Soils are best considered as the extremely heterogeneous entities they are. This requires that we "let the soil work for us" (Elliott and Coleman, 1988), and stratify, in a statistical sense, the regions of the soil that are "hot spots" of activity. These zones include the rhizosphere, aggregates, litter and organic detritus, and the "drilosphere," which is that portion of the soil volume influenced by secretions of earthworms (Bouché, 1975) (Fig. 6.4). Each region is a relatively small subset of the total soil volume, but may contain a preponderance of numbers, and more importantly, activity of the soil biota (Beare et al., 1995). Examples include: The 5-7% of the total soil that was root-influenced or rhizosphere in extensive pot trials of Ingham et al. (1985) contained a majority (greater than 70%) of the bacterial- and fungal-feeding nematodes. Ingham et al. (1985) also measured higher biomasses of rhizosphere bacteria in microcosms with large numbers of microbivorous nematodes (greater than 4000 per gram of rhizosphere soil) than in microcosms without these nematodes. Yet the extent of mineralization of nitrogen in the microcosms with nematodes reflected that they were ingesting large quantities of microbes as well. Thus there was a net enhancement of microbial production, in a fashion similar to that measured by Porter (1975), who found a net stimulation of phytoplankton growth after the cells had undergone transit through the guts of Daphnia sp. in freshwater incubations. As an example of the dynamic nature of shifting "hot spots," Griffiths and Caul (1993) found that more nematodes were active in the rhizosphere, and they moved readily to new concentrations of fresh organic matter (leaf litter) in short-term trials. Other examples of "hot spots" that have shown enhanced microbial activity include the drilosphere and worm castings, which show enhanced carbon and nitrogen (Syers et al., 1979a; Daniel and Anderson, 1992) and phosphorus mineralization (Syers et al., 1979b; Lavelle et al., 1992). Lumbricus ter-restris "middens" (small patches of plant litter and casts gathered around the burrow entrance) in experimental field sites in Ohio were found to be functionally different, with enhanced acetate incorporation and microbial cell synthesis, compared with surrounding non-earthworm-influenced soil (Bohlen et al., 2002). Another center of activity is the aggregatusphere (Fig. 6.4), or region of micro- and macroaggregates
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