Fertilization may influence the population abundance or composition of meso-faunal communities in soil. The outcome is a result of factors such as fertilizer quality and/or quantity (Verhoef and Brussard, 1990). Fertilization may, thus, affect the abundance and diversity of soil mesofauna directly or indirectly. These changes in community composition may, in turn, influence ecosystem function.

Nutrients applied to agricultural soils may be derived either from fossil fuels or plant and animal waste products. Nutrients are available in both forms, but organic amendments also contain microbes and their respective food resources. Additions of mineral fertilizers decrease populations of oribatid (cryptostigmatid) and prostig-matid mites, and root- and fungal-feeding, omnivorous and predaceous nematodes (Sohlenius and Wasilewska, 1984). Numbers of root-feeding nematodes may increase with increased nitrogen fertilizer (Wasilewska, 1989). Populations of astig-matid mites (Andren and Lagerlof, 1983) and bacterial-feeding nematodes (Sohlenius and Wasilewska, 1984) increase with additions of mineral fertilizer, but even more so when soils are fertilized with manure which simultaneously adds organic matter and microbes (Andren and Lagerlof, 1983; Weiss and Larink, 1991). However, in Dutch polder soil, abundances and biomass of nematodes, mites, and Collembola were similar between fields fertilized with manure, crop residues, and green manure and those field soils amended with crop residues and synthetic fertilizer (van de Bund, 1970).

Mesofauna aggregate around manure and plant litter (van de Bund, 1970). Populations of fungal-feeding nematodes (Weiss and Larink, 1991), potworms

(Enchytraeids), collembolans, and sometimes mesostigmatid mites increase with applications of manure (Andren and Lagerlof, 1983). Fratello et al. (1989) examined the effects of seven types of organic fertilizer on the microfauna in alfalfa fields. The reactions of populations of microarthropods to the different treatments varied with sample date, illustrating the highly complex interactions that occur in the soil. Poultry manure, sheep manure, worm compost, autoclaved urban sludge, urban sludge, vetch green manure, or straw were added to soil to provide a common level of 4% organic matter. Straw was the only additive that did not depress mite populations. Fewer mites and Collembola were found in plots treated with autoclaved urban sludge than those treated with non-autoclaved urban sludge.

The quality of plant and animal wastes as nutrient sources may be altered by composting. For example, applications of aged compost can increase suppression of plant pathogens by increasing the effectiveness of biocontrol agents. The plant-pathogenic fungus Rhizoctonia solani may cause damping-off disease in soil when fresh or immature compost material high in cellulose content is added. However, in aged compost, cellulose is degraded and the biocontrol fungus Trichoderma spp. can parasitize the pathogen effectively, thus suppressing disease (Chung et al., 1988).

Large doses of mineral or manure fertilizers can harm mesofauna because of toxicity (e.g., anhydrous ammonia) or high osmotic pressure due to salt (Andren and Lagerlof, 1983). The repellent nature of ammonium can affect soil invertebrates adversely (Potter, 1993). The potential for toxic effects can be decreased by applying composted manure and sludge (Ott et al., 1983). However, accumulation of heavy metals from repetitive sludge applications may kill omnivorous and predaceous nematodes (Weiss and Larink, 1991).

Fertilization affects soil microflora and, thus, indirectly impacts soil mesofauna by changing their food resources (Weil and Kroontje, 1979). Additions of nitrogen may acidify soil and inhibit microbial growth and activity. Nitrogen may also affect the quality of microbes as a food source for mesofuana. Booth and Anderson (1979) grew two species of fungi in liquid media with 2, 20, 200, or 2000 ppm nitrogen and determined the fecundity of the collembolan Folsomia candida while feeding on the fungi. Fecundity increased with increasing nitrogen content up to 200 ppm. F. candida did not show a preference for feeding on fungi with a greater or lesser nitrogen content.

The effect of fertilization on microarthropod species diversity and abundance within taxa and the subsequent impact on decomposition and nutrient mineralization processes are not well understood. For example, synthetic fertilizers increase nem-atode diversity, but applications of manure decrease nematode diversity (Wasilewska, 1989). The mechanism(s) explaining the differences are not understood. Applications of synthetic nitrogen fertilizer on Swedish arable soils growing spring barley (Hor-deum distichum L.) changed community composition, but not numbers and biomass of nematodes, Collembola and mites (Andren et al., 1988). Within a given environment, increased densities of microarthropods have been correlated with increased foliage, root and microbial productivity (Lussenhop, 1981), or increased food resource via fertilization. It is not known at what scale of resolution soil faunal communities respond to changes in ecosystem function.

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