MIGRATION OF BACTERIAL-FEEDING NEMATODES, BUT NOT PROTOZOA, TO DECOMPOSING GRASS RESIDUES

Populations of bacterial-feeding nematodes and protozoa developing in soil amended with dried grass powder or a nutrient solution were monitored in experimental systems designed to prevent migration from surrounding unamended soil. The addition of nutrient solution stimulated both microbial activity, as determined by dehydrogenase activity, and protozoa, but brought about no increase in nematode numbers. Amendment of soil with grass, however, caused an increase in both types of grazer, with the maximum biomass of protozoa (180 mug g-1) exceeding that of bacterial-feeding nematodes (42 mug g-1). The decomposing grass was rapidly colonised by rhabditid nematodes, mainly Caenorhabditis sp. Incubating grass-amended soil in the absence of any surrounding soil, to prevent migration, changed the microflora from predominantly bacterial to predominantly fungal, and so could not be used to compare treatments with and without migration. Surrounding the amended soil with sterilised soil prevented migration and caused no detectable change in the microflora. This treatment demonstrated that migration plays an important part in the colonisation of decomposing substrates by nematodes, but that protozoa do not migrate in soil. The nematodes migrated from a volume of unamended soil that was equivalent to eight times the volume of amended soil. The potential effects of the large grazing pressure on the subsequent decomposition of the grass residue are discussed.