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Fadly H. Yusran
AgroscientiaeISSN 0854- 233360
THE ROLE OF ORGANIC CARBON IN PHOSPHORUS AVAILABILITYOF
LATERITIC SOILS
PERANAN KARBON ORGAN IK DALAM KETERSEDIAA N FOSFORPADA TANAH
LATERITIK
Fadly Hairannoor YusranSoil Department, Faculty of Agriculture,
Lambung Mangkurat University,
Jl. Jend. A. Yani Km.36 PO Box 1028 Banjarbaru 70714e - mail:
[email protected]
ABSTRACT
The loss of organic - C has been neglected; especially in
lateritic soils which are usually exist in very highrainfall
tropical areas. As a highly influential and mobile substance, its
occurrence is very important in bio-availability of nutrients. Not
only in the decomposition process of soil organic matter - which is
the mainprocess of nutrient availability- , but also in sorption-
desorption process of P in solid- liquid phases of soils.
Inaddition, ligand exchange is another process which occurs due to
the existence of organic- C in Ptransformat ion. Another important
factor is the role of soil micro- organisms in mediation of P
dynamics in thesoil. In this role, soluble P immobilisation and its
mineralisation due to phosphatase enzyme are anotherimportant
biochemical processes related to organic - C. Therefore,
investigation to quantification of C and itsmobility in soils,
either horizontally or vertically, is really crucial in lateritic
soils, especially when thesemarginal soils are going to convert to
agricultural land.Key words: Lateritic soils, organic - C, organic
matter persistence, P mobility and availability.
ABSTRAK
Hilangnya C - organik selama ini tidak banyak mendapat
perhatian, terutama pada tanah- tanah lateritik yangbanyak terdapat
di wilayah tropika. Sebagai substansi dengan mobilitas tinggi dan
sangat berpengaruh,keberadaaanya menjadi sangat penting dalam
siklus ketersediaan unsur hara. Tidak hanya dalam prosesdekomposisi
bahan organik yang merupakan proses utama ketersediaan hara, tapi
juga dalam prosessorpsi- desorpsi pada fase padat dan fase cair
(larutan) tanah. Demikian pula halnya pada prosespertukaran ligan
dalam transformasi P. Faktor lainnya adalah peranan mikro-
organisma yang menjadimediator dinamika P. Pada mekanisme ini,
immobilisasi P dan mineralisasinya dengan enzim fosfatase
jugamempunyai pengaruh dalam reaksi biokimia C- organik. Oleh
karena ini, penelitian tentang C- organik danmobilitasnya, baik
secara horizontal maupun vertikal, sangat penting dilakukan di
tanah- tanah lateritik.Apalagi mengingat banyakn ya lahan marginal
ini dipilih sebagai lahan pertanian alternatif di masa depan.Kata
kunci: Tanah laterik, C - organik, bahan organic, mobilitas dan
ketersediaan P.
INTRODUCTIONThe loss of soil organic matter in agricultural
lands, usually by erosion and/or rapid mineralisation,is often
considered to be the most serious factor incausing soil degradation
(Craswell and Lefroy, 2001;Katyal et al. , 2001; Paustian et al.,
1997). T his losscan have detrimental effects on soil
physical,chemical, and biological properties. Indeed,maintaining
and improving soil organic mattercontent is generally accepted as
being an importantobjective of any sustainable system of
agriculture.In most developing countries, extending the area
ofagricultural land is a major problem becauseagricultural areas
are under the pressure of highrates of population growth and the
expansion ofurban areas into productive agricultural soils.
Consequently, s oils with low fertility tend to be themost
likely alternative for expanding agriculturaldevelopment. However,
in many developingcountries, the knowledge required for
exploitingthese soils is far behind that of existing
agriculturalsoils.
Lateritic so ils such as Ultisols and Oxisols, whichare commonly
low in soil organic matter, are acidic,have limited cation exchange
capacity, and have lownutrient status could become more suitable
for foodcrops if levels of soil organic matter are raised.
Thiscould be achieved by amending soil with varioussources of
organic matter. More specifically, thetransformation of soil
organic carbon during thedecomposition of organic matter allows
soil toprovide nutrients for plants, especially P. In thisrespect,
s oil organic carbon (C) has been known to
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The Role of Organic Carbon in & &
Agroscientiae Volume 19 Nomor 1 April 2012 61
influence phosphate adsorption (Brennan et al.,1994; Erich et
al., 2002; Leytem et al., 2002) and tobe positively correlated with
phosphatase activity(Baligar et al., 1999).
