Australia (Brennan et al., 1994). Phosphate buffering capacity was
also correlated with soil organic-C, Al, and Fe contents in Inceptisols in India
(Patiram et al., 1990). In other studies, soil organic-C was positively correlated
with the activity of phosphatase, especially in acid soils (Baligar et al., 1999;
Barrett et al., 1998; Canarutto et al., 1995). The increases of soil organic-P
content generally associated with increases in soil organic-C (Iyamuremye and
Dick, 1996; Reddy et al., 2000). Thus, if leaching of organic-C is prominent in
V. Soil Organic Carbon Losses: The Balance... 135
these soils, and organic-P affects P adsorption (Brennan et al., 1994), this
leaching will also have an effect on P mobility in the soil profile. Working with soil
columns, Chardon et al. (1997) found that 90% of total-P in leachate was in the
form of dissolved organic-P and that leaching was related to dissolved organic-C.
Rupp et al. (2002) suspected that decreasing redox potential might be
responsible for the increasing P solubility as well as dissolved organic-C, and
intensifying the leaching of both.
The decomposition of soil organic matter is associated with the release of P from
adsorption sites (Dalal, 1979; Gressel et al., 1996; Maroko et al., 1999). In this
process, mechanism such as organic ligand exchange (Hinsinger, 2001;
Iyamuremye and Dick, 1996) is responsible as part of dissolution and desorption
of phosphate (Burkitt et al., 2002; MacKay et al., 1986; McBride, 1994; Rhue and
Harris, 1999). For temperate forest soils, changes in soil organic-P are closely
related to changes in soil organic matter composition (Gressel et al., 1996),
although this study was limited to topsoil. Information on the relationship
between soil organic-C and soil organic-P in the profile of lateritic soils is limited,
especially in the tropics. Such information may be of special importance in the
tropics where soil organic matter decomposition is very rapid and may affect P
mobility. Additionally, P dynamics would be related to the activity of soil micro
organisms as indicated by the increase in phosphatase activity (Joner and
Jakobsen, 1995) and microbial biomass-P (Oberson et al., 1995) where organic
matter is applied to the soil.
As the adsorption and the release of P is related to soil organic-C (Brennan et al.,
1994), whilst aluminium (Al) and iron (Fe)-hydroxides are factors in phosphate
adsorption (Haynes and Mokolobate, 2001), the mobility of soil organic-C in
leaching may affect the sesquioxides concentrations through soil profile. In high
rainfall tropical areas, these mechanisms may occur to the deeper profile of
lateritic soils due to higher leaching.
In summary, organic-P has an impact on P cycles in soils. To a greater extent,
dissolved organic-P is the predominant form of P in subsoil of leaching columns
(Chardon et al., 1997). The turnover of dissolved organic-P, together with
dissolved organic-C and dissolved organic-N, is a major pathway in nutrient
cycling and may have more impact than previously assumed (Kalbitz et al.,
2000). In lateritic soils, these impacts could be more complicated due to the role
V. Soil Organic Carbon Losses: The Balance... 136
of Al and Fe-oxides in soils which may also change with leaching of organic-C.
These issues need to be explained in order to understand the mechanisms
involved in P mobility in lateritic soils.
The brief reviews leads to the following hypotheses:
• rainfall will alter the ratio between leaching and respiration from organic
matter mineralisation and to a certain extent the persistence of organic matter
itself;
• the loss of soluble-C due to the downward movement of water affects the
mobility of extractable-P, and;
• changes in the content of Al and Fe oxides with depth are closely related to
mobility of organic-C in soil profile.
The objectives of this experiment were: to compare fluxes of soil organic-C and
soil organic-P in two different lateritic soils, by:
• studying the effect of rainfall regime relevant to tropical and subtropical
environments on the loss of C from a soil profile, and
• measuring the effect of leaching on the change of soil organic-C, soil
extractable-P, and extractable-Al and Fe with depth in the soil.
5.2 MATERIALS AND METHODS
5.2.1 Design of the experiment This leaching column (PVC plastic, 90x5.7 cm) experiment was set up in factorial
using a completely randomised block design. The treatments were:
• Two soils: Ultisol (Typic kandiudult) and Oxisol (Plinthic eutrodox).
• Two rainfall simulations: tropical (3000 mm year-1 equivalent) and subtropical
(900 mm year-1 equivalent). These are about the same as in South
Kalimantan, Indonesia and Perth metro area, Western Australia, and
• Four organic matter sources: control, peat, wheat straw, and lucerne hay.
The organic matter rate was 80 ton ha-1, similar to the application rate for the
second experiment.
Table 5.2-1 shows the whole treatment combinations between soils, rainfall
simulations, and organic matter sources.
V. Soil Organic Carbon Losses: The Balance... 137
Table 5.2-1 Treatment combinations and their abbreviation from two types of soils (Ultisol and Oxisol), two rainfall simulations (tropical and subtropical), and four sources of organic matter (control, peat, wheat straw, and lucerne hay)
Rainfall Tropical Subtropical Soil
Control Peat Wheat straw
Lucerne hay Control Peat Wheat
straw Lucerne
hay Ultisol UTC UTP UTW UTL USC USP USW USL Oxisol OTC OTP OTW OTL OSC OSP OSW OSL U = Ultisol, O = Oxisol, T = tropical, S = subtropical, C = control, P = peat, W = wheat straw, and L = lucerne hay.
Data were analysed statistically with GenStat (Payne et al., 1987). Analysis of
variance was followed by mean comparison (orthogonal polynomial contrasts) if
the treatment was significant. Correlation and regressions were carried out using
SPSS (Coakes, 2001) to determine any relationships between parameters.
5.2.2 Procedures An Ultisol from Western Australia was collected near Jarrah (Eucalyptus
marginata) forest, near Jarrahdale, and an Oxisol from near Bunbury. Both soils
were selected due to their high content of extractable-Al and Fe (McArthur,
1991), in order to see their effects on phosphate adsorption and P mobility
throughout the leaching columns. Some soil characteristics are presented in
Table 4.2-4 in Chapter 4. Samples were taken from lower horizon in the profile
(20-25 cm) where the amount of soil organic matter was less but the amount of
extractable-Al and Fe were expected to be higher.
In order to simulate a combination of leaching process and wet-dry conditions,
water equivalent to one year’s rainfall was added to each column; the equivalent
annual rainfall was distributed in equal increments applied weekly. Gradual
watering was applied using drippers made from plastic bottles. With the bottom
open, the bottles were positioned upside down on the top of leaching columns.
A 0.2 µm millipore filter (25 mm, supor membrane, non sterile, Pall Gelman
Laboratory) was fixed inside the lid of every bottle to ensure a realistic rate of
water delivery. After 24 hours of water application, a suction mechanism (10 cm
water) was applied to all leachate containers for another 24 hours to avoid water
saturation and reducing conditions at the bottom of every column.
V. Soil Organic Carbon Losses: The Balance... 138
Soil in each column was divided into seven layers. The first layer (0-10 cm) was
the mixture of treatment and soil sample (Figure 5.2-1). Soil packing was
performed with soil bulk density as an indicator where Ultisol was 1.32 g cm3 and
for Oxisol was 1.13 g cm3. The internal diameter of a leaching column was 5.7
cm; total volume of water for the tropics was 7655 mL and for the subtropics was
2297 mL. Before the first application of water, soils were wet to 60% water
holding capacity. The edge-flow effect due to plastic column was reduced by
using funnel o-rings (0.6 cm thick) made from styrofoam on every layer of soil.
The o-rings fitted tightly inside the column. Soil mesh was applied on every layer
on the top of o-ring. To evenly distribute water dripping, a filter paper was placed
on the top of the organic matter layer. The filter paper needed to be changed
every two weeks to avoid decomposition.
Fresh, dry, and chopped (≤ 0.5 cm) organic matter (12 g of peat, wheat straw,
and/or lucerne hay for Ultisol; 10 g for Oxisol) were mixed with soils (270 g Ultisol
or 230 g Oxisol) as the top layer (0-10 cm) of soil in the column. With the
replicate of three, the overall experimental unit was 2x2x4x3 = 48 leaching
columns.
5.2.2.1 Soil sampling To follow the mobility of dissolved organic-C and other factors affecting it at the
end of the experiment, sampling was done on six layers of the leaching columns
(0-10, 10-20, 20-30, 30-40, 40-50, and 50-60 cm). With 48 leaching columns,
there were 288 soil samples. Leachate was collected for further analysis of
dissolved organic-C, dissolved reactive-P, dissolved-Al and Fe, and soil pH
according to the methods described in the following section. There were 48
leachate containers collected.
Soil samples were air-dried and sieved (≤ 2 mm) and kept in sealed plastic bags
for other chemical analyses. Methods of determination or chemical analyses of
all parameters are described below.
V. Soil Organic Carbon Losses: The Balance... 139
0-10 cm
Soil mesh (pore ∅ = 1 mm)
PV
C, ∅
= 5
.7 c
m
Silica sand
Sealed leachate container
Soil + OM
Dripper
Vacuum air
O-ring 0.6 cm thick
10 cm water suction
Filter paper
Figure 5.2-1 Leaching column with six layers of soil and one layer of soil + organic matter. Suction apparatus (right part of the graph) was set at 10 cm water.
5.2.3 Measurements
5.2.3.1 Bicarbonate phosphorus in soil Bicarbonate-P was extracted from soil using 0.5 M NaHCO3 at pH 8.5. This is a
modification from a method by Olsen et al. (1954) described by Rayment and
Higginson (1992). The manual colorimetric method for determination of
phosphate in the extract was based on that of Murphy and Riley (1962).
5.2.3.2 Non-extractable phosphorus Measurement of non-extractable-P was conducted by measuring total-P with
acid-persulfate digestion (Nelson, 1987) and then subtracting the results of the
bicarbonate-P content of the same soil calculated beforehand.
V. Soil Organic Carbon Losses: The Balance... 140
5.2.3.3 Dissolved reactive phosphorus in leachate In leachate, dissolved reactive-P was measured directly in leachate after filtration
with 0.2 µm millipore filter (25 mm, supor membrane, non sterile, Pall Gelman
Laboratory) using the method of Rayment and Higginson (1992).
5.2.3.4 Soil respiration The method used to measure soil respiration in this experiment was similar to
method used in Chapter 3. The only modification was made in the time period
that the alkaline trap (0.5 M KOH) was exposed to soil inside the headspace of
the leaching column. Twenty and 30 mL (at the beginning of the experiment) of
0.5 M KOH were exposed for only 24 hours. Plastic containers (50 mL) were
used and put inside the columns sitting on the top of soil surface. The top of the
columns was sealed with parafilm (American National Can) and rubber bands.
Sampling was performed in every other two-week period for six months.
Total respiration during the experiment was calculated as total area under the
of dissolved organic-C in leachate (Figure 5.3-1). Type of soil was also differing
as Ultisol had higher C in leachate compared with Oxisol. Wheat straw
application produced the highest dissolved organic-C content in leachate followed
by lucerne hay, control, and peat. Peat application, in other words, had lower C
loss due to leaching, especially with tropical rainfall. This effect of peat on
dissolved organic-C leaching occurred in both soils. The amount of rainfall
affected the amount of C loss by leaching. The higher rate of rainfall (tropical)
had higher dissolved organic-C concentrations in the leachate.
Table 5.3-1 Analysis of variance for dissolved organic carbon in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Only main factors and their interactions are shown
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Organic matter 3 396.29 132.10 8.76 < 0.001 Rainfall 1 182.91 182.91 12.13 0.001 Soil 1 2026.70 2026.70 134.44 < 0.001 OM x rainfall 3 118.56 39.52 2.62 0.068 OM x soil 3 196.00 65.33 4.33 0.011 Rainfall x soil 1 270.28 270.28 17.93 < 0.001 OM x rainfall x soil 3 155.22 51.74 3.43 0.028 Residual 32 482.41 15.08 Total 47 38328.37 OM = organic matter.
Soil types differed in producing dissolved organic-C in their leachate. The Ultisol
produced more C leaching over all organic matter treatments compared with the
Oxisol.
