AGGREGATE STABILITY IN RELATION TO ORGANIC CARBON CONTENT OF EIGHT DIFFERENT SOILS OF BANGLADESH A dissertation for the partial fulfillment of the requirements for the Degree of four years Bachelor of Science (Hon’s) in Soil Science A Project thesis by Sharif Sinthia Islam Roll No: 061331 Session: 2006-2007 Supervised by Md. Sadiqul Amin Assistant Professor Soil Science Discipline Khulna University Chairman of Examination Committee Afroza Begum Associate Professor and Head Soil Science Discipline Khulna University Khulna-9208, Bangladesh Soil Science Discipline Life Science School, Khulna University Khulna, Bangladesh. July, 2010
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AGGREGATE STABILITY IN RELATION TO ORGANIC CARBONCONTENT OF EIGHT DIFFERENT SOILS OF BANGLADESH
A dissertation for the partial fulfillment of the requirements for theDegree of four years Bachelor of Science (Hon’s) in Soil Science
A Project thesis bySharif Sinthia Islam
Roll No: 061331Session: 2006-2007
Supervised byMd. Sadiqul AminAssistant Professor
Soil Science Discipline Khulna University
Chairman of Examination CommitteeAfroza Begum
Associate Professor and HeadSoil Science Discipline
Khulna UniversityKhulna-9208, Bangladesh
Soil Science DisciplineLife Science School, Khulna University
Khulna, Bangladesh.July, 2010
CHAPTER 1: INTRODUCTION
CHAPTER 2: LITERATURE REVIEW 3
2.1. Aggregate stability 32.2. Forces involved in aggregation 32.3. Correlation of aggregate stability with potential causal factors 4
capacity, diversity and activity of soil organisms, both those that are beneficial and harmful to
crop production and nutrient availability (Fig:2.3). It also influences the effects of chemical
amendments, fertilizers, pesticides and herbicides. The interrelationship between SOC and soil
structure and other physical properties has been extensively studied, and excellent reviews can
be found in Tisdall and Oades (1982). It is well established that addition of SOM can not only
reduce bulk density (Db) and increase water holding capacity, but also effectively increase soil
aggregate stability. Angers and Carter (1996) noted that the amount of water-stable aggregates
(WSA) was often associated with SOC content, and that particularly labile carbon was often
positively related to macro-aggregate stability. Kay and Angers (1999) reported that a minimum
of 2% SOC was necessary to maintain structural stability and observed that if SOC content was
between 1.2-1.5%, stability declined rapidly. Haynes (2000) found that the mean weight
diameter (MWD) of aggregates exhibited a curvilinear increase with carbon content, suggesting
an upper limit of influence of SOC (Fig: 2.2). Carter (1992) found that maximum levels for an
agronomically designed aggregation index (AI) were obtained at SOC contents of > 2.5% and at
microbial biomass carbon contents of 250µg C/g soil, whereas maximum soil structural stability
(determined by MWD) was found at SOC levels of 4.5%. Carter (1992) suggested that 2.5% could
serve as a critical limit to define minimum concentrations of SOC required to provide optimum
structural stability in fine sandy loams.
Figure 2.2: Effect of increasing SOC content on aggregate stability, measured by wet-sieving (MWD, mm), using air-dried (●) and field moist (○) samples (modified after Haynes, 2000).
Boix-Fayos et al. (2001) showed that a threshold of 3-3.5% SOC had to be attained to achieve
increases in aggregate stability, no effects on aggregate stability were observed in soils below
this threshold. Fine-textured soils sequestered more macroaggregate- protected SOC near the
soil surface than coarse textured soils, due largely to greater macroaggregation (Franzluebbers
and Arshad, 1996). The stability of larger macro aggregates, in particular, is largely a function of
active soil organic matter fractions (Tisdall, 1996). These fractions have high turnover rates and
Functions of Soil organic matter
Biological functions-Provides source of energy (essential for biological process)-Provides reservoir of nutrients (N, P, S)-Contributes to resilience of soil /plant system
-contributes to resilience of soil /plant system
Physical functions-Improves structural stability of soils at various scales-Influence water retention properties of soils and thus water –holding capacity-Alter soil thermal properties
Chemical functions-Contribute to the cation exchange capacity-Enhances ability of soils to buffer changes in pH - Reduce concentrations of toxic cations - Promote binding of SOM to soil minerals.
are sensitive to management (Wander et al., 1994). Carter (1992) found that maximum levels for
an agronomically designed aggregation index (AI) were obtained at SOC contents of > 2.5% and
at microbial biomass carbon contents of 250µg C/g soil, whereas maximum soil structural
stability (determined by MWD) was found at SOC levels of 4.5%. Carter (1992) suggested that
2.5% could serve as a critical limit to define minimum concentrations of SOC required to provide
optimum structural stability in fine sandy loams.