Processes involved in interactions b etween soilsolutes and the
soil solution such as ligandexchange, sorption, and desorption, may
beconsidered as being directly involved in Ptransformation in
association with organic matter.Some soil micro- organisms play an
important role inmediating P dynamics in soils, especially where
Pinput from fertilisers is limited (Beck and Sanchez,1994; Yao et
al., 2002). Immobilisation of solublephosphate and the promotion of
P mineralisation byproduction of phosphatase are among
thebiochemical processes involving soil organic matterwhich affect
P transformation.
In forest soils, C leaching contributes between 6-46% of total-
C loss as DOC (Cronan, 1985; Magilland Aber 2000). Investigations
of C balance havefocused on the effect of temperature on
Cmineralisation (Liechty et al., 1995; Tate et al., 1993;Zogg et
al., 1997), which is only effective in surfacesoil (MacDonald et
al., 1999). The leaching of DOCfrom soil may return to the
atmosphere as CO
2 lossfrom streams, lakes, or oceans (Kling et al., 1991).For
soils in the tropics, the balance between upward(respiration) and
downward (DOC leaching) losscould be important, especially for
lateritic soils if theeffectiveness of SOM application is to
beunderstood. Hence, heavy rainfall may be anadditional factor in
increasing OC loss from soil, notonly in erosion but also in
infiltration of water throughthe soil profile.
Several studies have concluded that the C lossas dissolved
organic - C via leaching could reach 50% of the total C loss from s
oil (Cronan, 1985; Magilland Aber , 2000). However, these studies
mainlyconcern C loss at the soil surface. Investigation of
Cleaching within the soil profile is crucial, especially inareas
with very high annual rainfall. Carbon transferdue to leaching
through the soil profile is importantbecause soluble organic - C
may affect soil chemicalproperties such as sesquioxide
concentrations. Forlateritic soils in tropical areas, heavy
rainfall not onlycauses nutrient leaching (Duwi g et al ., 2000 ;
Less aand Anderson, 1996) but also removes organicsubstances
(Haberhauer et al., 2002) which mayaffect sorption and desorption
of nutrients such asphosphate.
Soil organic matter can change the phosphatefixing capacity of
some soils (Eric h et al., 2002;Iyamuremye and Dick , 1996; Kwabiah
et al., 2003;Ohno and Crannel, 1996). Several mechanismshave been
proposed to explain how soil organicmatter affects phosphate
adsorption, either due tobiotic or abiotic processes (Iyameremye
and Dick,1996). In biotic processes, soil organic matter
affects P mineralisation and transformation(Frossar d et al.,
2000; Magid et al ., 1996), andabiotic processes affect P dynamics
via mechanismssuch as organic ligand exchange (Hinsinger ,
2001;Violante and Gianfreda, 1993), dissolution (MacKa yet al.,
1986; Traina et al . , 1986), and desorption(Burkitt et al., 2002 ;
Rhue and Harris , 1999).
Organic - C leaching may lead to loss of appliedorganic matter
and, at the same time, may affect Pdynamics throughout the soil
profile. Interactionsbetween soil solutes and the soil solution
where theprocess of P sorption and desorption occur need tobe
investigated, especially when lateritic soils areused being used.
This is not only because of higherrainfall in the tropics where
lateritic soils arecommon, but also due to high phosphate
sorbingcapacity of the soils. Any novel information withinthe frame
of limiting factors of these soils is neededto be repeatedly
investigated for the final results inthe applied knowledge
according to the specificareas.
LATERITIC SOILSLateritic soils can be low in phosphorus (P)
availability for plant growth due to their high contentof
aluminium (Al) and iron (Fe) - oxides which areable to adsorb
phosphate from added fertilisers(Buol and Eswaran, 2000). Soil
organic matter candecrease the affinity of Al and Fe- oxides
forphosphate and provide biochemical conditionssuited for making P
more soluble (Dubus andBecquer, 2001; Haynes and Mokolobate,
2001;Maguire and Sims , 2002; Yusran, 2010a). However,the
persistence of organic matter in soil is animportant issue as
artificial sources of organic matterof agricultural origin often
decompose rapidly,especially in tropical areas (Chuyong et al.,
2000;Silva and Cook, 20 03). Furthermore, theeffectiveness of newly
applied organic matter inalleviating P deficiency is limited, as
earlydecomposition processes are not necessarilyfavourable for P to
be mineralised or transformedfrom organic - P to inorganic - P
(Iyamuremye andDick, 1996).
