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FIGURE 8.4. The functional role of soil organic matter within an ecosystem depends on the intensity with which that system is managed (from E.T. Elliott, personal communication; modified from Woomer and Swift, 1994).

FIGURE 8.4. The functional role of soil organic matter within an ecosystem depends on the intensity with which that system is managed (from E.T. Elliott, personal communication; modified from Woomer and Swift, 1994).

involved in the formation and turnover of macro- and microaggregates in a wide range of soil types worldwide (see e.g., Beare et al., 1994a, b; Six et al., 1999, 2002a). Indeed, as noted in Chapter 3, chemical, microbiological, and macrobiological characterization of physically isolated fractions may provide the best opportunity for identifying functional pools of soil organic matter. For example, each major category of soil biota has a significant effect on one or more aspects of soil structure, including production of organic compounds that bind aggregates, and hyphal entanglement of soil particles (microflora), producing fecal pellets and creating biopores (meso- and macrofauna) (Hendrix et al., 1990; Linden et al., 1994). A complete list of influences of soil biota is given in Table 4.12 (Hendrix etal., 1990).

Some recent developments have been made in conceptualizing SOM dynamics, which should have a considerable impact on the ways in which soils are viewed and managed for carbon sequestration. There are three principal mechanisms by which SOM is stabilized: (1) it is physically stabilized, or protected from decomposition through microaggregation; (2) SOM is closely associated with silt and clay particles; and (3) it is biochemically stabilized through the formation of recalcitrant SOM compounds. These stabilization mechanisms are strongly related to the ways in which SOM pools are protected (Six etal., 2002a). The protective capacity of soil has been represented graphically, in an ascending series (Fig. 8.5) (Six, 2002a) showing silt and clay at an asymptotic maximum that is the maximum protection possible, because anything above that is considered nonprotected. Aconceptual model of SOM dynamics with the aforementioned measurable pools follows a sequence beginning with above- and belowground inputs into unprotected soil carbon, moving either into microaggregate-associated soil carbon by aggregate turnover, or via adsorption/desorption into soil- and clay-associated soil carbon. It then moves subsequently into nonhydrolyzable soil carbon via condensation and complexation reactions, producing the biochemically protected soil carbon (Fig. 8.6) (Six et al., 2002a). In contrast with earlier conceptual and simulation models, this scheme is indeed measurable, hence meeting the criterion of Elliott (1994) of "modeling the measurable." It also has the virtue of reflecting realities in lightly versus heavily weathered soils. The former, with a greater proportion of 2:1 clay mineral dominated soils, have a greater silt- and clay-protected carbon pool than the 1 : 1 clay mineral dominated soils. The latter minerals, for example, kaolinite and gibbsite, tend to dominate in the typically heavily weathered tropical soils (Theng et al., 1989).

Six et al. (2002b) proceeded to test some of the assumptions in the conceptual model given above by performing a major synthesis of SOM dynamics in a wide range of temperate and tropical soils worldwide, comprising more than 32 sites. Six etal. (2002b) found a 1.8 times longer average MRT of carbon in the soil surface of temperate versus tropical soils (63 ± 7 versus 35 ± 6 years). This indicates that there is generally a faster carbon turnover in tropical than temperate soils. Interestingly, the range of MRT values was similar for both temperate and tropical soils, being 14-141 versus 13-108 years, respectively. The higher turnover rate for tropical soils is due primarily to faster turnover rates of the slow carbon pool in tropical soils (Feller and Beare, 1997, cited in Six et al., 2002b).

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