V. Soil Organic Carbon Losses: The Balance... 143
0.0
20.0
40.0
Control Peat Wheatstraw
Lucernehay
Oxisol Ultisol
0.0
20.0
40.0
Control Peat Wheatstraw
Lucernehay
Dis
sove
d or
gani
c ca
rbon
(mg
kg-1
)Tropical rainfall
(4000 mm year-1)
Subtropical rainfall(900 mm year-1)
Figure 5.3-1 Cumulative dissolved organic carbon in leachate expressed per mass of soil after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.
The percentage of C in leachate compared with the total-C in soil and organic
matter sources was calculated and is presented in Table 5.3-2.
Table 5.3-2 Cumulative carbon loss (%) from leaching following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical)
Rainfall Tropical Subtropical OM
Source Ultisol Oxisol Ultisol Oxisol Control 0.13c 0.15c 0.04a 0.05a
Peat 0.10b 0.10b 0.03a 0.04a
Wheat straw 0.22d 0.14c 0.04a 0.05a
Lucerne hay 0.16c 0.10b 0.04a 0.04a
Numbers followed by the same letters in superscript indicate differences according to LSD 5% (0.032).
V. Soil Organic Carbon Losses: The Balance... 144
5.3.1.2 Leachate pH Rainfall affected (p ≤ 0.001, Table 5.3-3) leachate pH and the effect was highly
significant. Tropical rainfall caused the increase of leachate pH with the about
0.6 point. In Ultisol the effect even more pronounced with the increase about 1.0
point. Even though the soil pH of both soils were about the same (Table 4.2-4,
Chapter 4), the amount of rainfall can make the leachate from those differ from
each other.
Organic matter source only had significant effect on leachate pH. Peat tended to
decrease pH of leachate as of pH of soils in previous experiment (I and II), whilst
wheat straw showed the opposite. Lucerne hay treatment showed inconsistency
within the two soils and rates of rainfall applied. Under tropical rainfall, lucerne
hay in both the Ultisol and Oxisol decreased leachate pH compared with control.
With subtropical rainfall, however, lucerne hay increased leachate pH in the
Ultisol and decreased the pH in the Oxisol.
Table 5.3-3 Analysis of variance for leachate pH after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Organic matter 3 1.8905 0.6302 3.97 0.016 Rainfall 1 3.5698 3.5698 22.50 < 0.001 Soil 1 0.5188 0.5188 3.27 0.080 OM x rainfall 3 1.7479 0.5826 3.67 0.022 OM x soil 3 0.3391 0.1130 0.71 0.552 Rainfall x soil 1 2.2925 2.2925 14.45 < 0.001 OM x rainfall x soil 3 2.4870 0.8290 5.22 0.005 Residual 32 5.0777 0.1587 Total 47 17.9232 OM = organic matter.
V. Soil Organic Carbon Losses: The Balance... 145
5.50
7.00
8.50
Control Peat Wheatstraw
Lucernehay
5.50
7.00
8.50
Control Peat Wheatstraw
Lucernehay
Oxisol UltisolLeac
hate
pH
Tropical rainfall(4000 mm year-1)
Subtropical rainfall(900 mm year-1)
Figure 5.3-2 Cumulative pH on leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.
5.3.1.3 Carbon throughout the leaching column Throughout the leaching column, dissolved organic-C concentration in soil
increased approximately linearly with depth (Figure 5.3-3). This linear trend was
calculated from the main effect of column layers with orthogonal polynomial
contrast (p ≤ 0.01). Test statistics with t-test0.01 also showed that dissolved
organic-C concentrations in column layers were different from the control. The
Oxisol had higher soil dissolved organic-C concentrations compared with the
Ultisol.
V. Soil Organic Carbon Losses: The Balance... 146
Table 5.3-4 Analysis of variance for dissolved organic carbon throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown
Figure 5.3-3 Dissolved organic carbon concentration throughout the leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.
5.3.1.4 Soil respiration There was no significant difference in soil respiration between tropical and
subtropical rainfall treatments. For both rates of rainfall, soil respiration from
V. Soil Organic Carbon Losses: The Balance... 147
wheat straw and lucerne hay treatments rapidly declined within the first four
weeks after a very high CO2 evolution at the beginning of measurement.
Table 5.3-5 Analysis of variance for soil respiration after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Organic matter 3 2.551283 0.850428 334.05 < 0.001 Rainfall 1 0.005633 0.005633 2.21 0.147 Soil 1 0.997633 0.997633 391.87 < 0.001 OM x rainfall 3 0.007117 0.002372 0.93 0.437 OM x soil 3 0.236083 0.078694 30.91 < 0.001 Rainfall x soil 1 0.000033 0.000033 0.01 0.910 OM x rainfall x soil 3 0.008217 0.002739 1.08 0.373 Residual 32 0.081467 0.002546 Total 47 3.887467 OM = organic matter.
V. Soil Organic Carbon Losses: The Balance... 148
0
10
20
30
2 8 14 20 26
0
10
20
30
2 8 14 20 26
Ultisol Oxisol
week
-1)
Figure 5.3-4 Soil respiration throughout the experiment with tropical rainfall. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph represent
LSD 5% for organic matter.
Figure 5.3-5 Soil respiration throughout the experiment with subtropical rainfall. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph represent LSD 5% for organic matter.
The results are presented in Table 5.3-6 as follow:
Table 5.3-6 Respiration estimates (mg CO2) during the experiment for every source of organic matter (control, peat, wheat straw, and lucerne) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical)
As for the first experiment, the amount of C loss from the total-C in soils can be
calculated from the above table, and the results are in Table 5.3-7.
Table 5.3-7 Cumulative carbon loss (%) from soil respiration following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical)
Lucerne hay 0.54de 0.94fg 0.46cd 0.88f Numbers followed by the same letters in superscript indicate differences according to LSD 5% (0.084).
5.3.2 Balance on carbon losses between respiration and leaching
Table 5.3-8 shows the balance in percentage between respiration (Table 5.3-7)
and leaching (Table 5.3-2) on dissolved organic-C. The lowest value was 1.4 in
the control Ultisol receiving tropical rainfall, meaning that C loss from respiration
was 1.4 times higher than that of leaching for the particular treatment. Carbon
loss was 21.1 times higher in respiration than that of leaching for Ultisol receiving
subtropical rainfall and wheat straw treatments.
Table 5.3-8 Balance between respiration and leaching on every treatment, calculated from loss (%) due to respiration divided by loss (%) due to leaching
5.3.3 Effect of leaching on phosphorus concentrations
5.3.3.1 Phosphorus concentration in columns and leachate As the purpose of this experiment was to follow the mobility of bicarbonate
extractable-P throughout the leaching columns, the analysis of bicarbonate-P as
V. Soil Organic Carbon Losses: The Balance... 150
of experiment I and II was carried out in the same method. Bicarbonate-P was
determined in columns that had high concentration of dissolved organic-C in their
leachate, which were columns with wheat straw treatments. They were columns
with Oxisol treated with tropical rainfall and wheat straw (OTW) and Ultisol
treated with tropical rainfall and wheat straw (UTW) (Table 5.2-1).
There was a significant (p ≤ 0.01, Table 5.3-9 ) effect of tropical rainfall on the
distribution of bicarbonate-P concentration throughout the leaching column
(Figure 5.3-6).
Table 5.3-9 Analysis of variance for bicarbonate phosphorus throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown
Figure 5.3-6 Bicarbonate phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.
Table 5.3-10 Analysis of variance for dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Organic matter 3 0.0013676 0.0004559 2.11 0.119 Rainfall 1 0.0000081 0.0000081 0.04 0.848 Soil 1 0.0000536 0.0000536 0.25 0.622 OM x rainfall 3 0.0000691 0.0000230 0.11 0.956 OM x soil 3 0.0000471 0.0000157 0.07 0.974 Rainfall x soil 1 0.0000263 0.0000263 0.12 0.730 OM x rainfall x soil 3 0.0000725 0.0000242 0.11 0.953 Residual 32 0.0069187 0.0002162 Total 47 0.0085628 OM = organic matter.
V. Soil Organic Carbon Losses: The Balance... 152
0.0000
0.0175
0.0350
Control Peat Wheatstraw
Lucernehay
Oxisol Ultisol
0.0000
0.0175
0.0350
Control Peat Wheatstraw
Lucernehay
Dis
solv
ed re
activ
e p
hosp
horu
s (m
g kg
-1)
Subtropical rainfall(900 mm year-1)
Tropical rainfall(4000 mm year-1)
Figure 5.3-7 Cumulative dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.
In leachate, there was no effect (p ≥ 0.05, Table 5.3-10) of soil and rainfall on
dissolved reactive-P concentrations, although the trend was about the same as of
the dissolved organic-C in leachate (Figure 5.3-7), especially in tropical rainfall.
As with dissolved organic-C, dissolved reactive-P concentration in leachate was
higher from columns treated with wheat straw, followed by lucerne hay and peat.
In case of total-P, the Oxisol had higher concentration than that of the Ultisol and
the difference was highly significant (p ≤ 0.001, Table 5.3-11). This could be due
to the initial total-P concentration (Table 4.2-4, Chapter 4). Rainfall also affected
total-P concentrations in every layer (p = 0.01, Table 5.3-11) of the leaching
column (Figure 5.3-8). As of total-P, non-extractable-P in the Oxisol was higher
than in the Ultisol, but rainfall had no effect on non-extractable-P concentrations
in either soil.
V. Soil Organic Carbon Losses: The Balance... 153
Table 5.3-11 Analysis of variance for total phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown
No amount of total-P concentration was detected in the leachate (Table 5.3-12) to
indicate the leaching of total-P from the columns.
Table 5.3-12 Analysis of variance for total phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Organic matter 3 0.21142 0.07047 1.31 0.289 Rainfall 1 0.00227 0.00227 0.04 0.839 Soil 1 0.00110 0.00110 0.02 0.887 OM x rainfall 3 0.00064 0.00021 0.00 1.000 OM x soil 3 0.00157 0.00052 0.01 0.999 Rainfall x soil 1 0.00152 0.00152 0.03 0.868 OM x rainfall x soil 3 0.00072 0.00024 0.00 1.00 Residual 32 1.72513 0.05391 Total 47 1.94438 OM = organic matter.
V. Soil Organic Carbon Losses: The Balance... 154
0
10
20
30
40
50
60
0 50 100
Total phosphorus (mg kg-1)
Col
umn
laye
rs (c
m)
Figure 5.3-8 Total phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.
Table 5.3-13 Analysis of variance for non-extractable phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown
There was a significant concentrations of non-extractable-P throughout the
leaching columns in every type of soil (p ≤ 0.05, Table 5.3-13). The trend was
linear (p ≤ 0.01, orthogonal polynomial contrast) as shown in Figure 5.3-9.
V. Soil Organic Carbon Losses: The Balance... 155
0
10
20
30
40
50
60
0 50 100
Col
umn
laye
rs (c
m)
Non-extractable phosphorus (mg kg-1)
R2 = 0.56
R2 = 0.70
Figure 5.3-9 Non-extractable phosphorus distribution in leaching column with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.
5.3.4 Factors affecting phosphorus transformation in the leaching columns
5.3.4.1 Extractable aluminium In the soil column treated with wheat straw and tropical rainfall, there was an
interaction effect (p ≤ 0.05, Table 5.3-14) of soil and column layer on oxalate
extractable-Al (Figure 5.3-12). In the Ultisol, Al showed a declining concentration
(p ≤ 0.01) to the lower layers of the column. On the contrary, in the Oxisol, Al
recorded an increasing concentration (p ≤ 0.01). Both in the Ultisol and the
Oxisol, these linear trends were significant according to t-test0.01. Oxisol had a
higher content of extractable-Al (206 mg kg-1) than the Ultisol (194 mg kg-1). The
content of extractable-Al in leaching column was between 194-217 mg kg-1.
V. Soil Organic Carbon Losses: The Balance... 156
Table 5.3-14 Analysis of variance for extractable aluminium throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown
In leachate, a fraction of dissolved-Al was recorded and showed a significant (p ≤
0.01, Table 5.3-15) response due to soils, sources or organic matter, and rainfall
simulations (Figure 5.3-10).