Lateritic soils are very common in tropical regions(Eswaran,
1993). Those in high rainfall areas arehighly weathered with
inorganic colloids beingmainly kaolinitic with significant amounts
of(hydr)oxides, especially those of iron (Fe) a ndaluminium (Al)
(Buol and Eswaran, 2000; Eswaran,1993; West et al., 1998). Because
of the highambient temperatures throughout the year and
theabundance of water, the turnover of soil organicmatter is rapid
(Rezende et al., 1999; Silva andCook, 2003; Silver, 1998), with a
half life of 9- 33days (Rezende et al ., 1999). However, soil
organicmatter associated with inorganic colloids in these
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Fadly H. Yusran
AgroscientiaeISSN 0854- 233362
soil environments appears to have a considerableturnover time in
soil environments, ranging from 14-275 years (Monreal et al .,
1997). This kind of soilorganic matter may have an important role
in thereactivities of the soil colloidal component and in
soilfertility, especially in lateritic soils in the tropics.
Ultisols are part of a group of soils with an argillicand/or
kandic horizon that have developed in ahumid climate (West et al.,
1998). Importantfeatures of these soils are: (1) the parent
materialcontains minerals which weather to form silicateclays, and
(2) the climate during soil developmentchar acteristically has more
precipitation thanevapotranspiration. Ultisols are common in
tropicaland subtropical areas between 40 N and 40 S(Eswaran,
1993).
The main characteristic of Ultisols thatdifferentiate them from
other soils is that they musthave 35 % or less base saturation in
the lower partof the subsoil (Soil Survey Staff, 1999).
Thisdefining characteristic is related to other propertiessuch as
low pH, low cation exchange capacity, andhigh Al saturation,
resulting in negative effects of theability of these soils to
sustain agricultural plantgrowth. Therefore, chemical properties
are the mostcommonly discussed limiting factors in managingUltisols
for plant production.
With regards to cation exchange capacity,Ultisols have permanent
negative charges fromisomorphic substitution of cations within the
clay, butthey also have variable charges (Barnhisel andBertsch,
1989; Wada and Wada, 1985), which arepositive at low pH.
Consequently, most Ultisols areexpected to have very limited net
negative chargeand this influences on nutrient retention,
especiallyfor cations.
Oxisols are characterised by the existence of oxichorizons which
usually have a minimum of 15% clay(Buol and Eswaran, 2000; El
Swaify, 1980).Physical properties of Oxisols are determined bytheir
sesquioxides and kaolinite mineralogy. Thefine and very fine
granular structure is very porousand leads to low bulk density
which is generallybetween 1.0- 1.3 Mg m - 3 (El Swaify, 1980).
Oxicmaterial with high oxide content is generally notsticky, and
can be hydrophobic to some extent.Water moves rapidly through the
large poresbetween aggregates. The combination of highporosity
(Tejedor et al., 2003) and low wettability(Scott, 2000) can make
these soils susceptible toerosion (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 andRichter, 2002). Cation exchange capacity for
thesesoils arises from kaolinitic clays and organic matter,it is pH
dependent, and effective cation exchangecapacity values are less
than cation exchange
capacity at pH 7. Oxisols with substantial content ofFe- oxides
have a high fixation capacity forphosphate applied from fertilizers
(Haynes andMokolobate, 2001; Leytem et al., 2002). Oxisolsalso have
low quantities of essential elements forplant growth (Melgar et
al., 1992; Moraghan andMascagni, 1991). As a consequence, Oxisols
andUltisols are usually the next optio ns in thereclamation program
for agricultural development formany countries in tropical regions.
Managing thesesoils requires comprehensive knowledge in order
togain profits from their management, not only foragricultural
products but also for sustainability of thesoil resource.
SOIL ORGANIC MATTERThe term soil organic matter has been
widely
used to describe the organic components in soils.The initial
concept of soil organic matter refers to thewhole of the organic
material in soils, including litter,light fraction, microbial
biomass, water solubleorganics, and stabilised organic matter
(humus)(Baldock and Nelson, 1998; Stevenson, 1994). Soilorganic
matter is defined as the total of all organicmaterials contained
within and on soils (Baldock andNelson, 1998), as well as non-
decayed plant andanimal tissues, their partial decomposition
productsand the living soil biomass (MacCarthy et al., 1990).