Fig 2.3: Functions ascribed to SOM note that interaction occurs between the different soil functions modified from
Badlock and Skjemstad, 1999.
2.5 Stabilization and destabilization of Soil organic matter
Stabilization of a portion of the litter C yields material that resists further transformation.
Stability of the organic C is viewed as resulting from three general sets of characteristics. It is
widely assumed, for example, that fresh plant detritus is converted gradually to more stable
forms (sometimes termed “humus”), and that this stabilization involves a variety of physical,
chemical, faunal, and microbial processes (Ladd et al.,1993).
Recalcitrance comprises molecular-level characteristics of organic substances, including
elemental composition, presence of functional groups, and molecular conformation, that
influence their degradation by microbes and enzymes.
Interactions refers to the inter-molecular interactions between organics and either inorganic
substances or other organic substances, that alter the rate of degradation of those organics or
synthesis of new organics.
Accessibility refers to the location of organic substances as it influences their access by microbes
and enzymes.
Stability is the integrated effect of recalcitrance, interactions, and accessibility. By definition, it
increases with recalcitrance and decreases with accessibility.
Destabilization yields material that is more susceptible to microbial respiration.
Destabilization is the overall process by which soil organic substances become less resistant to
degradation. By definition, it occurs by decreasing recalcitrance or by increasing accessibility.
Decreasing interactions may also promote destabilization.
2.6 Aggregate stability
Aggregate stability is often used as a measurement of soil structure. An aggregate is a group of
primary particles that cohere to each other more strongly than to other surrounding soil
particles. Most adjacent particles adhere to some degree. Stability of aggregates is a function of
whether the cohesive forces between particles withstand the applied disruptive force. However,
aggregate stability is often measured on a specific aggregate size class which is not a
measurement of whole soil structure. Furthermore, aggregate stability measurements are an
important parameter in determining the resistance of soil aggregates against environmental
factors (Hillel, 1982). Soil structure arises from the reciprocal arrangement and placement of
soil particles and aggregates (lal and shukla, 2004). It has a major influence on the ability of the
soil to receive, store, and transmit water and to favor C and nutrient cycles, and therefore, it
supports plant growth (kay, 1998).
2.6.1 Forces involved in aggregation
Two of the primary forces holding particles together in aggregates in moist soils are the surface
tension of the air and water interface and the cohesive tension (negative pressure) in the liquid
phase. Briggs (1950) has shown that the cohesive tension of water can have values up to 26MPa.
As a soil dries, the water phase recedes into capillary wedges surrounding particle-to-particle
contacts and films between closely adjacent platelets. The interfacial tension and internal
cohesive tension pull adjacent particles together with great force as soil dries. Soluble
compound such as silica, carbonates, and organic molecules are concentrated in the liquid phase
as soil dries. Many of these solute molecules and ions thus precipitate as inorganic
semicrystallaine compounds or amorphous organic compounds around these particle-to-
particle contacts, cementing them.
Soil aggregation may be determined by the mean weight diameter (MWD), the geometric mean
diameter (GMD) and the normalized stability index (NSI), which is obtained by breaking the soil
into aggregate classes by the wet sieving method (Kemper and Chepil, 1965).
