In recent years, interest has been shown by soil scientists and ecolo-gists in measuring "soil quality." This elusive concept has been the subject of entire symposia and volumes resulting from them (e.g., Doran et al., 1994). As defined by soil scientists, soil quality can be considered as the degree or extent to which a soil can: (1) promote biological activity (plant, animal, and microbial); (2) mediate water flow through the environment, and (3) maintain environmental quality by acting as a buffer that assimilates organic wastes and ameliorates contaminants (Linden et al., 1994). Many environmental scientists are attempting to use the concept of indicator organisms or indicator communities as a way to determine overall soil "health" (e.g., Bongers, 1990; Ettema and Bongers, 1993; Foissner, 1994; Linden et al., 1994; Neher et al., 1995; Ferris et al., 2001). Because of their large size and public awareness of them, earthworms are often considered a sign of soil "health" (Linden et al., 1994; Hendrix, 1995). All of the biota play important roles in affecting and influencing soil processes. As summarized in Table 4.12, each of the biotic groups has significant impacts. Among the fauna, microfauna have a principal role via interactions with the microflora. The mesofauna and macrofauna create fecal pellets, and produce biopores of various sizes, which affect water movement and storage as well as root growth and proliferation. Perhaps more important, over the longer term, they have marked effects on humification processes as well (Wolters, 1991). Based on biological characteristics, there are three general trophic systems: microtrophic (protozoa, nematodes, and some enchytraeids), mesotrophic (the mesofauna), and macrotrophic (the
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