Soil organic matter investigations, have beenestablished for
decades. The importance of soilorganic matter as a source and sink
of C has alsobeen known (Lal, 1999; Lal, 2001). Considerablerecent
research on soil organic matter has beenfocused 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 andpedogenesis
(Jansen et al., 2003; Kaiser and Zech,1998), and the availability
of dissolved organic - C tosoil micro-flora (Kalbitz et al., 2003;
Yano et al .,2000). More over, knowledge of dissolved
organicmatter, as well as dissolved organic nitrogen (N), iswell
documented (McDowell, 2003). However, therelationship between these
processes and soilphosphorus (P) is less well understood,
especiallydissolved organic - P. In agricultural soils
wherefertiliser input was regular, dissolved organic - P wasabout
70% of total- P in solution (Chardon et al.,1997), and dissolved
organic matter was responsiblefor redistribution and loss of P in
forest soils (Donaldet al., 1993). Dissolved organic - C is in
close contactwith soil particles that adsorb phosphate so it
isexpected that this pool is important in controlling Pdynamics in
soil solid and soil solution interactions.
In lateritic soils, soil organic matter is a majorcontributor to
the soil exchange capacity (Buol andEswaran, 2000; Pushparajah,
1998; West et al.,
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The Role of Organic Carbon in & &
Agroscientiae Volume 19 Nomor 1 April 2012 63
1998). Since the clays in lateritic soils are mainlykaolinitic
(Allen an d Fanning, 1983; Wes t et al.,1998), soil organic matter
is a major contributor tothe negatively charged colloids and plays
animportant role in soil chemical (Pushparajah, 1998;Schnitzer ,
2000) and biological properties (Marinar iet al., 2000). However,
the content is generally low(Buol and Eswaran, 2000; Pushparajah,
1998; Westet al., 1998). Critical factors include: climate,
wherewarmer temperature and high rainfall create the fastbreakdown
of organic residues, and erosion wheresoil organ ic matter is lost
(West et al., 1998).Erosion may also lead to leaching of some forms
oforganic matter. In this case, there must be anexcess of
precipitation in relation to the capacity ofthe soil to retain
water, so that water will alsopercolates through the solum (Miller,
1983). As aconsequence, highly weathered soils commonlyoccur in
warm areas from the humid tropics to humidwarm temperate
climates.
The application of organic matter to soil toimprove soil
physical, chemical, and biologicalproperties (Anikwe and Nwobodo,
2002; Khalilian etal., 2002; Larbi et al., 2002) has been a
practicesince prehistoric time (Kleber et al., 2003). In thepast,
the types of organic matter applied weregenerally manure, green
manure, compost, cropresidues , and to some extent sewage sludge
andbiosolids. Today, the term organic amendment hasbecome broader
in meaning and includes materialsfrom various terrestrial and
marine sources such asfish bone meal and crab meal. The use of
organicamendments is increasing with the development oforganic
farming and the increase is even higheramong conventional farmers
(Hart z et al., 2000).This is not only because of social pressure
forhealthy food under conditions that protect theenvironment, but
also as a result of pressure forrecycling organic resources
(Thuries et al ., 2001).
Among physical properties, organic matterenhances soil particle
aggregation for better waterpermeability and gas exchange (Poch et
al., 2000).Organic matter increases water retention bypreventing
shrinking and drying (Hajnos et al ., 2003;Stehouwer, 2003). The
black colour of organicmatter may facilitate soil warming in
temperateregions by balancing the radiative heat (Schmidt andNoack,
2000). In relation to chemical character -ristics, organic matter
increases cation exchangecapacity and buffering capacity to
minimise changesin solute concentrations and pH. Soil organic
mattercan also adsorb and buffer trace soil components(Barancikova
et al., 2004; Burt et al., 2003; Minkinaet al., 2000). In addition,
an improved biologicalenvironment in soil results from organic
matteraddition increasing microbial activity, leading
tomineralisation and enhanced availability of nutrientssuch us N,
P, potassium (K), and sulphur (S) for
plant growth (Fortuna et al., 2003; Krishna, 2002;Williamson and
Wardle, 2003).
Organic matter affects nutrient availability forplants directly
and indirectly (Stevenson, 1994).Organic matter is a source of N
for plants whenmineralised (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 duringmineralisation, and the rate
of release, depend onthe content of the element and elemental
ratios inthe biomass, which reflects the origin of the
organicmatter. Indirectly, organic matter contributes to themineral
nutrition of plants in soils throughincorporation of N and S into
humic substancesduring decomposition, or by complexation of
calcium(Ca), Al, and Fe from their respective phosphates byhumic
substances to increase concentrations ofsoluble phosphate
(Stevenson, 1994).