Table 5.3-15 Analysis of variance for dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Organic matter 3 0.202717 0.067572 12.79 < 0.001 Rainfall 1 0.086700 0.086700 16.41 < 0.001 Soil 1 0.294533 0.294533 55.75 < 0.001 OM x rainfall 3 0.057117 0.019039 3.60 0.024 OM x soil 3 0.219617 0.073206 13.86 < 0.001 Rainfall x soil 1 0.100833 0.100833 19.09 < 0.001 OM x rainfall x soil 3 0.052471 0.017472 3.31 0.032 Residual 32 0.169067 0.005283 Total 47 1.183000 OM = organic matter.
V. Soil Organic Carbon Losses: The Balance... 157
0.0
0.4
0.8
Control Peat Wheat Lucerne
Oxisol Ultisol
0.0
0.4
0.8
Control Peat Wheat Lucerne
Dis
solve
d al
umin
ium
(mg
kg-1
)
Tropical rainfall(4000 mm year-1)
Subtropical rainfall(900 mm year-1)
Figure 5.3-10 Cumulative dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.
5.3.4.2 Extractable iron As for Al, there was a significant interaction effect between soil type and colum
layer (p ≤ 0.05, Table 5.3-16) on oxalate extractable-Fe (Figure 5.3-12). The
trends of Fe concentration throughout the leaching column were linear (p ≤ 0.01).
In the Ultisol, Fe showed a declining concentration to the lower layers of the
column. On the contrary, in the Oxisol, Fe recorded an increasing concentration.
Similar to Al, these linear trends were different from the control according to t-
test0.01. The content of extractable-Fe ranged between 1534-1708 mg kg-1.
There was a significant difference (p ≤ 0.01) in Fe content between Ultisol and
Oxisol which were 902 and 2228 mg kg-1.
V. Soil Organic Carbon Losses: The Balance... 158
Table 5.3-16 Analysis of variance for extractable iron throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown
In leachate, a fraction of dissolved-Fe was measured and showed a significant (p
≤ 0.01, Table 5.3-17) response due to soil type, sources or organic matter, and
rainfall simulations (Figure 5.3-10).
Table 5.3-17 Analysis of variance for dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment
Source of variation Degree
of freedom
Sum of squares
Mean square F P
Organic matter 3 1.71389 0.57130 18.92 < 0.001 Rainfall 1 0.43510 0.43510 14.41 < 0.001 Soil 1 3.80250 3.80250 125.90 < 0.001 OM x rainfall 3 0.41427 0.13890 4.57 0.009 OM x soil 3 1.74874 0.58291 19.30 < 0.001 Rainfall x soil 1 0.57860 0.57860 19.16 < 0.001 OM x rainfall x soil 3 0.41397 0.13799 4.57 0.009 Residual 32 0.96647 0.03020 Total 47 10.07355 OM = organic matter.
V. Soil Organic Carbon Losses: The Balance... 159
0.0
1.0
2.0
Control Peat Wheat Lucerne
Oxisol Ultisol
0.0
1.0
2.0
Control Peat Wheat Lucerne
Dis
solve
d iro
n (m
g kg
-1)
Tropical rainfall(4000 mm year-1)
Subtropical rainfall(900 mm year-1)
Figure 5.3-11 Cumulative dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.
V. Soil Organic Carbon Losses: The Balance... 160
0
10
20
30
40
50
60
150 200 250
Aluminium (mg kg-1)
0
10
20
30
40
50
60
0 1500 3000
Iron (mg kg-1)
Col
umn
laye
rs (c
m)
R2 = 0.53
R2 = 0.36
R2 = 0.73
R2 = 0.82
Figure 5.3-12 Extractable aluminium and iron distribution in leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.
5.3.4.3 Soil pH There was no difference (Table 5.3-18) on soil pH throughout the leaching
column. Water movement from rainfall treatments had no effect on the balance
between OH- and H+ in leaching columns.
Table 5.3-18 Analysis of variance for soil pH throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown
However, the amount of water from subtropical rainfall made the leachate more
acid than that of tropical rainfall (Table 5.3-3), either in the Ultisol or Oxisol.
5.3.4.4 Correlations between all parameters in the leaching column
Table 5.3-19 Correlation among all parameters in the leaching column. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation
TP BP NP Al Fe pH DOC TP BP 0.47** NP 0.97** Al Fe 0.54** 0.59** 0.42* pH DOC 0.49** 0.56** 0.76** TP = Total Phosphorus, BP = Bicarbonate Phosphorus, NP = Non-extractable Phosphorus, and DOC = Dissolved Organic Carbon.
5.4 DISCUSSION
The high leaching of dissolved organic-C in the Ultisol treated with tropical rainfall
following application of wheat straw may have been related to the higher C
organic compounds from peat are likely to remain in soil for longer period of time,
in contrast to the easily-decomposable, simple organic acids from lucerne hay or
wheat straw. However, C loss from leaching was not as much as C loss from
respiration. Even though the respiration decreased over time (Figure 5.3-5), the
overall C loss was much higher than that of leaching process (Table 5.3-2 and
Table 5.3-7). Subtropical rainfall showed a higher respiration rate in both soils
compared to tropical rainfall. The magnitude of C loss from respiration with
tropical rainfall ranged between 1 to 10 times higher in control and in soil with
wheat straw treatment than that of leaching. In columns receiving subtropical
rainfall, C loss from respiration ranged between 7 to 36 times higher in control
and in soil with wheat straw treatment (Table 5.3-4). Columns receiving tropical
rainfall also had higher C loss due to leaching, but columns receiving subtropical
rainfall had more C loss due to respiration.
Not only did dissolved organic-C leach from the columns, but soluble organic
matter concentrations increased in the deeper layers of soil in columns (Figure
5.3-3). This effect was also observed by Rumple et al. (2002) and Brye et al.
V. Soil Organic Carbon Losses: The Balance... 162
(2001), and may represent a mechanism for podzolisation (Egli et al., 2003;
Ugolini et al., 1987). Podzolisation involves the loss of some cations (Al3+ and
Fe2+/Fe3+) from surface soil horizons (Egli et al., 2003), and this experiment also
showed this effect on extractable-Al and Fe content in the leachate, especially in
the Oxisol (Figure 5.3-10 and Figure 5.3-11). In the leaching columns, there was
an interaction (p ≤ 0.01) between type of soil and column layers on extractable-Al
and Fe contents (Figure 5.3-12) indicating the mobility of those cations.
Consequently, longer exposure to a high rate of rainfall (more than the time of the
experiment) may also have leached extractable-Al and Fe through the Ultisol
columns.
In spite of a significant effect on dissolved organic-C concentration in leachate
due to organic matter treatment, rainfall, and type of soil, there was no significant
effect on dissolved reactive-P content in leachate (Table 5.3-10). However, the
non-significant trend in Figure 5.3-7 was similar to the significant trend in Figure
5.3-1 of dissolved organic-C, where wheat straw showed the greatest effect
followed by lucerne hay and peat. This trend in dissolved reactive-P could be the
explanation why this P pool contributes to the problem of water body, i.e.
eutrophication (Liikanen et al., 2004; McDowell and Sharpley, 2004). Moreover,
there was a significant difference with depth (p ≤ 0.01) in bicarbonate-P
concentrations throughout the leaching column. There was an increase of
concentration at the top layer of the column. This occurred in columns treated
with wheat straw with tropical rainfall (Figure 5.3-6). A similar trend was also
observed for total-P (Figure 5.3-8). Observations by Kleinman et al. (2003) who
also recorded that there was an increase in P concentrations (oxalate and
Mehlich 3-extractable) in the deeper layers of leaching columns, and there was
poor correlation of soil P fractions with leachate P. Kleinmann et al. (2003)
suggested that translocation of P due to preferential flow of P via macropores
(bypass leaching) might have taken place.
A closer inspection of the distribution pattern of bicarbonate-P along the leaching
column reveals that there was an initial deposition of bicarbonate-P at the first 10
cm from the soil surface. Working with soil samples from 0-5 cm depth,
Koopmans et al. (2003) found an accumulation of inorganic-P (extracted with
EDTA) after manure applications. This could be similar to the process of
bicarbonate-P deposition in this experiment, but with intermittent occurrence with
V. Soil Organic Carbon Losses: The Balance... 163
deposition. According to Kleinman et al. (2003), P leaching losses often occur in
discrete events or pulses that can be easily missed and to monitor this effect
continuously requires expensive equipment. The fluctuation of P might have
been due to the process of mobilisation and resorption at the wall of macropores
(Jensen et al., 1999). Working with glucose as a source of C in soil column,
Jensen et al. (1999) predicted the role of high C content (1000 mg glucose-C L-1),
Al and Fe(III)(hydr)oxide, and redox condition in creating the fluctuation. This
mechanism might have taken place in our experiment, even though no easily
metabolisable-C source was present.
An interesting result was observed for non-extractable-P throughout the soil in
leaching column. Non-extractable-P was correlated with dissolved organic-C in
Ultisol leaching column (Table 5.3-19) and the distribution of non-extractable-P
(Figure 5.3-9) was similar to that of dissolved organic-C (Figure 5.3-3). Both
showed linear trends with depth (p ≤ 0.05, orthogonal polynomial contrast). More
non-extractable-P and dissolved organic-C were recorded at greater depth in the
columns indicating leaching and deposition processes. In the Oxisol, the
deposition of non-extractable-P took place at 20-30 cm depth in the column. The
high content of extractable-Fe in the Oxisol (Table 4.2-4, Chapter 4) might
represent amorphous Fe sesquioxide which can adsorb this pool of P by ligand
exchange with surface hydroxyls, especially organic-P from inositol (Turrion et
al., 2001). Considering the effect of mobilisation and resorption on bicarbonate-P
and total-P in this experiment, non-extractable-P on the other hand, was not
affected by such mechanisms, but transported more freely to the deeper layers of
the column.
As for non-extractable-P in leaching columns, there were significant linear trends
(p ≤ 0.01) in Al and Fe distribution throughout the leaching column. In the Oxisol,
probably due to higher content of extractable-Al and Fe (Table 4.2-4, Chapter 4),
both Al and Fe were more mobile. Soil pH might also responsible for their
mobility. Even though soil pH was not affected, soil organic matter
decomposition created more acid leachate, especially at a subtropical rainfall
rate. This acid leachate was probably more easily to transport extractable-Al and
Fe throughout the leaching columns. As a result, extractable-Al and Fe
deposited at the deeper layers or even leached away (Figure 5.3-10 and Figure
5.3-11). This mechanism confirms the hypothesis that organic matter addition
V. Soil Organic Carbon Losses: The Balance... 164
will affect not only soil organic-C content of the soil, but also non-extractable-P,
extractable-Al, extractable-Fe, and pH (Chapter 4).
Figure 5.4-1 summarises average C mobility and loss in Ultisol columns treated
with tropical rainfall and 80 ton ha-1 wheat straw. Respiration was the highest C
flux, followed by leaching and deposition. The total C loss from the original pools
(soil organic C + wheat straw treatment at 80 ton ha-1) was about 0.8%.
Before After
Tropicalrainfall
(4000 mm year-1)
Ultisol = Wheat = 113.5 g C114.4 g C
252 mg C
66 mg C
646 mg C Respiration
Residual
Translocationwithin column
Leaching
Figure 5.4-1 Carbon losses (respiration and leaching) and deposition due to tropical rainfall (4000 mm year-1) in soil (Ultisol) column treated with wheat straw (80 ton ha-1).
5.5 CONCLUSION
Respiration contributed to C loss ranging from 0.2-1.00% of total-C in soils,
regardless of the rainfall rate. A small percentage of C loss occurred via leaching
(0.03-0.05% of total-C for subtropical and 0.10-0.22% of total-C for tropical
rainfall regimes). Respiration therefore contributed 1-36 times higher C loss than
leaching. However, at a tropical rate of rainfall, the leaching process was as
important as respiration as a C loss mechanism, and could reduce the
persistence of added organic matter.
V. Soil Organic Carbon Losses: The Balance... 165
There was no evidence of bicarbonate-P and non-extractable-P leaching from the
columns, but there was an inconsistent distribution of bicarbonate-P and linear
distribution of non-extractable-P within the leaching columns.
Added organic matter had an impact on the distribution of soil bicarbonate-P,
non-extractable-P and dissolved organic-C as well as sesquioxides down the
leaching columns. Dissolved organic-C was the most readily leached from within
the six month period. There was an indication that bicarbonate-P and non-
extractable-P will be leached away with longer period of time, especially in sandy
soil with higher rainfall.
