Plant-feeding nematodes can become abundant in agricultural ecosystems (Wasilewska, 1979; Popovici, 1984; Neher and Campbell, 1996). These nematodes may affect primary productivity of plants by altering uptake of water and nutrients. These abnormalities may result from changes in root morphology and/or physiology. For many agricultural crops, a negative relationship between crop yield and populations of plant-feeding nematodes, such as Meloidogyne, Heterodera, and Praty-lenchus spp., has been observed (Mai, 1985; Barker et al., 1994). However, when entire nematode communities, including free-living nematodes, are examined, a positive association has been observed between plant biomass production and total nematode populations in grassland ecosystems (Yeates and Coleman, 1982). This relationship holds for plant production measured as harvested hay and root biomass (King and Hutchinson, 1976). A negative relationship between total nematode populations and plant productivity has been observed in tropical forests (Kitazawa, 1971). The relationship between soil nematode communities and row crop yield has yet to be determined.
Microarthropods rarely harm crop plants. However, soil mesofauna may become pests when a preferred food source is absent. Some Collembola, e.g., sminthurids and onychiurids, may feed on roots. For example, root-grazing injury on sugar beet is caused by Onychiurus spp. (Collembola) rubbing their bristled bodies against roots (Curl et al., 1988). However, root injury decreases if specific weed species and certain kinds and amounts of organic matter are present and, thus, provide the preferred microbial food supply. Few groups of soil mites are adapted to feeding on live plant tissues in soil. Some examples occur in the Tarsonemidae (Prostigmata) and Periohmanniidae (Oribatida). Most soil mites feed on plant material only after decomposition has begun. Often, increasing vegeta-tional diversity and the quality and quantity of organic matter in soil increases potential benefits by soil mesofauna.
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