Incorporation of N (Kelley and Stevenson, 1995)and S (Brown,
1986; Xia et al., 1998) int o humicsubstances, as well as P (Cooper
et al., 1998; Singhand Amberger, 1990) keeps the nutrients
fromvolatilising (except P) and leaching and alsoprovides those
nutrients to plants for longer periodsof time. Furthermore, there
is a relationshipbet ween those nutrients within organic matter
whichhas been described as a definite ratio. The C:Nratio of
organic matter has been used as an indicatorfor the maturity of
compost (Contreras- Ramos et al.,2004; Priya and Garg, 2004) and
the stage of Csequestration in soils (Tan et al ., 2004).
Theaverage C/N/P/S ratio of 140:10:1.3:1.3 was claimedto be an
optimum for those nutrients to sustain plantgrowth (Stevenson,
1994).
Another key reason why organic matter canparticipate in a wide
range of chemical reactions insoils is due to the presence of
oxygen- containingfunctional groups (- CO
2H, - OH, C=O). Thesefunctional groups are capable of
enhancingdissolution of soil minerals by complexing anddissolving
metals, transporting them throughout thesoil solution, and making
them available for plantsand microbes (Schnitzer, 2000).
Interactionsbetween soil organic matter and metal ions includeion
exchange, surface adsorption, chelation,peptisation, and
coagulation reactions.
PHOSPHORUS CYCLE IN RELATI ON TO SOILORGANIC MATTER
Phosphorus is an essential nutrient for livingorganisms due to
its vital role in life processesincluding photosynthesis in green
plants andtransformation of energy in all forms of life (Sanyaland
De Datta, 1991). Compared with other essentialnutrients, P is by
far the least mobile and leastavailable to plants under most soil
conditions
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Fadly H. Yusran
AgroscientiaeISSN 0854- 233364
(Hinsinger, 2001). Therefore, P often becomes amajor limiting
factor for plant growth.
In the soil solution, P usually occurs at fairly
lowconcentration as ortho- phosphate or organicphosphate, while a
large proportion is more or lessstrongly held by soil minerals
(Frossard et al., 2000).Some phosphate ions can be adsorbed
toaluminosilicate clays and/or Fe and Al oxides.Phosphate ions can
also form a range of minerals incombination with metal cations such
as Ca2+ , Fe3+ ,and Al3+ . These sorption- desorption
andprecipitation- dissolution equilibria control
phosphateconcentration and the same time both the chemicalmobility
and bio- availability. According to Hinsinger(2001) the major
factors that determine thoseequilibria are: (1) soil pH, (2) the
concentration ofanions that compete with phosphate ions for
ligandexchange reactions. Including in these anions isphosphate ion
itself, and; (3) the concentration ofmetals (Ca, Fe, and Al) that
can precipitate withphosphate ions.
Other factors affecting phosphate availability arethe amount and
type of adsorbing phases, such asdominant clay mineral and various
oxides (Hue,1991; Wahba et al., 2002). Therefore, byconsidering
these factors, the effect of organicmatter on phosphate ions must
relate to the secondfactor due to organic ligands such as carboxyl.
Ingeneral, the effect of soil organic matter on the Pcycle is
related to the effect of biotic processes thatcontrol P release to
soil solution (Frossard et al.,2000). In this process, P turnover
from organicmatter plays an important role despite the fact
thatorganic matter also contributes via abiotic processessuch as
adsorption- desorption and dissolution-precipitation. In biotic
processes, soil organic matterplays the central role in
mineralisation andimmobilisation.
Considering phosphate rock as a non- renewableresource and the
availability of P is relatively low insoils, P supply to plant
growth must be rationalised.This is true especially for Oxisols and
Ultisols thathave Fe and Al- oxides which strongly adsorb
solublephosphate from fertilisers. To improve the efficiencyof P
supply in soils, it is imperative to maximise Precycling from crop
residues or even from organicand mineral fertilisers.
The P cycle is very dynamic and involves bothgeochemical and
biochemical reactions (Figure 1).The cycle of P is different from
that of C, N, and S.This is because P has no pathways to
atmosphericpools. The overall cycle ranges from solubilisationand
fixation at clays and oxide surfaces in the soilsolution to
mineralisation- immobilisation processesmediated by micro-
organisms. The roles of soilorganic m atter and soil micro-
organisms are verysignificant in the P cycle. Microbial activity is
anagent that functions as a reversible sink for P,
continuously consuming and releasing P to the soilsolution
(Stewart, 1981).