In these scenarios, if organic matter amendments were to be applied to lateritic
soils, climatic factors, i.e. rainfall, need to be taken into account in order to make
amendments more effective and efficient. Moreover, to test the effectiveness of
organic matter amendments, in situ research is likely to be the most
advantageous approach to be employed.
V. Soil Organic Carbon Losses: The Balance... 166
C h a p t e r 6
THE RELATIONSHIP BETWEEN PHOSPHATE ADSORPTION AND SOIL ORGANIC CARBON
FROM ORGANIC MATTER ADDITION
6.1 INTRODUCTION
The addition of organic matter to soils may increase phosphate availability by
decomposition and mineralisation of organic phosphorus, or by abiotic processes
such as ligand-exchange effects on phosphate adsorption. It is difficult to
separate the effects of biotic and abiotic processes on the release of phosphate.
This chapter describes experimental work which investigated the direct effect of
organic matter addition on adsorption of phosphate in four types of lateritic soils.
The organic matter content of highly weathered soils has been shown to be
negatively correlated with phosphate adsorption capacity. For example, Singh
and Gilkes (1991) found a negative correlation between phosphate sorption by
soil (quantified by the Freundlich KF parameter) and soil organic matter content
for 97 mainly highly weathered soils of Western Australia. Based on Freundlich
KF values, Dubus and Bacquer (2001) also found a significant negative
correlation between phosphate sorption and soil organic matter content.
Three abiotic mechanisms are proposed to explain how organic matter additions
increase P availability in soils (Iyamuremye and Dick, 1996). First, soluble
organic molecules may specifically adsorb to soil minerals by ligand exchange in
competition with phosphate (Erich et al., 2002; Ohno and Crannel, 1996; Singh
and Jones, 1976). Second, the soluble organic matter may react with bound Al3+
or Fe3+ at the surface of mineral phases to form soluble complexes of these
elements and release phosphate which was previously sorbed or which was
present as insoluble Al and Fe-phosphate (Haynes and Mokolobate, 2001).
Third, organic matter may be sorbed to soil particles at non-specific adsorption
sites, resulting in higher negative charge of the particles (Haynes and Naidu,
1998; Naidu et al., 1997). This process reduces the electrostatic attraction of
phosphate and decreases phosphate anion activity at the reactive surface. All
VI. The Relationship between Phosphate Adsorption … 167
three abiotic mechanisms may occur simultaneously, and in combination with
biotic release of phosphate by mineralisation.
The experiment described in Chapter 3 showed that phosphate extracted with 0.5
M NaHCO3 at pH 8.5 (bicarbonate-P) was significantly higher in soil amended
with organic matter than in unamended soil, especially with green manure
amendment (lucerne hay), followed by peat. The effect of soluble organic matter
on phosphate release from Oxisols has been found to be transient (Afif et al.,
1995). Moreover, fresh organic matter, which produces soluble organic
compounds, has more active (particulate) pools prior to decomposition (Brenner
et al., 2001; Gijsman et al., 2002). The findings in this work therefore lead to the
question of whether peat (the passive pool) would have a longer abiotic effect
due to its resistance to decomposition, and possible slow release of soluble
organic ligands which can compete for adsorption sites with phosphate. This
issue is poorly understood. The abiotic effects of organic matter additions could
be determined by analysing phosphate adsorption isotherms for soil incubated
with lucerne hay and peat for varying lengths of time up to one year.
Organic matter addition also releases phosphate to the soil solution as a result of
biotic mineralisation processes (Cobo et al., 2002; Kwabiah et al., 2003; Lupwayi
et al., 2003). These two mechanisms may be particularly difficult to separate in
soils with high sorbing capacity for phosphate, such as lateritic soils with high
sesquioxide or 1:1 clay content. By (i) observing net release of extractable
phosphate (Chapter 3 and 4) and (ii) determining phosphate adsorption
isotherms, the two processes (inorganic effects on adsorption, and mineralisation
of organic-P) can be separated as being more and less important for releasing
soluble phosphate into soils.
In summary, it is expected that organic matter addition will reduce phosphate
adsorption in soils, and this hypothesis will be addressed by determining
phosphate adsorption at a range of organic matter application rates. This work
will also test the novel hypotheses that (i) any reduction in phosphate adsorption
will be temporary depending on the persistence of organic matter added (for
example, peat is expected to reduce phosphate adsorption for longer than
lucerne hay) and (ii) newly added organic matter will decrease phosphate
adsorption, in the short-term, more than pre-existing soil organic matter.
The objectives of this experiment were therefore:
VI. The Relationship between Phosphate Adsorption … 168
• to determine the effect of organic matter application rate on phosphate
adsorption to soil;
• to determine the interactive effect of (i) application of different types of
organic matter (varying in persistence following application) and (ii) organic
amendment–soil incubation time on phosphate adsorption to soil;
• to determine the effect of pre-existing soil organic matter and newly added
organic amendments on PO4-P adsorption in soils;
• to study the effect of phosphate adsorption and P mineralisation after
organic matter addition on the process of bicarbonate-P release;
• to study the changes in dissolved organic-C after organic matter treatment
on closely related factors of phosphate adsorption in soil, e.g. Al, Fe, and
pH.
6.2 MATERIALS AND METHODS
6.2.1 Design of the experiments Soil samples for the experiments in this chapter were from four lateritic soils (two
Oxisols, an Ultisol, and a deep regolith material or subsurface soil). The
experiments were divided into three subsets which had been subjected to the
following treatments:
1. Soil incubation with peat, wheat straw, and lucerne hay treatment at 80
ton ha-1 for different times (3, 6, 9, and 12 months).
2. Different organic matter levels (40, 80, 120, and 160 ton ha-1) for peat,
wheat straw, and lucerne hay, incubated with soil for six months.
3. Type of soil organic matter (existing and new). The existing refers to
inherent soil organic matter plus additional fresh organic matter as in
point 1 above. ‘New’ refers to equal (in organic-C concentration)
replacement of soil organic matter plus additional fresh organic matter to
soil in which the intrinsic organic matter has been removed by
combustion.
The first subset of this experiment was in a factorial design experiment with two
factors: incubation time (3, 6, 9, and 12 months), and source of organic matter
(nil (control), peat, wheat straw, and lucerne hay). The second subset was also
VI. The Relationship between Phosphate Adsorption … 169
factorial with two factors: source of organic matter (nil (control), peat, wheat
straw, and lucerne hay), and organic matter levels (40, 80, 120, and 160 ton
ha-1). The third subset was a factorial design with three factors: soils (an Ultisol,
an Oxisol, and a subsurface soil), type of soil organic matter (existing and new),
and organic matter source (peat and lucerne hay).
Statistical analyses were performed either with GenStat (Payne et al., 1987) for
analyses of variance, or SPSS (Coakes, 2001) for regression analyses.
6.2.2 Procedures The preparations for the first and the second subset of the experiment are
described in Chapter 3. In brief, soil (Oxisol, Typic hapludox; “Balkuling”) was
collected from a farmland near York, Western Australia. One kg of soil was
placed into each pot and the appropriate treatment was applied by mixing the soil
and respective organic material. Deionised water was added weekly during
incubation (12 months) to reach a moisture equivalent of 60% of the maximum
water holding capacity of the soil (42%).
The preparations for the third subset are described in Chapter 4. In brief, soil
samples (Ultisol from Jarrahdale, Oxisol from Bunbury, and subsurface soil —
deep regolith material— from Boddington Gold Mine, all in Western Australia)
were air-dried and sieved to ≥ 2 mm. To generate soils having only newly-added
organic matter, soils were combusted at 450° C for 12 hours using a pottery kiln
(Kiln West, model 6191Z) as described in Chapter 4. A temperature of 450° C is
hot enough to deplete organic-C content, but has a minimal effect on other
physical and chemical properties of soils such as porosity, textural class, and soil
acidity (Giovannini et al., 1988; Kang and Sajjapongse, 1980).
6.2.3 Measurements
6.2.3.1 Phosphate adsorption isotherms Phosphate adsorption isotherms were measured using methods of Morel et al.
(1996) and Erich et al. (2002). To determine phosphate adsorption isotherms, six
amounts of added P (30, 40, 50, 60, 80, 100 mg P kg-1) were applied as KH2PO4
to one g of soil in 25 mL of 0.01 M CaCl2. These concentrations had been
established in a preliminary experiment to check for the appropriate concentration
range, given the high phosphate sorbing capacity of the soils. Suspensions were
VI. The Relationship between Phosphate Adsorption … 170
shaken in 50 mL centrifuge tubes at 25° C for 17 hours. Tubes were centrifuged
at 3000 rpm for five minutes and the supernatants were filtered (Whatman #42)
prior to analysis of phosphate.
Phosphate remaining in solution after equilibrium process was measured using
the methods of Murphy and Riley (1962) as described in Rayment and Higginson
(1992).
6.2.3.2 Numerical methods Two common adsorption isotherm equations, Langmuir and Freundlich, were
used to fit the phosphate adsorption data. The Langmuir equation, which is
where c = P concentration in equilibrium solution (µg P mL-1), cs = the
concentration of the solute (i.e. soil), xm = adsorption maximum (µg P g-1 soil),
and KBET = a constant for energy of interaction with soil particles (Burau and
Zasoski, 2002). A plot of c/(cs-c)(1/x) versus c/(cs-c) therefore has a slope
(KBET-1)/(KBETxm) and intercept 1/(KBETxm).
In order to determine the statistical significance of differences between
treatments, linear regression analysis with grouped data was performed on data
transformed to give linear Langmuir or Freundlich relationships, as described
above (Section 6.2.3.1). This procedure allowed statistical significance to be
assessed without replication of points on adsorption isotherms. Grouped
regressions were performed using GenStat (Payne et al., 1987).
6.2.3.3 Dissolved organic carbon Dissolved organic-C in soil samples was measured using the method by Wagai
and Solin (2002). Briefly, dissolved organic-C in soil was extracted from 10 g air-
dry soil with 25 mL deionised (MilliQ), shaken for 30 minutes on an end-over-end
shaker, and then centrifuged at 10,000 rpm for 10 minutes. Filtered supernatant
(0.2 µm filter, 25 mm, supor membrane, non sterile, Pall Gelman Laboratory) was
used to measured soluble organic-C using a TOC (Shimadzu TOC-5000A)
machine as the difference between total-C and inorganic-C in the solution.
VI. The Relationship between Phosphate Adsorption … 172
6.2.3.4 Total phosphorus Total-P was measured by persulfate digestion as described by Nelson (1987). A
mixture of 0.5 g soil, 1.0 mL deionised water, 1.0 mL 5.5 M H2SO4, and 0.400 g
K2S2O4 in screwcapped Pyrex tubes (100 mL) was autoclaved for one hour at
130° C. Samples were diluted to 50 mL with deionised water, mixed, and
allowed to stand overnight to settle particulate materials. Total phosphate in
extracts was measured colorimetrically as described by Rayment and Higginson
(1992).
6.2.3.5 Bicarbonate phosphorus Bicarbonate-P was measured following extraction of soil with 0.5 M NaHCO pH
8.5. The method used was a modification of a method by Olsen et al. (1954)
described by Rayment and Higginson (1992). The manual colorimetric
determination of phosphate at the end of the method was based on the method of
Murphy and Riley (1962).
3
6.2.3.6 Non-extractable phosphorus Non-extractable-P was measured by assessing total-P with acid-persulfate
digestion as described by Nelson (1987) (see above) and then subtracting the
bicarbonate-P content.
6.2.3.7 Extractable aluminium and iron Oxalate extractable-Al and Fe, considered to represent amorphous Al and Fe
oxyhydroxides, were measured using an acid oxalate extraction as explained by
Rayment and Higginson (1992), modified to use 0.250 mg of soil in 25 mL
reagent. The extraction was conducted in the dark with minimal room light to
avoid photo-reduction of Fe (Schwertmann and Taylor, 1989). A black plastic
sheet was also used to cover sample batch during transportation from shaker to
centrifuges. Aluminium and Fe concentrations in extracts were determined by
atomic absorption spectrophotometry. Caesium chloride solution 0.01 M (1+4)
was used for dilution to reduce ionisation effects.