2.6.2 Correlation of aggregate stability with potential causal factors
2.6.2.1 Soil texture
The more clay present in the soil, the more likely the soil is to form aggregates (clays carry an
electric charge and can stick together). However, the clay is also the part of the soil that
disperses if aggregate stability is poor. In generally considered that as the silt (0.002-0.05mm)
or silt + very fine sand (0.05-0.10 mm) fraction increases and clay decreases in consequences
aggregate stability decreases erodibility increases. This is because of
1) The aggregation and bonding effect of clay
2) The transportability of fine and non aggregated particles (i.e. silt)
3) The detachability of sand and silt
According to Edwards and Bremner (1967), the only highly stable aggregates are fine sand- and
silt-sized microaggregates (<250_m) consisting of clay–polyvalent metal–organic matter
complexes. Microaggregates are formed by bonding of C–P–OM clay sized units, where C clay
particle, P polyvalent metal (Fe, Al, Ca) and OM organo-metal complex, and are represented as
[(C–P–OM). It is evident that the C–P–OM units are equivalent to the clay domains of Emerson.
However, Edwards and Bremner (1967) envisioned C–P–C and OM–P–OM units too. They also
postulated that the organic matter complexed into the microaggregates would be inaccessible to
microorganisms and physically protected. In the aggregate hierarchy concept it is postulated
that the different binding agents (i.e. transient versus temporary versus persistent binding
agents) act at different hierarchical stages of aggregation. Free primary particles and silt-sized
aggregates (<20_m) are bound together into microaggregates (20–250_m) by persistent binding
agents (i.e. humified organic matter and polyvalent metal cation complexes), oxides and highly
disordered aluminosilicates.
2.6.2.2 Organic carbon
Aggregation generally increases with increasing soil organic matter, which is connected to clay
surfaces through positively charged polyvalent cat ions, thus overcoming the repulsion between
the negative charges of both clay and organic matter (Edwards and Bremner, 1967). In this
process, not only the amount of clay but also the clay type is important (Kay, 1998), because
mineralogical species differ in surface charge. Isomorphic substitutions, resulting in negative
permanent charges, prevail in layer silicates, whereas pH-dependent charges form on surface
hydroxyls and dominate in the case of oxides and hydroxides. The latter clays are present in the
soil with a wide range of crystallinity, and their effect on aggregation varies. Amorphous iron
oxides (Feo) are the most effective because they not only carry positive charges but also block
the negatively charged sites on layer silicates (Shao and Wang, 1991). The soil colloidal fraction
is thus characterized by a total charge, resulting from the complex interactions among all the
components. Igwe and Nwokocha (2006) investigated the role of SOC in the restoration of soil
fertility and stability of soil micro-aggregates, which is of special importance in soils that
degrade rapidly. They reported that soils were coarse-textured, deep and low in soil nutrients
and SOC, probably due to high mineralization rates. Micro-aggregate-associated SOC was also
low with most of the SOC protected by the <63 mm fractions. Principal component analysis
revealed that SOC fractions associated with 2000—63 mm aggregate sizes were the SOC
fractions that best explained the variance in aggregated silt + clay, indicating their contribution
to microaggregate stability. This was attributed to the production of polysaccharides and
materials released by microbial activities from this recently deposited or incompletely
decomposed SOC.
2.6.2.3 Chemical dispersing agent
The di-valent and trivalent cations, such as Ca2+ and Al3+, are tightly adsorbed and can effectively
neutralize the negative surface charge on clay particles; these cations can also form bridges that
bring clay particles close together. Monovalent ions, especially Na+, with relatively large
hydrated raddi, can cause clay particle to repel and each other and create a dispersed condition.
Two things contribute to the dispersion
1) The large hydrated sodium ion does not get close enough to the clay to effectively
neutralize the negative charges, and
2) The single charge on sodium is not effective in forming a bridge between clay particles.
Calcium ions associated with clay generally promote aggregation, whereas sodium ions promote
dispersion. Exchangeable sodium - can cause very poor aggregate stability. Soils with a high
percentage of exchangeable sodium are very likely to disperse and need to be managed
carefully.
Fe-oxide rich soils (e.g., many Oxisols) and allophanic soils are among the most stably micro-
aggregated (El-Swaify, 1980;). This is generally interpreted as evidence that oxides and
hydroxides of Al and Fe, as well as amorphous aluminosilicates, are important in aggregation.
Evidence for the importance of various materials as binding agents comes from studies of
disaggregation upon exposure to chemical extractants (e.g., Bartoli and Philippy, 1990;
Wierzchos et al., 1992) correlation of aggregate stability with soil properties (Molope et al.,
1985), and addition of binding agents to soil.