In highly weathered soils, which are commonlyacidic (Eswaran,
1993; West et al., 1998),phosphate is usually adsorbed to Al and Fe
oxidesor precipitated as insoluble Al- and Fe-
phosphates(Iyamuremye and Dick, 1996; Lindsay et al.,
1989;Stevenson and Cole, 1999; Yusran, 2005; Yusran,2010 b). Both
forms are poor sources of P for plantsand P deficiency is common in
soils rich in Fe andAl, such as Oxisols and Ultisols of the tropics
andsub tropics. The application of organic matter tothose soils may
complex Al and Fe, in either ionicform or as oxides. Furthermore,
in many soils, theavailability of P may depend more on the turnover
ofeasily decomposable soil organic matter than on therelease of
adsorbed phosphate.Dissolution and precipitation
The beginning of the P cycle involves parentmaterials, climate,
and time as factors affecting theexistence and amount of P in
soils. Dissolution of Pfrom its origin can usually be explained
asdissolution from apatite [Ca
10X 2(PO 4) 6, where X =OH - or F - ; Ca may also be substituted
with Na or Mg,and PO
4 with CO 3] which are the most commonprimary phosphate
minerals.
Precipitation of phosphate with Ca carbonatesand its adsorption
on Al and Fe hydrous oxides hasbeen known since the mid- nineteenth
century.Calcium phosphate is formed following phosphateadsorption
to calcite (Syers and Curtin, 1989). Asphosphate is adsorbed to the
surface of calcite,monocalcium phosphate [Ca(H
2PO 4) 2] precipitatesand transforms to become dicalcium
phosphatedihydrate (CaHPO
4.2H2 O), to octocalcium[Ca
8H 2(PO 4) 6.5H 2O] and finally to hydroxyapatite[Ca
10(PO 4) 6(OH) 2]. Adsorption of phosphate with Aland Fe oxides
resulted in the formation ofamorphous Al- phosphate and Fe-
phosphate whichmay later transform to variscite (AlPO
4.2H 2O) andstrengite (FePO
4.2H 2O) af ter prolonged aging(Lindsay et al., 1989). Phosphate
adsorption is notonly 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; Sanya l et al., 1993;
Uehara, Gillman, 1981).
Apatite dissolution requires a source of H+ , whichoriginates
from plant roots, micro- organisms, or fromthe 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 organicresidues are added to
soils. Dissolution of bothgroups of solid phases relies on organic
acids suchas citrate and formic acid (Traina et al ., 1986) as
thesource of H+ . The process is the replacement of
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The Role of Organic Carbon in & &
Agroscientiae Volume 19 Nomor 1 April 2012 65
phosphate sorbed on metal hydroxides (Fox et al.,1990).Sorption
and desorption
The term sorption is used to describe the surfaceaccumulation of
phosphate on soil componentswhich can be accompanied by penetration
ofadsorbed P by diffusion into the adsorbent body,resulting in
further adsorption of the adsorbedspecies (Sanyal and De Datta,
1991). These twoprocesses take place simultaneously. Desorption
isdefined as the adsorbed phosphate ions beingreleased as a
reciprocal action of sorption. Thesorption of phosphate ions has
been interpreted as abiphasic reaction (Rhue and Harris, 1999),
i.e. theinitial and rapid sorption which is believed to last inthe
order of minutes or hours. The second sorptionis slo w reaction
lasting weeks or months. There aretwo mechanisms responsible for
the process:1. Ion exchange, mechanism from the electrostatic
attraction of phosphate anions to positivelycharged sites exist
on variable- charge surfacesbelow the zero point char ge, and
2. Ligand exchange, mechanism by which aphosphate anion replaces
a surface hydroxyl thatis coordinated with a metal cation in a
solidphase (Rhue and Harris, 1999).The last mechanism, ligand
exchange, is also
referred as specific adsorption and is characterisedby: a)
adsorption is accompanied by the release ofOH - ; b) ligand
exchange shows a high degree ofspecificity; c) adsorption step
often occurs muchmore rapidly than the desorption step, leading
toapparent hysteresis in the isotherm, and; d)adsorption is
accompanied by an increase in surfacenegative charge (McBride,
1994).
