Aggregate stability of different aggregate sizes

by T.B.S. Christopher 1996

Aggregate stability of whole soils are usually measured, disregarding the analysis of individual aggregate sizes. Even then, researchers usually concentrate on the macroaggregate size fractions above 250 m m (Dormaar, 1983; Voronin and Sereda, 1976). This is understandable considering the amount of work needed to individually analyze each aggregate size fraction. However, by analyzing whole soils, one assumes that different aggregate sizes have similar properties to each other. This is not so according to Garey (1954) who discovered that the chemical properties of aggregates vary with their sizes, and are different from the whole soil. Garey observed that the smaller aggregates (0.15-0.42 mm) have higher cation exchange capacities, and have larger amounts of clay and organic matter than of the whole soil.

Although there are many models of aggregation, the aggregate hierarchy proposed by Hadas (1987) and reviewed by Dexter (1988) is perhaps one of the most interesting because of its simplicity and conformity with other models. The lowest hierarchical order is the combination of single mineral particles, like clay plates, into a basic type of compound particle, such as a floccule or domain of clay plates (Dexter, 1988). The next hierarchical order is the larger compound particles such as clusters of domains, and when this clusters combine into microaggregates, they form the next hierarchical order, and so on. However, not necessarily all of these hierarchical orders must exist in all soils.

This hierarchy model states that the larger aggregates are assemblages of small aggregates which in turn, are assemblages of even smaller aggregates. This follows statements by Baver (1956), Tisdall and Oades (1982), and Elliott (1986) that microaggregates are building blocks or templates of macroaggregates. This model is also just another way to express the well-known model C-P-OM proposed by Edwards and Bremner (1967), where C represents clay, OM is organic matter, with bridging by P or polyvalent cations. To flocculate, (C-P-OM) binds with other (C-P-OM)s, yielding large particles, and this bonding continues with other units of (C-P-OM) until progressively larger complexes are formed. This aggregation process (from left to right) is represented as follows (Voronin and Sereda, 1976):

xy(C-P-OM) « y(C-P-OM)x « [(C-P-OM)x]y

Note that this process is also reversible (from right to left) by dispersion.

This aggregate hierarchy model also conforms partly to the fractal theory (Mandelbrot, 1982) as introduced recently into the study of aggregation by several workers (Bartoli et al., 1991; Perfect et al., 1992; Rasiah et al., 1992, 1995). This theory states that no matter how complex a structure may be, it is built by repeating continuously, using a same pattern, a very much simpler structure or unit. Large aggregates, for example, look similar to small aggregates. Likewise, if an aggregate breaks up into a hundred pieces, each piece (even if one has to look at it under a microscope) would look similar to each other and to the larger piece from which they came from.

Although aggregates of various sizes may all look similar, their composition is different. Because macroaggregates are built from microaggregates, the composition of microaggregates has the stronger influence on both the physical and chemical properties of soils, including the character of macroaggregates (Voronin and Sereda, 1976). Dissimilarities among aggregate sizes or hierarchy levels have either been demonstrated or been hypothesized. Compound particles of lower hierarchical order, for example, are denser than those of the higher hierarchical order because each order excludes the pore spaces between particles of the next higher order (Currie 1966; Dexter, 1988). Lower hierarchical orders are also internally stronger than of the higher orders (Braunack et al., 1979; Hadas, 1987). North (1976) added that successively more energy is required to breakdown progressively smaller aggregates.

Because smaller aggregates are progressively more difficult to breakdown, Hadas (1987) and Dexter (1988) hypothesized that the destruction of a given hierarchical order automatically destroys all higher hierarchical orders as well. Dispersion of clay is then the ultimate destruction of aggregates. If a given energy can breakdown aggregates of, say, 1 mm then this energy already exceeds the amount of energy required to breakdown aggregates larger than 1 mm. Therefore, all aggregates larger than 1 mm would be destroyed, but those smaller than 1 mm would persist.

Oades and Waters (1991) and Stevenson (1982), however, discovered that this kind of destruction is absent in highly weathered soils of Oxisols, but present in Alfisols and Mollisols. They explained aggregate hierarchy is applicable to Alfisols and Mollisols because the influence of organic matter in these soils is the greatest; whereas, for Oxisols, it is the iron oxides, not organic matter, that is most important. But such claims are refuted by several researchers (Greenland et al., 1992; Sanchez, 1976; Moormann, 1981). To suggest, or to even imply that organic matter in tropical soils has a poorer quality and lower quantity than in temperate soils is wrong; therefore, a myth (Greenland et al., 1992). Many red Oxisols can have higher carbon contents than black Vertisols (Gabriels and Michiels, 1991).

Perhaps then why this concept of automatic destruction is not universal to all soils is this concept is derived from a model that describes aggregation not aggregate stability. The process of aggregation, though similar, is nevertheless different from the process of aggregate stability because aggregate stability is more concerned with the forces within the aggregates with relation to outside forces (Allison, 1968).

Ultimately, to break up an aggregate, the exerted force must always exceed the binding force within the aggregate (Allison, 1968). Voronin and Sereda (1976) discovered that the adhesive forces of different aggregate sizes were identical. However, as an aggregate increases in size, its mass will rise more rapidly than its surface area because surface area increases with the square of the radius, and the volume with the cube of the radius. Hence, gravity, counteracting adhesion, rises in proportion to the aggregate mass. So as the aggregate size increases, the adhesive forces remain constant but the counteracting forces increase. Consequently, progressively larger aggregates will become increasingly less stable (Brady, 1990; Tisdall and Oades, 1982).

Why the difference in stability among the various aggregate sizes is due to several factors. The stability of microaggregates is insensitive to changes in either the soil organic matter content or soil management practices (Tisdall and Oades, 1982). Microaggregates are very stable because they are bound by persistent aromatic humic material associated with amorphous Fe and Al compounds. The stability of macroaggregates, on the other hand, varies with the changes in organic matter content or in management practices. This is because macroaggregates are stabilized by transient or temporary binding agents such as roots, hyphae, and microbial- and plant-derived polysaccharides.

Although organic matter decreases with decreasing aggregate size (Cambardella and Elliott, 1993; Elliott, 1986; Gupta and Germida, 1988; Puget et al., 1995), the organic matter in the smaller aggregate size fractions are older, less liable and more highly processed than the organic matter in larger aggregates (Elliott, 1986; Gupta and Germida, 1988; Paul, 1984; Parton et al., 1983; van Veen and Paul, 1981). Also, Monreal et al. (1995) found that the macroaggregate stability is correlated to many types of organic matter structures such as lignin dimers, alkylaromatics, lipids, sterols, organic carbon and nitrogen. The microaggregate stability, however, failed to correlate to any of the organic matter structures. They concluded that soil organic matter and its entities may be less important than the inorganic components in stabilizing microaggregates.

Cohesion between clay particles, mediated by humic substances linked to polyvalent cations, may play a stronger role in microaggregate stability (Krishna Murthi et al., 1977; Monreal et al., 1995). Principal component analysis (a variant of factor analysis) revealed that the microaggregate stability of seven Italian soils of different pedogenesis was related mainly to soil mineralogy; on the other hand, organic carbon had little effect (Nwadialo and Mbagwu, 1991).

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