2.6.2.4 Clay mineralogy
Clay mineralogy influences aggregate stability but the effect is difficult to asses because soils
most often contain a mixture of clay minerals. Using pure clay minerals Emerson(1964) showed
that swelling clays like montmorillonite are less subject to slaking than kaolinite or illite because
the pressure which is developed by entrapped air is released by swelling ; however ,fissuring of
montmorillonite may occur, due to the combination of stress of entrapped air and swelling of
aggregates. As aggregating particles, the smectiteic clays should be more efficient than other
clays because of their large specific surface and high physiochemical interaction capacity .In
accordance with this statement, Young and Mutchler (1977) found that montmorillonite was
highly correlated with aggregate stability. The effects of clay and organic-matter content can be
seen in Fig.2. The soil highest in clay content had the highest aggregate stability at all water
contents and constrainment levels. Mostaghimi et al. (1988) predicted that aggregate stability
would increase with clay content. More clay implies more or stronger clay bridges between soil
particles. This suspected high degree of bridging was apparently little affected by water content
or constrainment in (Fig.2.4)
Fig. 2.4.Aggregate stability as a function of water content for both constrained and unconstrained samples of each soil.
2.6.2.5 Porosity
Elliott and Coleman (1988) adopted the concept of microaggregate formation within
macroaggregates from Oades (1984) and ascribed this microaggregate formation to the
anaerobic and resulting reducing conditions in the center of the macroaggregates. They also
described, as a mirror image of the aggregate hierarchy, four hierarchical pore categories in (Fig:
2.5)
(1) Macropores;
(2) Pore space between macroaggregates;
(3) Pores between microaggregates but within macroaggregates; and
(4) Pores within micro aggregates.
The concept of aggregation as a process involving different organic binding agents at different
scales was pioneered by Tisdall and Oades (1982) and based on their work, Oades and Waters
(1991) introduced the concept of aggregate hierarchy. Large aggregates (>2000µm) were
hypothesized to be held together by a fine network of roots and hyphae in soils with high SOC
content (>2%), while 20-250µm aggregates consist of 2-20µm particles, bonded together by
various organic and inorganic cements. Water stable aggregates of 2-20µm size, in turn, consist
of <2µm particles, which are an association of living and dead bacterial cells and clay particles.
The concept aggregate hierarchy degradation of large (relatively unstable) aggregates creates
smaller, more stable aggregates. Stabilization of macro-aggregates occurs mainly via binding by
fungal hyphae and roots.
Fig.2.5. The opposing chronology of the formation of the hierarchical aggregate orders
2.7 Different mechanisms of aggregate breakdown
Aggregate breakdown can result from several physico-chemical-physical mechanisms. Le
Bissonnais (1996a) reviewed four main mechanisms of breakdown (i) Slaking; (ii) Differential
swelling; (iii) Raindrop impact and (iv) Physico-chemical dispersion due to osmotic stress stress.
Swelling causes the volume of the aggregate to increase, and is often followed by the soil
slaking.
Slaking is when the air-dried aggregate breaks into smaller aggregates when immersed in
water. This indicates that the aggregates are not strong enough to withstand the pressures
involved in wetting. Some soils are strong enough to withstand this pressure, and increasing the
organic matter content of the soil may increase aggregate stability. Slaked soils can also
disperse.
Dispersion is caused by breakdown of the clay aggregate into individual clay particles.
Ashman et al. (2003) reviewed two of the most commonly used aggregate fractionation
methods: The slaking method is commonly used to simulate wetting stresses in the field and the
shaking method to simulate mechanical disruption followed by wet sieving. Slaking refers to the
disintegration of large aggregates (2-5mm diameter) into finer aggregates when placed in water.
Rapid disintegration is believed to be due to a lack of organic bonding between particles. They
found that slaking resulted in macro-aggregates being enriched in SOC and, after incubation to
measure microbiologically-available carbon, showed a faster respiration rate than in shaken
treatments. Here, micro-aggregates (<250µm) had more soil SOC and faster respiration rate.