The second sorption or the slow phase is thoughtto have two
mechanisms, i.e. diffusion (either intosoil particles or to surface
sites of limitedaccessibility), and pr ecipitation (either by
directheterogenous nucleation or following the dissolutionof the
host solid, following an initial adsorptionreaction) (Rhue and
Harris, 1999). Phosphorussorption and desorption are important
mechanismscontrolling soil phosphate partitioning between thesorbed
and solution phases (Burkitt et al., 2002) andhave crucial
implication for P management. Thismechanism is commonly referred to
phosphatebuffering capacity, which describes a soil s capacityto
moderate changes in phosphate solutionconcentration when P is added
or removed from thesoil (Ozanne, 1980).
In relation to soil organic matter, the release ofphosphate by
mineralisation may be difficult toseparate from the sorption
mechanisms, especiallyin soils with high sorbing capacity such as
lateriticsoils. This is not only because of the high content
ofsesquioxides and 1:1 clay content in these soils, butalso due to
negligible amounts of soil organic matter.However, by observing the
net release ofextractable- P and determining phosphate
adsorptionisotherms, the two processes can be separated asmore and
less important in releasing phosphates tothe soil solution.
Furthermore, as Afif et al . (1995)found that the effect of soluble
organic matter onphosphate release from Oxisols to be transient,
thisleads to the question of whether peat would have alonger - term
effect on P adsorption due to itsresistance to decomposition. At
the same time peatcan possibly slowly release soluble organic
ligandswhich compete for adsorption sites with phosphate.This also
could be determined by analysingphosphate adsorption isotherms.
Figure 1 . Soil phosphorus cycle, its components and measurable
fractions (adapted from Stewart and Sharpley (1987) .Arrows
represent fluxes between reservoirs.
The Role of Organic Carbon in & &
Agroscientiae Volume 19 Nomor 1 April 2012 65
phosphate sorbed on metal hydroxides (Fox et al.,1990).Sorption
and desorption
The term sorption is used to describe the surfaceaccumulation of
phosphate on soil componentswhich can be accompanied by penetration
ofadsorbed P by diffusion into the adsorbent body,resulting in
further adsorption of the adsorbedspecies (Sanyal and De Datta,
1991). These twoprocesses take place simultaneously. Desorption
isdefined as the adsorbed phosphate ions beingreleased as a
reciprocal action of sorption. Thesorption of phosphate ions has
been interpreted as abiphasic reaction (Rhue and Harris, 1999),
i.e. theinitial and rapid sorption which is believed to last inthe
order of minutes or hours. The second sorptionis slo w reaction
lasting weeks or months. There aretwo mechanisms responsible for
the process:1. Ion exchange, mechanism from the electrostatic
attraction of phosphate anions to positivelycharged sites exist
on variable- charge surfacesbelow the zero point char ge, and
2. Ligand exchange, mechanism by which aphosphate anion replaces
a surface hydroxyl thatis coordinated with a metal cation in a
solidphase (Rhue and Harris, 1999).The last mechanism, ligand
exchange, is also
referred as specific adsorption and is characterisedby: a)
adsorption is accompanied by the release ofOH - ; b) ligand
exchange shows a high degree ofspecificity; c) adsorption step
often occurs muchmore rapidly than the desorption step, leading
toapparent hysteresis in the isotherm, and; d)adsorption is
accompanied by an increase in surfacenegative charge (McBride,
1994).
The second sorption or the slow phase is thoughtto have two
mechanisms, i.e. diffusion (either intosoil particles or to surface
sites of limitedaccessibility), and pr ecipitation (either by
directheterogenous nucleation or following the dissolutionof the
host solid, following an initial adsorptionreaction) (Rhue and
Harris, 1999). Phosphorussorption and desorption are important
mechanismscontrolling soil phosphate partitioning between thesorbed
and solution phases (Burkitt et al., 2002) andhave crucial
implication for P management. Thismechanism is commonly referred to
phosphatebuffering capacity, which describes a soil s capacityto
moderate changes in phosphate solutionconcentration when P is added
or removed from thesoil (Ozanne, 1980).
In relation to soil organic matter, the release ofphosphate by
mineralisation may be difficult toseparate from the sorption
mechanisms, especiallyin soils with high sorbing capacity such as
lateriticsoils. This is not only because of the high content
ofsesquioxides and 1:1 clay content in these soils, butalso due to
negligible amounts of soil organic matter.However, by observing the
net release ofextractable- P and determining phosphate
adsorptionisotherms, the two processes can be separated asmore and
less important in releasing phosphates tothe soil solution.
Furthermore, as Afif et al . (1995)found that the effect of soluble
organic matter onphosphate release from Oxisols to be transient,
thisleads to the question of whether peat would have alonger - term
effect on P adsorption due to itsresistance to decomposition. At
the same time peatcan possibly slowly release soluble organic
ligandswhich compete for adsorption sites with phosphate.This also
could be determined by analysingphosphate adsorption isotherms.