6.3 RESULTS
The Langmuir equation fitted experimental data better than the Freundlich
equation in describing phosphate adsorption (Table 6.3-1 and Table 6.3-2). This
VI. The Relationship between Phosphate Adsorption … 173
conclusion was based on the R2 values from the Langmuir equation that tended
to be higher than that of the Freundlich equation. These were applied to the first
two sampling (subset 1 and 2, Section 6.2.1). For the last subset (3), the
Brunauer-Emmett-Teller equation provided a better fit to the experimental data,
due to the sigmoidal shape of the adsorption relationship.
6.3.1 The effect of incubation time The effect of prior incubation time with organic amendments on phosphate
adsorption varied, depending on the source of organic matter. Figure 6.3-1
shows the trends in adsorption isotherms at different incubation times.
Increasing incubation time had a tendency to reduce the affinity of adsorption up
to six or nine months after organic matter application. One-year after application,
organic amendment was less or not effective in reducing phosphate adsorption in
the soils. After this period of time, adsorption capacity seemed return to the initial
position or was similar to adsorption at three months after organic matter
addition.
These effects correspond with the trends KL and KF values presented in Table
6.3-1. Values of KL and KF decreased from three to nine months incubation, and
approached the initial values (three-month incubation) at the end of incubation.
Soil amended with peat recorded its lowest KL (0.42) and KF (338) values after
nine months of incubation. Similar trends were also recorded for lucerne hay.
Therefore, for peat and lucerne hay applications, the nine-month incubation
effectively reduced phosphate adsorption (p ≤ 0.01 on linear regression with
grouped data) more than three-month incubation (Figure 6.3-2).
VI. The Relationship between Phosphate Adsorption … 174
0
500
1000
0.0 10.0 20.0
0
500
1000
0.0 10.0 20.0
0
500
1000
0.0 10.0 20.0
0
500
1000
0.0 10.0 20.0
Peat
Control
Wheat
Lucerne
P in solution (mg L-1)
P so
rbed
(mg
kg-1)
Figure 6.3-1 The effect of incubation and organic matter sources on phosphate adsorption at 80 ton ha-1 organic matter in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Lines are a logarithmic trend of observations and a visual guide only.
VI. The Relationship between Phosphate Adsorption … 175
0.00
0.01
0.02
-1 7 14
c
2.4
2.8
3.1
-0.5 0.4 1.3
log c
log
x
BAc/
x
Figure 6.3-2 Linearised Langmuir (A) and Freundlich (B) plots of incubation effect on phosphate adsorption for peat treatment (80 ton ha-1) in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).
6.3.2 The effect of organic matter sources The effectiveness of organic matter source in reducing phosphate adsorption was
in the order of lucerne hay>wheat straw>peat>control (p ≤ 0.01). This order
occurred at almost every level of organic matter applied (Figure 6.3-3).
For wheat straw alone, phosphate adsorption was lower than the control until six
months after organic matter treatment (Table 6.3-1). After that, phosphate
adsorption returned to the initial condition for all sources of organic matter.
These conclusions are supported the magnitude of the Langmuir’s bonding
energy (KL) and KF at Freundlich. Both Langmuir KL and Freundlich KF
parameters were lower for soil samples having organic matter applications
compared with controls (Table 6.3-1).
VI. The Relationship between Phosphate Adsorption … 176
0
500
1000
0.0 9.0 18.0
0
500
1000
0.0 9.0 18.0
0
500
1000
0.0 9.0 18.0
0
500
1000
0.0 9.0 18.0
P in solution (mg L-1)
P so
rbed
(mg
kg-1) 80 ton ha-1
40 ton ha-1
120 ton ha-1
160 ton ha-1
Figure 6.3-3 The effect of organic matter sources and their levels on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Lines are a logarithmic trend of observations and are a visual guide only.
VI. The Relationship between Phosphate Adsorption … 177
VI. The Relationship between Phosphate Adsorption … 178
0.00
0.01
0.02
-3 8 18
c
2.4
2.7
3.0
-0.6 0.5 1.5
log c
log
x
BAc/
x
Figure 6.3-4 Linearised Langmuir (A) and Freundlich (B) plots of organic matter sources (120 ton ha-1) on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).
6.3.3 The effect of organic matter levels Figure 6.3-3 shows that the higher the amount of organic matter applied the more
soluble phosphate were released in the soils. This could be an indication that the
higher the organic matter application, the lesser the phosphate adsorption, as all
adsorption parameter showed decreasing values, especially KL and KF (Table
6.3-2). Lucerne hay, for example, had KL = 0.63 at 40 ton ha-1 application and
the value of this parameter decreased regularly until KL = 0.41 at 160 ton ha-1.
Similar trends were recorded for the Freundlich equation parameter for
adsorption after addition of peat and wheat straw.
Increasing amounts of peat application also decreased (p ≤ 0.01) phosphate
adsorption (Figure 6.3-5). Increasing the level of peat from 40-120 ton ha-1
decreased phosphate adsorption more regularly compared with wheat straw and
lucerne hay. The graphs from linear regression with group were similar either
from linearised Langmuir or Freundlich equation.
0.000
0.007
0.013
-2 5 12
c
2.40
2.70
3.00
-1.0 0.3 1.5
log c
log
x
BAc/
x
Figure 6.3-5 Linearised Langmuir (A) and Freundlich (B) plots of peat levels at six months incubation time on phosphate adsorption. = 40, = 80, = 120, and = 160 ton ha-1 in “Balkuling” Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).
Table 6.3-1 The effect of organic matter sources (80 ton ha-1) and incubation time on phosphate adsorption parameters based on Langmuir and Freundlich equations
VI. The Relationship between Phosphate Adsorption … 179
Table 6.3-2 The effect of organic matter sources and levels on phosphate adsorption parameters at six months incubation time based on Langmuir and Freundlich equations
6.3.4 The effect of existing and new soil organic matter The effect of existing and new soil organic matter depended on both the source
of organic matter and incubation time. Neither the Langmuir nor Freundlich
equations provided a good fit in describing phosphate adsorption from the
treatments, as the adsorption relationships were sigmoidal. The Brunauer-
Emmett-Teller equation described this type of data better. The new soil organic
matter treatments of peat and lucerne hay resulted in an increase in phosphate
adsorption in soils compared with the existing soil organic matter (Figure 6.3-6).
This effects occurred in all soils (subsurface, Ultisol, and Oxisol), no matter how
high the initial bicarbonate-P content of any particular soil. New soil organic
matter treatment increased phosphate adsorption intensity, despite more
bicarbonate-P release in soils. The KBET parameter (Table 6.3-3) for new soil
organic matter treatments had higher values compared with existing soil organic
matter treatments.
For subsurface soil, due to negligible pre-existing soil organic matter content
(Table 4.2-5, Chapter 4), the effect of soil organic matter type (existing and new)
VI. The Relationship between Phosphate Adsorption … 180
VI. The Relationship between Phosphate Adsorption … 181
can be assumed to represent the effect of heating soil. The difference between
phosphate measured in solution from new and existing soil organic matter can be
assumed to represent the increase in adsorption capacity. The increase was
significant (p ≤ 0.05 on linear regression with grouped data) when using
Brunauer-Emmett-Teller equation to parameterise the adsorption data (Figure
6.3-8).
Figure 6.3-6 The effect of soils, existing and new soil organic matter on phosphate adsorption in peat and lucerne hay treatments at 80 ton ha-1 and six-month incubation time. = subsurface soil, = Ultisol, and = Oxisol. Lines are polynomial trend for observations and are a visual guide only.
ween Phosphate Adsorption … 182
0
450
900
0.0 40.0 80.00
450
900
0.0 40.0 80.0
0
450
900
0.0 40.0 80.00
450
900
0.0 40.0 80.0
P so
rbed
(mg
kg-1)
Existing New
P in solution (mg L-1)
Peat
Lucerne
VI. The Relationship bet
Table 6.3-3 The effect of soil organic matter types (existing and new) and soil types (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption parameters based on fitting data to the Brunauer-Emmett-Teller equation
6.3.5 The phosphate adsorption capacity of the three soils Phosphate adsorption capacity was different among the three soils. The Oxisol
had the highest adsorption capacity, followed by the Ultisol and subsurface soil
(Figure 6.3-6). This conclusion was supported by the higher magnitude of KBET
for the Oxisol compared with the other two soils (Table 6.3-3). Regression
analysis with grouped data also indicated that the Oxisol had the highest
phosphate adsorption capacity compared with Ultisol and subsurface soil (p ≤
0.01) (Figure 6.3-7).
VI. The Relationship between Phosphate Adsorption with… 183
0.0000
0.0001
0.0002
-0.01 0.03 0.07
c/cs
c/(c
s-c)
.1/x
Subsurface soil
Ultisol
Oxisol
Figure 6.3-7 Linearised Brunauer-Emmett-Teller equation of soil type (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption in new soil organic matter + peat treatment. = subsurface soil, = Ultisol, and = Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.01).
In subsurface soil, new soil organic matter treatments, of either peat (p ≤ 0.01) or
In the Ultisol and Oxisol, the same increase was recorded for new soil organic
matter + peat (p ≤ 0.01) and new soil organic matter + lucerne hay (p ≤ 0.01)
(Figure 6.3-9 and Figure 6.3-10).
VI. The Relationship between Phosphate Adsorption with… 184
0.0000
0.0007
0.0013
0.00 0.04 0.08
c/cs
c/(c
s-c)
.1/x
Existing SOM + peat
New SOM + peat
Existing SOM + lucerne
New SOM + lucerne hay
Figure 6.3-8 Linearised Brunauer-Emmett-Teller equation of soil organic matter type (existing and new) on phosphate adsorption in subsurface soil. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).
0.00000
0.00007
0.00014
0.000 0.021 0.041
c/cs
c/(c
s-c)
.1/x
Existing SOM + peat
Existing SOM + lucerne
New SOM + lucerne hay
New SOM + peat
Figure 6.3-9 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Ultisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).
VI. The Relationship between Phosphate Adsorption with… 185
0.00000
0.00003
0.00006
0.000 0.017 0.034
c/cs
c/(c
s-c)
.1/x
Existing SOM + lucerne
Existing SOM + peat
New SOM + lucerne hay
New SOM + peat
Figure 6.3-10 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Oxisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).
6.3.6 Correlation of phosphate adsorption parameters with dissolved organic carbon, extractable aluminium, and extractable iron
In order to see the effect of peat and lucerne hay on phosphate adsorption
capacity of the soil in the third subset of the experiment, we assessed the data
from uncombusted soil samples. Bicarbonate-P was positively correlated with
dissolved organic-C in soil treated with existing soil organic matter + lucerne hay
only. For existing soil organic matter + peat and existing soil organic matter +
lucerne hay treatments, bicarbonate-P was negatively correlated with extractable-
Al and Fe (Table 6.3-4). Only in existing soil organic matter + lucerne hay
application did bicarbonate-P show a positive correlation with dissolved organic-
C.
VI. The Relationship between Phosphate Adsorption with… 186
Table 6.3-4 Correlation between inorganic phosphorus, dissolved organic carbon, extractable aluminium, and extractable iron in uncombusted soils treated with peat and lucerne hay treatments. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation
Existing SOM + peat Existing SOM + lucerne hay BP DOC Al BP DOC Al
DOC 0.63** Al -0.83** -0.72** -0.69** Fe -0.64** -0.44* 0.44* -0.61** -0.82** 0.56** SOM = soil organic matter, DOC = dissolved organic-C, BP = bicarbonate-P.
Further correlation analysis of adsorption parameters with soil chemical
properties in the first subset of the experiment showed that dissolved organic-C
and pH had negative correlations (Table 6.3-5) with KL and xm for the Langmuir
equation; and KF for the Freundlich equation. Even though KL represents the
bonding energy and KF relates to adsorption capacity, the coefficient correlations
for both on dissolved organic-C and soil pH were almost similar.