While the general concept of aggregate hierarchy (depending on the size of aggregates, different
organic binding agents are active in aggregate stabilisation) (Oades, 1991) is generally accepted,
when reviewing the literature there are often different and conflicting results, depending on the
kind of fractionation scheme used (Ashman et al., 2003). The different results suggest that
chemical and biological properties of aggregates depend on the fractionation scheme used.
Accordingly, observed relationships can only be interpreted with respect to the specific
disruptive mechanism used and aggregate size can only be related to ‘energy inputs’. The results
from fractionation schemes therefore provide information with regard to the resistance of soil
to disruption, which is different from information about the “true” structure of the soil (Fig. 2.6).
Figure 2.6: Influence of fractionation procedures on biological and chemical properties of different aggregate sizes
(Ashman et al., 2003).
2.8 Soil aggregate stability and Organic matter
Soil aggregate stability was highly correlated with soil organic matter content but the addition of
crop residues and manure were not alone sufficient to restore soil physical quality. Organic
byproducts proceeding from industrial processes represent an important source of nutrients,
especially for organic fertilization. Aggregation generally increases with increasing soil organic
matter, which is connected to clay surfaces through positively charged polyvalent cations, thus
overcoming the repulsion between the negative charges of both clay and organic matter
(Edwards and Bremner, 1967). In this process, not only the amount of clay but also the clay type
is important (Kay, 1998), because mineralogical species differ in surface charge. Isomorphic
substitutions, resulting in negative permanent charges, prevail in layer silicates, whereas pH-
dependent charges form on surface hydroxyls and dominate in the case of oxides and
hydroxides.
Organic matter is known to stabilise aggregates by at least two different mechanisms:
(i) by increasing the inter-particle cohesion within aggregates and thus decreasing their
breakdown to the four above-mentioned breakdown mechanisms and
(ii) by increasing their hydrophobicity and thus decreasing their breakdown by slaking.
Kay and Angers (1999) and Greenland et al. (1975) observed relationships between SOC content
and aggregate stability. Using the Emerson crumb test, Greenland et al. (1975) found that at SOC
<2%, soil aggregates were considered unstable, moderately stable at 2-2.5% and very stable at
SOC contents >2.5%. Carter (1992) also found that maximum structural stability was obtained at
4.5% SOC.
2.8.1 Inorganic binding agents
2.8.1.1 Oxides
The aggregating and SOM stabilizing effect of oxides has been emphasized in many studies. The
aggregating effect of oxides is mainly at the microaggregate level (Oades et al., 1989) but also
macroaggregation has been related to oxide content (Imhoff et al., 2002). Shang and Tiessen
1998) reported that the stabilization of C in tropical soils is highest in stable microaggregates
consisting of oxides, soil organic matter and minerals. Oxides can act as binding agents in three
ways
(1) Organic materials adsorb on oxide surfaces (Oades et al., 1989);
(2) An electrostatic binding occurs between the positively charged oxides and negatively
charged clay minerals (El-Swaify and Emerson, 1975); and
(3) A coat of oxides on the surface of minerals forms bridges between primary and secondary
particles (Muggler et al., 1999).
In a kaolinitic soil, this binding of oxides to minerals will reduce the cation exchange capacity of
the kaolinite and increases the positive charge property of the kaolinite, further promoting the
aggregation through electrostatic binding (Dixon, 1989).
2.8.1.2. Calcium
It is generally accepted that calcium is a critical element for the stabilization of SOM and
aggregates through its role in the formation clay–polyvalent cation–organic matter complexes
(Clough and Skjemstad, 2000). Because calcium exerts its influence at the scale of the organo-
mineral complexation, its stabilization effect is mostly observed at the microaggregate level, but
it can also indirectly increase macroaggregation through a stimulation of microbial activity in
acidic soils (Chan and Heenan, 1999). Additions of calcium to field soils, in the form of lime or
gypsum, increased (∼10%) the aggregation level (Chan and Heenan, 1998, 1999). However, an
initial temporary decrease (1–3%) in aggregate stability has been observed upon the application
of lime to variable charged soils. This temporal decrease in aggregation has been related to an
increase in soil pH (Roth and Pavan, 1991) and microbial activity (Chan and Heenan, 1998,
1999) upon lime application to these acidic soils. An increase in pH of a variable charge soil
leads to an increase of negative charges (Roth and Pavan, 1991), resulting in a dominance of
repulsive forces over edge-to-face flocculation of kaolinite or oxide–kaolinite coagulation. The
dominance of repulsive forces causes dispersion. Nevertheless, this decrease in aggregation
seems to be reversed in the longer-term (Roth and Pavan, 1991; Chan and Heenan, 1998) and is
more pronounced if the calcium is added together with an organic matter source (such as wheat
straw) (Baldock et al., 1994). The latter suggests that the process of calcium bridging is the
dominant factor for the long-term positive effect of calcium addition on the structural stability of
a soil.