Figure 1 . Soil phosphorus cycle, its components and measurable
fractions (adapted from Stewart and Sharpley (1987) .Arrows
represent fluxes between reservoirs.
The Role of Organic Carbon in & &
Agroscientiae Volume 19 Nomor 1 April 2012 65
phosphate sorbed on metal hydroxides (Fox et al.,1990).Sorption
and desorption
The term sorption is used to describe the surfaceaccumulation of
phosphate on soil componentswhich can be accompanied by penetration
ofadsorbed P by diffusion into the adsorbent body,resulting in
further adsorption of the adsorbedspecies (Sanyal and De Datta,
1991). These twoprocesses take place simultaneously. Desorption
isdefined as the adsorbed phosphate ions beingreleased as a
reciprocal action of sorption. Thesorption of phosphate ions has
been interpreted as abiphasic reaction (Rhue and Harris, 1999),
i.e. theinitial and rapid sorption which is believed to last inthe
order of minutes or hours. The second sorptionis slo w reaction
lasting weeks or months. There aretwo mechanisms responsible for
the process:1. Ion exchange, mechanism from the electrostatic
attraction of phosphate anions to positivelycharged sites exist
on variable- charge surfacesbelow the zero point char ge, and
2. Ligand exchange, mechanism by which aphosphate anion replaces
a surface hydroxyl thatis coordinated with a metal cation in a
solidphase (Rhue and Harris, 1999).The last mechanism, ligand
exchange, is also
referred as specific adsorption and is characterisedby: a)
adsorption is accompanied by the release ofOH - ; b) ligand
exchange shows a high degree ofspecificity; c) adsorption step
often occurs muchmore rapidly than the desorption step, leading
toapparent hysteresis in the isotherm, and; d)adsorption is
accompanied by an increase in surfacenegative charge (McBride,
1994).
The second sorption or the slow phase is thoughtto have two
mechanisms, i.e. diffusion (either intosoil particles or to surface
sites of limitedaccessibility), and pr ecipitation (either by
directheterogenous nucleation or following the dissolutionof the
host solid, following an initial adsorptionreaction) (Rhue and
Harris, 1999). Phosphorussorption and desorption are important
mechanismscontrolling soil phosphate partitioning between thesorbed
and solution phases (Burkitt et al., 2002) andhave crucial
implication for P management. Thismechanism is commonly referred to
phosphatebuffering capacity, which describes a soil s capacityto
moderate changes in phosphate solutionconcentration when P is added
or removed from thesoil (Ozanne, 1980).
In relation to soil organic matter, the release ofphosphate by
mineralisation may be difficult toseparate from the sorption
mechanisms, especiallyin soils with high sorbing capacity such as
lateriticsoils. This is not only because of the high content
ofsesquioxides and 1:1 clay content in these soils, butalso due to
negligible amounts of soil organic matter.However, by observing the
net release ofextractable- P and determining phosphate
adsorptionisotherms, the two processes can be separated asmore and
less important in releasing phosphates tothe soil solution.
Furthermore, as Afif et al . (1995)found that the effect of soluble
organic matter onphosphate release from Oxisols to be transient,
thisleads to the question of whether peat would have alonger - term
effect on P adsorption due to itsresistance to decomposition. At
the same time peatcan possibly slowly release soluble organic
ligandswhich compete for adsorption sites with phosphate.This also
could be determined by analysingphosphate adsorption isotherms.
Figure 1 . Soil phosphorus cycle, its components and measurable
fractions (adapted from Stewart and Sharpley (1987) .Arrows
represent fluxes between reservoirs.
-
Fadly H. Yusran
AgroscientiaeISSN 0854- 233366
CONCLUSIONSThere are two main processes are governing P
transformation, i.e. biotic and abiotic processes.The biotic
process is mainly responsible formineralisation and immobilisation
of P. Thisprocesses involve the role of a wide range of soilmicro-
organisms. While abiotic process isresponsible for maintaining the
equilibrium ofsorption- desorption with precipitation
andsolubilisation mechanisms. In this process, either inrapid or
slow sorption, ion and ligand exchanges areresponsible with their
typical characteristics inrelation to other available factors in
lateritic soils.Above all, organic - C role in P availability in
lateriticsoils is diverse, ranging from very rapid to very
slowreactions which taking hundred years in the process.All
processes have many implications inmanagement of the soils.
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