Table 6.3-5 Correlation between phosphorus adsorption parameters and soil chemical properties in “Balkuling” Oxisol. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation
capacity due to the combustion of the soils (Figure 6.3-6) as also indicated by the
increase of KBET in Table 6.3-3. In other words, changes in phosphate adsorption
appear to be less important as a bicarbonate-P release mechanism when soils
are treated with organic matter. According to Osiname et al. (Osiname et al.,
2000) a sizable proportion of P released from organic-P mineralisation might be
used to satisfy phosphate adsorption capacity of the soils. In applied practices,
however, P mineralisation from organic matter should be accounted for when the
adsorption isotherm technique is applied to soils that have been amended with
organic materials (Berton and Pratt, 1997). In our experiment, although
combustion plus treatment of soils with new soil organic matter increased
phosphate adsorption capacity, the bicarbonate-P release to soil solution showed
no sign of decreasing. Contributions from incomplete oxidation of organic-P
during combustion and the changes in acid solubility of soil inorganic-P
(Anderson, 1960; Condron et al., 1990; Williams et al., 1970) as a result of
combustion might also have occurred in soil samples.
Increasing the level of organic matter in all treatments (i.e. peat, wheat straw, and
lucerne hay) did not show the same trend in phosphate adsorption for all organic
matter sources. Wheat straw and lucerne hay, according to their values of KL in
Table 6.3-2, consistently decreased phosphate sorption as the level of their
applications increased. Peat, however, showed almost similar KL values as the
level of application increased. In other words, increasing level up to 180 ton ha-1
VI. The Relationship between Phosphate Adsorption with… 190
was ineffective for the purpose of decreasing phosphate adsorption in an Oxisol,
when peat was applied.
Decreasing phosphate adsorption due to the increasing level of organic matter
addition may also be explained as a consequence of P content in their biomass.
When phosphate is released during decomposition of organic matter, it is then
rapidly adsorbed onto sorption sites. Therefore, more adsorbed P is present
before the equilibration by CaCl2 solution (Haynes and Mokolobate, 2001;
Iyamuremye and Dick, 1996; Li et al., 1990). As a result, there is less phosphate
adsorption capacity of the soil with respect to subsequently added phosphate.
Different types of organic matter had different effects on phosphate adsorption.
Lucerne hay decreased phosphate adsorption more than wheat straw and wheat
straw decreased phosphate adsorption more than peat (Figure 6.3-3 and Table
6.3-1). Comparable values in Table 6.3-1 (row six months) and Table 6.3-2 (row
80 ton ha-1) appeared to be slightly different due to different sub samplings.
However, declining trends in parameter KL, xm, b and KF were similar to the effect
of different organic matter sources, indicating similar trend of declining phosphate
adsorption capacity. At the same time, increasing the amount of organic matter
application also reduced phosphate adsorption more. Even at 180 ton ha-1, the
order of effect was peat<wheat straw<lucerne hay. The fact that peat increased
bicarbonate-P content in soils (Chapter 3 and Chapter 4) higher than wheat straw
application again suggests that phosphate adsorption was not the dominant
factor affecting bicarbonate-P release after peat addition. Though peat has lower
P content than wheat straw (Table 3.3-2, Chapter 3), it could provide more
bicarbonate-P if it is applied to soil. Peat probably could facilitate organic-P
transformation to inorganic-P better than wheat straw by other mechanisms,
except decreasing phosphate adsorption.
6.5 CONCLUSION
Organic matter addition could reduce phosphate adsorption and the effect could
last up to nine months after application. Lucerne hay was more effective than
peat and wheat straw addition in reducing phosphate adsorption capacity.
Langmuir equation fitted better than Freundlich in describing phosphate
adsorption in this experiment.
VI. The Relationship between Phosphate Adsorption with… 191
Soil combustion in new soil organic matter addition treatments increased
phosphate adsorption capacity, especially in the Oxisol and Ultisol. At the same
time, only the Brunauer-Emmett-Teller equation could describe the sigmoidal
effect of treatments on phosphate adsorption. Consequently, comparing the
effect of existing and new soil organic matter treatment by comparing equation
parameters was impossible.
The new soil organic matter addition released more bicarbonate-P into soil
solution. The experiment revealed the reduction of phosphate adsorption
occurred during the first year of organic matter applications and long term release
of bicarbonate-P in soil contributed to mineralisation, P hydrolysis, or P
transformation.
VI. The Relationship between Phosphate Adsorption with… 192
C h a p t e r 7
GENERAL DISCUSSION AND CONCLUSION
7.1 BRIEF SUMMARY OF THE MAJOR FINDINGS AND THEIR INTERPRETATIONS
Lateritic soils, which have been generally considered marginal for food
production, occupy large areas in tropical regions. With rapidly increasing
populations in developing countries, these soils are under increasing pressure to
become agricultural land. Soil fertility management of these soils is required to
overcome problems such as low in cation exchange capacity, low and declining
soil pH, and low nutrient availability. Organic matter addition is identified to be
effective in reducing phosphate adsorption in soils high in aluminium (Al) and iron
(Fe) oxides. This study was therefore conducted to investigate the changes
chemical and biological properties of examples of lateritic soils when amended
with fresh organic materials. The emphasis on soil phosphorus (P) was essential
as most lateritic soils have abundant hydroxides which can intensively fix this
nutrient in the form of phosphate ions.
The major findings in the research were:
• Application of organic matter enhanced the availability of nutrients, especially
P in the lateritic soils studied. Its persistence, to some extent, depended on
the C:N ratio of the material applied. Peat, in spite of its low concentrations of
C and P, released more P compared with wheat straw. For the longer term
(more than a year), peat would release more P than lucerne hay.
• The stronger correlation between bicarbonate-P and organic-C over time (12
months) indicated the importance of organic-P hydrolysis, transformation, or
displacement of adsorbed bicarbonate-P by soluble organic compounds.
Therefore, respiration and leaching which were the keys to C loss from soil,
contributed to the P dynamics, especially in tropical environment.
• Despite the ability of organic amendment (especially lucerne hay) to decrease
phosphate adsorption up to nine months after treatment, the application of
lucerne hay to soil with no organic matter increased phosphate adsorption. At
VII. General Discussion and Conclusion 193
VII. General Discussion and Conclusion 194
the same time, the increase in P mineralisation was greater than the increase
in phosphate adsorption, releasing more P into soil solution.
• Even though addition of organic material such as wheat straw and lucerne hay
can reduce phosphate adsorption in lateritic soils, exceptions may be revealed
in very specific circumstances. On one hand, as such organic materials
increased phosphate adsorption in soils with very low content of organic
matter, studies need to be done in order to understand the circumstances in
which organic matter addition is effective in reducing, and increasing,
phosphate adsorption. Or on the other hand, the artefact of combustion needs
to be scrutinised further, especially in soil without added organic matter.
Further major findings and reasoning for the whole experiments in this thesis
presents in Table 7.1-1
Table 7.1-1 Summary of findings in all experiments in this thesis
Chapter Major finding Comment • Despite a low content of P, peat increased bicarbonate-P
more and for longer than did lucerne hay with higher P content.
• Peat decomposition created conditions (either chemically or biologically) which enabled P to be released from sources other than the peat itself. 3
• Bicarbonate extractable phosphate was positively correlated with total extractable-C, extractable organic-C, NH4-N, and soil pH.
• This highlights a role of organic-C in phosphate release.
• New organic matter increased bicarbonate-P more than the pre-existing organic matter, especially in Ultisol.
• Freshly added organic matter may produce specific type of organic-C which was more reactive in releasing phosphate than that of the existing soil organic matter.
• The stronger positive correlation between bicarbonate-P and soil organic-C over time indicated the importance of organic-P hydrolysis, transformation, or displacement of adsorbed bicarbonate-P by soluble organic compounds.
• Even though correlations might indicate the formation of sesquioxides precipitates with organic compounds, there was no indication that precipitates could adsorb more bicarbonate-P. The two processes (organic-P hydrolysis and displacement) perhaps involved in releasing more P. However, the experiment could not single out one specific process.
4
• Contrary to the hypothesis, microbial biomass-P and phosphatase were not correlated with bicarbonate-P.
• The two parameters were not reliable in predicting P dynamics in lateritic soils with organic amendment, suggesting that other factors contributed to observed increases in extractable phosphate.
VII. General Discussion and Conclusion 195
usion 196
Table 7.1-1 Continued Chapter Major finding Comment
• Respiration contributed between 1-36 times more C losses than leaching.
• The numbers are precise estimation for lateritic soil (Ultisol and Oxisol) from Western Australia.
• For simulated tropical rainfall, leaching was as important as respiration in reducing the persistence of soil organic matter.
• In the aspect of soil organic matter persistence in top soil, leaching might be more pronounced in tropical lateritics than that of sub-tropical.
• Leaching of dissolved organic-C influenced the mobility of bicarbonate-P and sesquioxides in soil leaching columns.
• More research needs to be done, especially on erratic total- and bicarbonate-P concentrations due to P mobilisation/resorption process proposed by other researchers for an Alfisol.
5
• There was a tendency of bicarbonate-P leaching, especially in sandy lateritic soil with longer expose of higher rainfall.
• Limited time and the amount of rainfall simulation was the cause why there was no dissolved reactive phosphorus in the leachate. Hence the results were not contradicted with P leaching in other experiments.
• Organic matter addition could reduce phosphate adsorption for up to nine months. The Langmuir adsorption isotherm equation fitted better than the Freundlich.
• The effect of organic matter additions on desorption of phosphate is transient, making reapplication necessary at intervals depending on organic matter source.
• The soil combustion procedure for removing the pre-existing soil organic matter might have altered physical and chemical properties of the soil and increased phosphate adsorption capacity. The Brunauer-Emmett-Teller equation described data showing sigmoidal adsorption isotherms.
• Under very limited soil organic matter, phosphate sorption isotherm could not be explained with the previous equations as the Brunauer-Emmett-Teller equation did. However, comparing the latest with the previous two was impossible as they were in different samplings.
6
• Recalcitrant sources of C such as peat, and low C:N ratio source of organic matter such as lucerne hay, were probably a suitable combination in reducing phosphate adsorption and its release from fixation sites through mineralisation.
• Suggestion for future research in order to optimise organic amendments in lateritic soils, especially in tropical environment.
VII. General Discussion and Concl
In the preliminary experiment (Chapter 3), as soon as fresh organic matter was
incorporated into soil (Typic hapludox, “Balkuling”; from Western Australia),
decomposition process starts immediately. Cumulative CO2 emission reached
1600 mg CO2 kg-1 year-1, representing about 6% of total organic-C in the soil after
organic matter addition. This was one of the causes why soil organic matter
disappears from soils and reduces its persistence (Curtin et al., 2000; Mapa,
1996). Surprisingly, peat application showed a greater effect in releasing more
bicarbonate-P to soil than wheat straw application, despite the fact that wheat
straw biomass had higher P content that peat. Peat persisted longer to make P
more available and related to total extractable-C, total extractable organic-C, and
soil pH through out the experiment. Furthermore, organic matter mineralisation
enhanced nutrient availability, especially ammonium, nitrate, P, and potassium
(K). This process occurred soon after organic matter applied to the soil. The
maximum mineralisation took place within the first three months and there was
indication of positive priming effect, especially C mineralisation (measured with
soil respiration) from soil samples treated with lucerne hay. The results
supported all hypotheses with most of them in agreement with previous
experiments by many authors (Hartemink and O'Sullivan, 2001; Haynes and
Mokolobate, 2001). As a preliminary experiment, these findings were expected
to develop the following ideas in conducting more advanced research, especially
in P dynamics of lateritic soils.
In accordance with the first experiment, in the second experiment (Chapter 4),
bicarbonate-P also increased in soil with applied organic matter (peat, wheat
straw, and lucerne hay), especially in treatments without pre-existing soil organic
matter. The Ultisol (Typic kandiudult) showed a larger increase in bicarbonate-P
than the Oxisol (Plinthic eutrodox). In the case of the subsurface soil, P
immobilisation might have been taking place after lucerne hay addition. The
higher inherent bicarbonate-P content in subsurface soil may have increased the
population of soil micro-organisms more than in the Ultisol and Oxisol (even in
combusted soil samples) causing P immobilisation. However, the results showed
that, by the end of the experiment, P immobilisation had declined indicated by
smaller microbial biomass-P. Similar to bicarbonate-P, total-P content in the soils
showed an increase after organic matter was applied to soil from which pre-
VII. General Discussion and Conclusion 197
existing soil organic matter had been removed, especially in the Ultisol and the
Oxisol. This increase leads to the speculation that the combustion process for
removing existing organic matter solubilised the occluded P and P which was
stabilised in the existing soil organic matter (Giardina et al., 2000; Serrasolsas
and Khanna, 1995). Before combustion, this fraction of P in soil samples was not
extractable. As soon as the existing soil organic matter was removed by
combustion, P was more readily extracted. The increase in total-P consequently
increased non-extractable-P content in soils. This fraction of soil P, which was
greater than bicarbonate-P, is presumed to be beneficial in P dynamics since
some of this fraction is organic-P. A higher content of organic-P in the soil will
have a greater chance to be transformed to inorganic-P which is then available
for plants. Peat, as in the first experiment, increased non-extractable-P more
than wheat straw, despite the fact that wheat straw had higher P content in its
biomass. At the beginning of the experiment, there was an indication that non-
extractable-P (expected to be mostly organic-P) was correlated with microbial
biomass-P as hypothesised earlier in the proposal. Nevertheless, the correlation
was not observed at the end of the experiment.
