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FIGURE 3.12. Aggregate formation and degradation mechanisms in temperate and tropical soils. Fungal and bacterial activity, active root growth, and earthworm activity are the biological aggregate formation agents in both temperate and tropical soils, whereas the mineral-mineral interactions in tropical soils are the physicochemical aggregate formation agents. UA = unstable aggregates; WSA = water-stable aggregates (from Six et al., 2002b).

cementing agents, and the effects of the accompanying stimulated microbial activity.

In addition, roots influence aggregation physically both by exerting lateral pressures inducing compaction, and by continually removing water during plant transpiration, leading to drying of the soil and cohesion of soil particles around the roots. Note that this process is likely to be enhanced or intensified by mycorrhizal hyphae associated with the plant roots.

During macroaggregate stabilization (t1 to t2), the intra-aggregate particulate organic matter (POM) is further decomposed by microorganisms into finer POM (Six et al., 1998) (Fig. 3.13). This fine POM is increasingly encapsulated with minerals and microbial products, forming new microaggregates (53-250 Mm) within the macroaggregates (Six et al., 1999). Similar processes may arise by stimulation from root exudation and mycorrhizal products, causing further encrustation of micro-bial products and mineral particles, forming microaggregates around the root-derived POM. Note that this microaggregate formation within macroaggregates is crucial for the long-term sequestration of carbon because microaggregates have a greater protective capacity to shield carbon against decomposition compared with macroaggregates.

The final phase of the aggregate turnover cycle (t2 to t3) occurs when the macroaggregates break down, releasing microaggregates and microbially processed soil organic matter (SOM) particles. The macroaggregates are more liable to break up over time, as the labile constituents of the coarse-sized SOM are consumed, microbial production of binding agents decreases, and the degree of association between the soil matrix and SOM decreases. Fortunately, microaggregates are still stable enough and not as sensitive to disruptive forces as the macroaggregates, and therefore survive (see Fig. 3.13). This is borne out by a table of mean residence time (MRT) (in years) of macro- and microaggregate-associated carbon (Table 3.3) (Six et al., 2002b). Note the essentially fivefold greater MRT for microaggregates (m) compared with macroaggregates (M).

The combined influences of physical, chemical, and biological factors in soil aggregate formation reach a peak when one includes the effects of arbuscular mycorrhizal fungi (AMF), both directly by physical binding, and indirectly by the production of the glycoprotein glomalin, as noted briefly in Chapter 2. In a controlled field plot study, five species (three grasses, one forb, and one legume) were grown in monocultures. Soil aggregate water stability (1-2 mm size class) was correlated with plant cover, root weight and length, AMF soil hyphal length, and glomalin concentrations (Rillig et al., 2002). Root length, soil glomalin, and percent cover contributed equally to water-stable aggregation using path

FIGURE 3.13. Effect of management on the normalized stability index (NSI) at the 0-, 5-, and 5- to 20-cm depths. NS = native sod; NT = no-tillage; CT = conventional tillage. Values followed by a different uppercase letter within a depth are significantly different. Values followed by * for the 5- to 20-cm depth are significantly different from corresponding values in the 0- to 5-cm depth. Statistical significance determined at P > 0.05 according to Tukey's HSD mean separation test (from Six et al, 1998).

FIGURE 3.13. Effect of management on the normalized stability index (NSI) at the 0-, 5-, and 5- to 20-cm depths. NS = native sod; NT = no-tillage; CT = conventional tillage. Values followed by a different uppercase letter within a depth are significantly different. Values followed by * for the 5- to 20-cm depth are significantly different from corresponding values in the 0- to 5-cm depth. Statistical significance determined at P > 0.05 according to Tukey's HSD mean separation test (from Six et al, 1998).

analysis as the structural equation modeling approach. The direct effect of the glomalin was much stronger than the direct effect of the AMF hyphae alone, suggesting that this protein from the .AMF is a very important hyphae-mediated mechanism of soil aggregate stabilization, at least for the larger macroaggregates of the 1-2 mm diameter size class.

For an extensive account of the many physical, chemical, and biological interactions involved in the dynamics of creation and dissolution of soil structure, refer to the masterful review by Baldock (2002).

TABLE 3.3. Mean Residence Time (MRT) (in years) of Macroaggregate- and Microaggregate-associated Carbon.

Ecosystem

Aggregate Size class"

(Mm)

0 0

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