2.8.2 Soil texture
Soil organic matter tends to increase as the clay content increases. This increase depends on two
mechanisms. First, bonds between the surface of clay particles and organic matter retard the
decomposition process. Second, soils with higher clay content increase the potential for
aggregate formation. Macroaggregates physically protect organic matter molecules from further
mineralization caused by microbial attack (Rice, 2002). For example, when earthworm casts and
the large soil particles they contain are split by the joint action of several factors (climate, plant
growth and other organisms), nutrients are released and made available to other components of
soil micro-organisms. Under similar climate conditions, the organic matter content in fine
textured (clayey) soils is two to four times that of coarse textured (sandy) soils (Prasad and
Power, 1997). Kaolinite, the main clay mineral in many upland soils in the tropics, has a much
smaller specific surface and nutrient exchange capacity than most other clay minerals.
Therefore,
kaolinitic soils contain considerably fewer clay-humus complexes. In addition, the unprotected
labile humic substances are vulnerable to decomposition under appropriate soil moisture
conditions. Thus, high levels of organic matter are difficult to maintain in cultivated kaolinitic
soils in the wet-dry tropics, because climate and soil conditions favour rapid decomposition. In
contrast, organic matter can persist as organo-oxide complexes in soils rich in iron and
aluminium oxides. Such properties favour the formation of soil microaggregates, typical of many
fine-textured, oxide-rich, high base-status soils in the tropics (Uehara and Gilman, 1981). These
soils are known for their low bulk density, high microporosity, and high organic-matter
retention under natural vegetation, but also for their high phosphate fixation capacity on the
oxides when used for crop production. Current knowledge suggests that whereas organic matter
contributes to the dark colour of Vertisols, it is not considered important in determining either
the development, robustness or resilience of structure in these soils. Organic matter levels tend
to be low in Vertisols, even as low as 10 g/ kg (Coulombe et.al, 1996).
2.9 Effects of aggregation on OM stability
Aggregation can influence accessibility of substrate to microbes and fauna and rates of diffusion
of reactants and products of extracellular synthesis reactions. Theoretical calculations suggest
that aggregation should limit access to organic matter. Direct evidence for effects of aggregation
on accessibility is limited. Adu and Oades (1978) produced synthetic aggregates that were
labelled uniformly with 14C substrates. Aggregates of a sandy loam soil respired less starch than
did unaggregated soil, which they took as evidence of the presence of inaccessible micropores in
the aggregates. This pattern was not observed for a clayey soil, or when the substrate was
glucose. Bartlett and Doner (1988) incorporated lysine and leucine either homogeneously
throughout sterilized synthetic aggregates or only on their surfaces. After adding inoculum,
more of the amino acid was respired from aggregate surfaces than from within aggregates
indicating delay in microbial access to substrate within the aggregates.
*Correlation is significant at the 0.05 level**Correlation is significant at the 0.01 levelCell contents: Pearson correlation, P-ValueThe value of NSI can vary between 0 to 1 (Six et al. 2000a).The NSI of studied soil varied from
0.38 to 1.01 for different series of soil under different textural classes. The NSI was higher in
silty clay loam soil and lower in silty clay soil. The mineralogy of studied soils may play
important role in aggregate stability (Six et al., 2000a).
CONCLUSIONS/Summury
As soil aggregation strongly affected by soil organic carbon content and type of soil. It can be
conclude that yet soil aggregate is badly affected by soil organic carbon content rather than type
of soil also play a vital role in soil aggregate status. From my result, Harta soil series contain
large amount of % OC, but as it was peat basin soil so aggregate status was poor. Similarly Sara
soil contain little amount of % OC but aggregate status was rich as it was Ganges meander