As for the first experiment, the increasing C content due to organic matter
application was also accompanied by an increase in bicarbonate-P. This was
followed by the changes in Al, Fe, and soil pH. These reactions were similar to
the previous experiment and could be very common in lateritic soils when
amended with fresh organic matter. Contrary to the initial hypothesis; there was
only weak evidence that microbial biomass-P played a role in providing more
bicarbonate-P content. Meanwhile, phosphatase activity was not related to
bicarbonate-P release in soil for this experiment. The enzyme may have been in
steady state, or pre existing in soil, but in this experiment phosphatase was not a
reliable parameter to estimate phosphate release. However, other mechanisms,
either chemical or biological might have been involved in the release of
bicarbonate-P, but the experiment was not able to reveal these. One thing in
common from both experiments was that the increasing level of organic matter
increased dissolved organic-C as well as bicarbonate-P. For this, we can
conclude that stronger correlation between dissolved organic-C and bicarbonate-
P towards the end of the experiment could be from organic-P hydrolysis or
displacement of adsorbed bicarbonate-P by soluble organic compounds
produced by decomposition of added organic matter.
VII. General Discussion and Conclusion 198
In the leaching columns in the third experiment (Chapter 5), a small percentage
of C loss (1.4%) occurred by leaching from soil. Simulated tropical rainfall,
indeed, resulted in higher leaching in C. Leaching contributed about one tenth of
the C loss due to respiration but, no doubt, leaching process reduced the
persistence of added organic matter. Therefore, most C was lost by respiration
on this situation, especially in lateritic soils treated with subtropical rainfall.
Moreover, organic-C was also deposited in the deeper layers of the column
indicating a podzolisation or similar process (Egli et al., 2003; Rumpel et al.,
2002).
There was no evidence of bicarbonate-P and non-extractable-P leaching from the
columns. Furthermore, the decomposition of added organic matter had an
impact on the distribution of soil bicarbonate-P, non-extractable-P and dissolved
organic-C as well as sesquioxides in the leaching columns. This was in
accordance with the previous experiments on the relationship between
bicarbonate-P and dissolved organic-C. Soil dissolved organic-C was the most
readily leached within the six months period. There was an indication that
bicarbonate-P and possibly non-extractable-P will be leached away with longer
period of time, especially in sandy soil with higher rainfall.
Another interesting record on this experiment was the apparent intermittent
leaching of bicarbonate-P and total-P. This rare finding was observed by few
authors (Jensen et al., 1999; Kleinman et al., 2003) and is still hard to
understand. The proposed mechanisms which involve mobilisation, resorption on
the wall of macropores (Jensen et al., 1999), redox condition, and the role of Al
and Fe need to be scrutinised separately in the future research of P dynamics,
especially in lateritic soils of the tropics.
In the fourth experiment (Chapter 6), organic matter addition could reduce
phosphate adsorption and the effect could last up to nine months after
application. Lucerne hay was more effective than peat and wheat straw addition
in reducing phosphate adsorption capacity. Langmuir equation fitted better than
Freundlich in describing phosphate adsorption. However, both equations could
not describe phosphate adsorption in samples where inherent or existing soil
organic matter was negligible due to combustion process. The temperature of
450° C probably has altered physical and chemical properties of the minerals in
soils. Besides, inherent high P content in subsoil may restrict their adsorption to
VII. General Discussion and Conclusion 199
soil particles. As a consequence, the Brunauer-Emmett-Teller equation was
applied to fit the data where lower concentration of absorbate and its precipitation
with other soil component may have been occurred. This equation can also
explain sigmoidal isotherms (Hinz, 2001), as the data for some treatments in this
experiment revealed.
Bicarbonate-P release mechanism through mineralisation was still apparent
despite the fact that combustion of the soil samples has increased phosphate
adsorption capacity. Therefore, for soils with very low organic matter content, it
was possible that applying fresh organic matter could increase phosphate
adsorption capacity of lateritic soils. Peat treatment affected more bicarbonate-P
in soil and factors related to its release such as dissolved organic-C, Al, and Fe,
compared with lucerne hay treatment. This mechanism is supporting the idea
that organic matter addition will bring about the formation of metal ion complexes
with soluble organic components.
Some general soil mechanisms were noticed from the beginning of organic
matter application to the availability of P for plant in lateritic soils. Mineralisation
of P from added organic matter might be classified as the initial process followed
by immobilisation and adsorption. In these two processes, biological and
physical component of the soil were involved. The next processes were the
release of phosphate followed by the possible uptake by plants. While organic
matter persists in soil, these processes can perform their roles in turn or in unity,
indicating soil dynamics. The whole systems, of course, were dictated by
environmental factors such as water supply and temperature which in turn can be
increase or decrease the availability of P. But from all the experiments, selected
sources of organic matter can manage the whole system and become the
solution for lateritic soils.
In summary, addition of organic matter on lateritic soils was investigated as an
alternative solution to managing these marginal soils. High phosphate adsorption
capacity due to clay type and Al and Fe hydroxides was manageable by fresh
organic matter amendment. In this case, mineralisation of organic matter
addition not only released more bicarbonate-P and non-extractable-P to soil but
was also capable of satisfying phosphate adsorption, either with phosphate or
with adsorbed organic anions. Moreover, the problem related to C loss and
persistence, especially in tropical areas, can be overcome with the application of
VII. General Discussion and Conclusion 200
resistant organic matter such as peat or a mixture of peat and lucerne hay.
However, this possibility needs to be assessed further; either from a more
detailed soil chemistry study or from economical and environmental point of
views. Excavation and removal of large quantities of peat may create additional
environmental problems, such as changes in hydrology or accelerated
acidification.
7.2 LIMITATIONS AND FUTURE RESEARCH NEEDS
Several limitations have been considered throughout the experiments and need
to be addressed in the future by related research.
Greater effects of peat treatment in releasing bicarbonate-P in these experiment
initiated a further thought that peat may beneficial in applied to lateritic than any
other soils. Complex reactions, either in releasing more P pools, its persistence
in soils, and in reducing phosphate adsorption; led to the prediction that peat has
better chemical properties in dealing with poor characteristics of lateritic soils.
More research on peat, not only its composition, but also its characteristics and
distribution in the world, may help considerably in making lateritic soils more
productive.
The apparent intermittent effect of leaching on bicarbonate-P and total-P in
macropores through preferential pathways of both Ultisol and Oxisol needs to be
investigated more carefully. Due to redox condition, phosphate sorption, and the
role of Al and Fe (hydr)oxides, this unusual process might be of importance in the
P dynamics of tropical lateritic soils where leaching is significant.
The validity of combustion of soils in order to eliminate the inherent soil organic
matter has to be assessed case by case. However, using other methods such as
hydrogen peroxide may only produce limited amount of soil for experiment and
may also change soil chemical properties related to organic matter cycling.
Hence, combustion method is not generally recommended to study physical and
chemical characteristic of soils, but may provide useful information in carefully
designed experiments. We need to pay more attention on the potential errors in
inorganic-P extraction from combusted samples (Condron et al., 1990; Oniani et
al., 1973; Williams et al., 1970), especially when highly weathered soils are the
VII. General Discussion and Conclusion 201
subject of the experiment. Nevertheless, the specific design of the experiments
described in this thesis allowed the effects of combustion to be assessed
independently using a soil material with negligible organic content. The results of
this thesis are therefore interpretable by careful analysis of all data, taking the
independently-measured effects of combustion into account.
In terms of a more specific part of the system, the role of non-extractable-P, the
pool containing the biggest proportion of P in tropical lateritic soils, is still not well
understood. Even though the effect of fresh organic matter on P availability was
beneficial, but in terms of the proportion it was not only a small quantity. More
research therefore needs to be done in order to understand more about the
importance of non-extractable-P, especially in environment with a limited supply
of P. Any pathway that leads to the conversion of organic-P prior to uptake by
plants has to be resolved to maximise the benefit of the whole P pools. By doing
this, dependency on inorganic P fertilisers can be minimised.
In terms of the bigger scope of C dynamics in the tropics, due to continuous high
temperatures, losses as well as gains of C occur more rapidly. Understanding
the mechanisms, especially in relation to P, will help to realise the importance
and potential of lateritic soils in the tropics. Balanced mixture between green
manure and peat, which is abundant in Indonesia, maybe useful as a starting
point in managing P supply in this area. However, to be more accurate in
predicting these processes, in situ experiments needed to be carried out under
comparable condition to minimise variability due to environmental factors.
VII. General Discussion and Conclusion 202
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Appendix A. Linearisation of Brunauer-Emmett-Teller equation
Appendix B Article for 17th World Congress of Soil Science, Bangkok 14-21 August 2002
TRANSFORMATION OF ORGANIC MATTER IN AN OXISOL
Fadly H. Yusran1,2, Andrew W. Rate2, and Lynette K. Abbott2.
1Soil Department, Faculty of Agriculture, Lambung Mangkurat University, Banjarbaru 70714, Indonesia
2School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia
Abstract The application of various types of organic matter (OM) could assist in managing Oxisols. However, key questions relate to how long recently added OM persists in the soil and whether the addition of organic-C (OC) has any advantage in the long term. The main objectives of this study were to determine: (1) the persistence of soil organic matter in an Oxisol, (2) the effect of additional OC on the soil characteristics, especially chemical and biological aspects, and (3) the long-term effect of additional OC to the extractability of nutrients, especially N, P, and K, which may be affected by C dynamics in the soil. An Oxisol from Western Australia was treated with peat, wheat straw (Triticum aestivum), and lucerne hay (Medicago sativa), and incubated wet and dry (weekly watering, up to 60% water holding capacity) for 3, 6, 9, and 12 months in a greenhouse. The rates of OC sources were 40, 80, 120, and 160 ton ha-1. Soil microbial biomass C increased over time suggesting an increase in the population of soil micro-organisms, while soil respiration increased then showed a decrease after six months. The loss of OC as CO2 ranged from 0.4-5.5% of total-OC during a 12-month incubation time, and the higher OC applied, the more C loss as CO2. Net mineralisation of N decreased over time. There was also a decrease in P but not K mineralisation. Above all, the maximum mineralisation of N, P, and K occurred within the first three months of incubation. Almost all variables measured increased in the order control<peat<wheat<lucerne, except for P, for which wheat was less than peat. The effect of increasing the amount of OC increased the content of all variables measured in the soil. There was a delay in the increase in NO3-N during experiment and this was probably due to immobilisation by soil micro-organisms. Each source of OC added to the soil persisted for at least twelve months irrespective of the amount applied, and the addition of OC affected N, P, and K dynamics.
Oxisols, together with Ultisols, which are considered marginal for agricultural production, are very highly weathered soils, low in CEC, base saturation, and pH (Buol and Eswaran, 2000; West et al., 1998) These lead to the low availability of N, P, and K for plant growth. In making this soil more productive, inputs and proper management practices should be applied.
Soil organic matter, the key to soil fertility, is involved in many reactions in creating favourable conditions for the availability of nutrients due to its capacity to retain nutrients, water retention, and buffering capacity. It also promotes soil aggregation and improves biological characteristics of soils. The application of various sources of OM could be an important solution for managing Oxisols in farming systems. However, different sources of OM vary in their persistence in soil, so the addition some forms of OM could have an advantage in maintaining C-storage in the long term in Oxisols.
