Micro- and mesofauna contribute directly to ecosystem processes such as decomposition and nutrient cycling in complex and interactive ways (Swift et al., 1979).
Bacteria, actinomycetes, fungi, algae, and protozoa are primary decomposers of organic matter. These microorganisms are involved directly with production of humus, cycling of nutrients and energy, elemental fixation, metabolic activity in soil, and the production of complex chemical compounds that cause soil aggregation.
Microbial-grazing mesofauna affect growth and metabolic activities of microbes and alter the microbial community, thus regulating decomposition rate (Wasilewska et al., 1975; Trofymow and Coleman, 1982; Whitford et al., 1982; Yeates and Coleman, 1982; Seastedt, 1984) and nitrogen mineralization (Seastedt et al., 1988; Sohlenius et al., 1988). Nematodes feed on bacteria and fungi on decaying organic matter, but not on the organic matter itself. Nematode species with a buccal stylet (spearlike structure) feed on cell contents and juices obtained by piercing the cellular walls of plant roots or fungal mycelium. Other species have no stylets and feed on particulate food such as bacteria and small algae (Vinciguerra, 1979). Microarthro-pods fragment detritus and increase surface area for further microbial attack (Berg and Pawluk, 1984). For example, collembolans and mites may enhance microbial activity, accelerate decomposition, and mediate transport processes in the soil. Even when they do not transform ingested material significantly, they break it down, moisten it, and make it available for microorganisms.
There is evidence that plants benefit from increased mineralization of nitrogen by soil mesofauna. Shoot biomass and nitrogen content of plant shoots grown in the presence of protozoans and nematodes were greater when compared with plants grown without mesofauna (Verhoef and Brussard, 1990). Soil fauna are responsible for approximately 30% of nitrogen mineralization in agricultural and natural ecosystem soils. The main consumers of bacteria are protozoa and bacterial-feeding nematodes which account for 83% of nitrogen mineralization contributed by soil fauna (Elliott et al., 1988). Nematodes also excrete nitrogenous wastes, mostly as ammonium ions (Anderson et al., 1983; Ingham et al., 1985; Hunt et al., 1987). Collembola excrete nitrate in concentrations 40 times more than their food source (Teuben and Verhoef, 1992). Furthermore, large collembolan species increase mineralization by selective feeding on fungi, whereas smaller species aid in the formation of humus by nonselective scavenging and mixing of the mineral and organic fractions of soil (van Amelsvoort et al., 1988). Microfauna constitute a reservoir of nutrients. When microfauna die, nutrients immobilized in their tissues are mineralized and subsequently become available to plants.
Soil fauna transport bacteria, fungi, and protozoa (in gut or on cuticle) across regions of soil and, thus, enhance microbial colonization of organic matter (Seastedt, 1984; Moore et al., 1988). For example, Collembola and sciarid fly larvae transmit root-infecting fungi and fungal parasites (Anas and Reeleder, 1988; Whipps and Budge, 1993). Microarthropods are surrounded by and, therefore, may disseminate propagules of insect-parasitic fungi including Beauveria spp., Metarhizium spp., Paecilomyces spp., and Verticillium spp. and facultative pathogens of insects in the genera Aspergillus and Fusarium spp. Under laboratory conditions, Collembola and mites transport spores of the insect-parasite M. anisopliae (Zimmerman and Bode, 1983). The impact of insect-parasitic fungi on natural populations of microarthropods is unknown.
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