Cultivation affects biogeochemical cycling by physically rearranging soil particles and changing pore size distribution, patterns of water and gas infiltration, and gas emission (Klute, 1982). Tillage disrupts soil aggregates, closes soil cracks and pores, and promotes drying of the surface soil. Soil fauna become sparse in top layers of cultivated soil because moisture content fluctuates widely and the original pore space network in this layer is destroyed. These physical alterations of the surface layers of soil may persist for many years after cultivation has ceased.

Soils managed by conventional — or reduced — tillage practices have distinct biological and functional properties (Doran, 1980; Hendrix et al., 1986). Plant residue is distributed throughout the plow layer in fields managed with conventional tillage. Under these conditions enhanced by cultivation, organisms with short generation times, small body size, rapid dispersal, and generalist feeding habits thrive (Steen, 1983). These soils are dominated by bacteria and their predators such as nematodes and astigmatid mites (Andren and Lagerlöf, 1983; Yeates, 1984; Hendrix et al., 1986; Beare et al., 1992) and are considered in an early stage of succession. Oribatid and mesostigmatid mites decrease while other groups such as prostigmatid mites and Collembola tolerate, but do not benefit, from cultivation (Crossley et al., 1992). However, prostigmatid mite communities can be more diverse, containing both fungal- and nematode-feeding taxa in cultivated soils (van de Bund, 1970). Many microarthropods have omnivorous feeding habits in systems cultivated frequently (Beare et al., 1992).

Conservation — or no-till — practices generate more biologically complex soils than conventional tillage; however, in general, no-tillage cultivation does not appear to result in greater concentrations of microarthropods than conventional tillage except under drought stress (Perdue and Crossley, 1989). However, many studies comparing tillage effects are short term. Our knowledge about tillage effects may change as more long-term studies are implemented. Reduced tillage leaves most of the residue of the previous crops on the soil surface, and results in changes in physical and chemical properties of the soil (Blevins et al., 1983). Surface residues retain moisture, dampen temperature fluctuations, and provide a continuous substrate which promotes fungal growth. The increased fungal abundance can be attributed to the ability of fungi to translocate nutrients from soil into surface residues, their tolerance of lower pH and water potentials that often occur in surface residues, and their ability to penetrate and use large detritus particles (Hendrix et al., 1986; Holland and Coleman, 1987). Relative abundances of fungi and their predators, such as nematodes and many microarthropods groups (e.g., uropodid mesostigmatid mites; tarsonemid, eupodid, tydeids, and pygmephorid prostigmatid mites; oribatid mites) (Walter, 1987), in no-till soils represent a more mature successional state than one dominated by bacteria (Yeates, 1984; Bostrom and Sohlenius, 1986; Hendrix et al., 1986; Holland and Coleman, 1987; Neher and Campbell, 1994). Fungal feeding by microarthropods may stimulate microbial growth and enhance decomposition and nutrient immobilization (Seastedt, 1984). However, nutrient mineralization rates are relatively slow with stratification of debris and soil; nutrients are immobilized inside plant debris on the soil surface (Hendrix et al., 1986; Holland and Coleman, 1987).

Was this article helpful?

0 0

Post a comment