Many experiments have been carried out on the mineralization of OM in soil, especially with organic-C and N. These have determined the appropriate rate of the OM application such as manure, green manure (Dunn and Beecher, 1994; Lupwayi and Haque, 1998), straw (Mueller et al., 1998; Strong et al., 1987), and sewage sludge (Boucher et al., 1999; Iakimenko et al., 1996) on various soils. The appropriate source and rate of application of OM to Oxisols in Western Australia in relation to the its persistence and the availability of N and P has not been studied.
The objectives of this experiment were (1) to measure the persistence of OM added to soil, (2) to measure the contribution of additional OM on the characteristics of soil, especially chemical and biological aspects, and (3) to measure the long-term effect of additional OM to the availability of nutrient, especially N and P which affected by C dynamics in the soil in an Oxisol.
Materials and methods Soil samples (Typic hapludox; Balkuling) were taken from farmland near York. Peat Richgro Garden Products, Canning Vale, WA was air dried and passed through a sieve (≤ 2 mm). Lucerne as well as wheat straw was chopped with a miller (RetschMühle) and passed through a sieve (0.5 cm). For chemical characteristics, green manure and wheat straw were oven-dried (60° C) for two days, ground and passed through a 2 mm sieve.
One kg of soil was placed in each undrained pot and the appropriate treatment was applied by mixing the soil and the respective treatment. Deionized-water was added weekly during incubation (12 months) to restore a moisture equivalent of 60% of the maximum water holding capacity of the soil (42%).
Soil sample characteristics are presented in Table 1, and C source characteristics are presented in Table 2. The sandy and slightly acid soil had total-C content of 3%. Lucerne had a smaller C:N ratio than wheat straw and peat.
VIII. Appendices 246
All parameters were measured 3, 6, 9, 12 months after treatment application. Methods of determination of all parameters were as follows.
Microbial biomass carbon Microbial biomass carbon (MBC) was measured using a modification of the fumigation-extraction methods of Vance et al. (1987) and Sparling (1991). Fumigation was done by adding chloroform (CHCl2) as proposed by Jenkinson (1966).
Twenty g (equivalent dry weight) of soil was weighed into a 50mL beaker glass. The soil was watered up to 60% water holding. The water holding capacity of soil in each treatment was different due to differences in the form of OM added. The beakers were placed in a desiccator with one empty beaker in the middle. Twenty five mL CHCl2 was poured into the empty beakers together with some anti-bumping granules. A lining of damp paper towel was placed between the beakers to control the moisture level inside the desiccator. Vaseline was applied around dessicator lid, and the dessicator was vacuumed until CHCl2 was boiling. The dessicator was covered with black plastic around and allowed stand for exactly one week.
Fumigated soils were extracted with 80 mL 0.5 M K2SO4. The extraction was done by transferring 20 g of soil into 120 mL plastic container. The rinsing process was done by dispensing 40 mL extractant twice. The mixture was shaken for 30 minutes and filtered with Whatman #40 into a 30 mL plastic container and kept in freezer until further analysis.
Six-fold dilution of the extract was used to measure C content using Shimadzu TOC-5000A. Total organic carbon (TOC) was calculated by subtracting IC (inorganic carbon) from TC (total carbon). Maximum number of injection used was six with two minutes sparging time for TC and no sparging time for IC. The volume of extract injected to the machine was 26 µL for TC and 33 µL for IC. 0, 50, and 100ppm working standards were applied to construct a calibration curve. Stock solution for TC was made from 2.125 g potassium hydrogen phthalate in one L MilliQ water, while stock solution for IC was made from 3.50 g NaHCO3 and 4.41 NaCO3 in one L MilliQ water. NaCO3 was heated at 285°C for one hour and cooled in desiccator before weighting.
Soil respiration The method used for measuring soil respiration was based on Anderson (1982) with a slight modification. The method used alkali trapping, followed by an acid-base titration for remaining OH from CO3
2- formation. This product is formed when the CO2-laden gas contacts an alkali absorber. The amount of CO3 was measured by titrimetric using a known concentration of HCl.
Fifty g (dry weight) of soil was placed in a 120 mL plastic container. Deionised water was added to make moisture content exactly 40%. The container was placed in a moist plastic bag. The open end was wrapped with a tissue paper and
VIII. Appendices 247
a rubber band and left for one week at room temperature. The respiration jar (one L volume, air-tight sealed jar) containing five mL of DI water was prepared for incubating the soil samples. Ten mL 0.5 M KOH in a smaller plastic container was also put into the jar as a CO2 trap. Samples in the 120 mL container was arranged together with KOH container inside the jar and closed tightly. Incubation time was one week at room temperature. Blanks were also prepared for CO2 calculation.
After one week incubation, the KOH containers were removed from the jar and tightly closed with the lid. Four mL of the KOH solution was transferred to an erlenmeyer flask and mixed with 10mL BaCl2 and three drops of phenolphthalein indicator. Titration was employed against 0.1 M HCl until the pink colour disappeared. Total CO2 was calculated from the difference of HCl volume to neutralize the samples and the blanks.
Ammonium and nitrate Ammonium ions were measured by a modified Berthelot indophenols reaction that utilizes the Griess-Ilosvay reaction (Searle, 1984). Ammonia was chlorinated to monochloramine to form 5-aminosalicylate after reaction with salicylate. The absorbance of a green colour formed after oxidation was measured colorimetrically at 660 nm.
The NO3 measurement was based on the hydrazinium reduction method. The amount of NO2 was measured by diazotiting with sulphanilamide and coupling with α-naphthylethylenediamine dihydrochloride to form an azo dye measured at 540 nm.
The extract used was 2 M KCl. Five g moist soil was shaken with 50 mL extract in a 125 mL plastic container for one hour. Whatman #40 filter paper was used for filtration and the filtrates were kept at 4o C for up to four months prior to further analysis. The moisture content of the soils was assessed before the extraction process to determine the weight of soil samples. NH4-N and NO3-N were measured by a Skalar SANplus Segmented Flow Analyzer using a dual-channel system.
Inorganic phosphorus Inorganic-P was measured using 0.5 M NaHCO3 at pH 8.5. This is a modification from a method by Olsen et al. (1954) described by Rayment and Higginson (1992). The manual colorimetric procedure at the end of the method was based on that described by Murphy and Riley (1962).
Fifty mg dry soil and 50 mL 0.5 M NaHCO3 in plastic centrifuge containers were shaken end-over-end for 16 hours. Centrifusion and filtration with Whatman #42 filter paper was employed to ensure the mixture free from particulate OM. 2.5 mL aliquots of the filtrate were diluted to 10 mL with 5 mL DI water and 0.2 mL 1 M H2SO4. The mixture was leave for one hour to cease the effervescence.
VIII. Appendices 248
Another 0.5 mL 0.5 M NaHCO3 was added to the system and mixed well. The samples were left to stand overnight to complete the removal of excess CO2.
A day later, 0.8 mL of colour reagent (ascorbic acid in Reagent A (ammonium molybdate-sulphuric acid-potassium antimony tartrate)) was added and after 30 minutes the absorbance of the solution was measured with a spectrophotometer at 882 nm.
Extractable potassium Extractable K was assessed from the same extract as inorganic-P (0.5 M NaHCO3 pH 8.5) using atomic absorption spectrophotometer. A solution of 1% CsCl was used in all samples and standards to avoid ionisation effects.
Results and discussion Effect of time on organic carbon When OM is added to soil, it is colonised by micro-organisms that need C as their source of energy. This is supported by the increase in MBC content in the soil (Figure 1). The application of peat, but not wheat and lucerne, to the Oxisol increased soil MBC throughout the year. This prolonged increase could be due to the considerable resistance of peat C to microbial decomposition (Brake et al., 1999; Chesson, 1997; Handayanto et al., 1997), making it more slowly available to microorganisms than the other forms of OM.
Soil respiration declined with incubation time (Figure 2). The decomposition of OM from wheat and lucerne was greatest during the first three and six months of application. For lucerne, the most active period of decomposition might have taken place before the first measurement was taken in this experiment. In other studies with green manure (rice straw) and legumes, the highest peaks in respiration were reached after four to six weeks after application (Villegas-Pangga et al., 2000; Zaharah and Bah, 1999).
In order to estimate the persistence of OM in soil, the total soil respiration was compared to the total-C at the beginning of the experiment. The higher the amount of OM applied, the more CO2 produced from soils, with lucerne being the most productive (Table 3).
As hypothesised, lucerne, which had the smallest C:N ratio, disappeared at the fastest rate, followed by wheat and peat. The higher and balanced C and N content of green manure such as lucerne has a narrow C:N ratio and a faster rate of decomposition (Bremer et al., 1991; Curtin et al., 2000; Mapa, 1996). Both nutrients are required for the growth of soil microorganisms.
The highest CO2 evolution for higher level of OM occurred either three or six months after OM was added. The same pattern was observed on soil MBC (r = 0.80 and 0.88 for three and six months, respectively). For lucerne, the decrease in release of CO2 was steep (Figure 2) and C content estimated to be similar to the control several months after the end of the experiment.
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Extractability of N, P, and K Ammonium was formed within the first three months after OM adding, especially with lucerne application (Figure 3). The greatest ammonification process for OM low in C:N ratio may had been initiated as soon as OM was applied to the soil and continued until six months.
As OM decomposition enhances nutrient availability (Hartemink and O'Sullivan, 2001; Lupwayi and Haque, 1999; Maharudrappa et al., 2000; Tian, 1998; Villegas-Pangga et al., 2000; Zaharah and Bah, 1999), this process is beneficial for marginal soils such as Oxisols. Mineralisation of P and K showed the same trend for all OM applications. The extracts of both P and K were greatest at the three-month sampling time, and subsequently declined. Thus, the first three months of incubation time was the most active decomposition process for P and K as well as for NH4, especially for the higher levels of OM applied. There was little evidence of nitrification until 12 months, indicating that immobilisation of N in the microbial biomass was an important pool for storage of N.
The order of mineralisation was control<peat<wheat<lucerne for NO3-N and K, whilst for NH4-N and P the order was control<wheat<peat<lucerne. Peat was more effective than wheat in releasing NH4-N maybe because peat produced more amine functional groups (Landgraf et al., 1998). A possible explanation higher P release might be an effective chelating process of Al-P and Fe-P in soils from peat functional groups (Alvarez-Fernandez et al., 1997; Jonge et al., 1996).
Conclusions
Organic matter persistence in an Oxisol from Western Australia depended on its C:N ratio. Peat was expected to persist longer than lucerne and wheat straw. Organic matter application to this Oxisol needs to be made on a regular basis, with one- or two- yearly application for lucerne green manure and less regularly for peat.
Acknowledgement This research was supported by Australian Development Scholarship ADS-AusAID for Indonesia.
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Tables and Figures
Table 1 Some physical and chemical characteristics of soil samples from York
Figure 1 Soil microbial biomass carbon with OM addition at 160 ton ha-1. = control, = peat, = wheat, and = lucerne.
0
100
200
300
400
500
3 6 9 1
month
mg kg -1
= LSD 5%
2
Figure 2 Soil respiration with OM addition at 160 ton ha-1. = control, = peat, = wheat, and = lucerne.
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0
250
500
750
3 6 9 12
mg kg-1
months
Figure 3 Concentrations of NH4-N (bars) and NO3-N (line) for lucerne application (80 ton ha-1).
255VIII. Appendices
Appendix C Pictures from some experiments
Figure 8-1 Benches and pots in arrangement for the first experiment.
Figure 8-2 Pottery kiln to combust (450° C) inherent soil organic matter in the second experiment.
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Figure 8-3 Half-burned soil samples.
Figure 8-4 Completely burned soil samples.
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Figure 8-5 Pot arrangement for the second experiment.
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Figure 8-6 Control for Oxisol (ONC) and Oxisol with peat (ONP) treatments for combusted soil samples.
Figure 8-7 Leachate containers under the leaching columns for the third experiment.
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Figure 8-8 The bench and the overall view of leaching column. Wood pole clear plastic tube on the right was the indicator for suction mechanism (10 cm water).
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Figure 8-9 The water dripper. The blue filter at the bottom was a 45 µm millipore filter (25 mm, supor membrane, non sterile, Pall Gelman Laboratory).
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Figure 8-10 Suction apparatus for the columns. Each plastic tube represents one leachate container under one leaching column.