THE ECOLOGY OF BACTERIVOROUS PROTOZOANS AND NEMATODES IN ARABLE SOIL

Protozoans and nematodes were studied as part of extensive investigati ons into the functioning of ecosystems in arable soil under low and hi gh input management regimes. Bacterial-feeding soil protozoans and nem atodes largely share the same niche in the soil ecosystem, the water f ilm surrounding soil particles and filling soil pores. This paper pres ents general information on the ecology of protozoans and nematodes co mbined with various results from field observations and laboratory exp eriments. Biomass (B) dynamics of protozoans and nematodes in the arab le fields of the Lovinkhoeve Experimental Farm and their estimated ann ual production rates (P) under winter wheat in 1990 amounted on averag e to 16 kg C ha(-1) and 105 kg C ha(-1) respectively (0-25 cm) for pro tozoans and to 0.330 kg C ha(-1) and 11.6 kg C ha(-1) for bacterivorou s and omnivorous nematodes. Experiments are described on the dependenc e of protozoans and nematodes on soil moisture. It was demonstrated th at considerable differences exist with respect to drought tolerance am ong different protozoan and nematode taxa: taxa such as the flagellate Spumella sp. and the bacterivorous Rhabditidae (Nematoda) proved to b e more vulnerable to drought than the flagellate Cercomonas sp. and th e bacterivorous Cephalobidae (Nematoda). It is assumed that in the ara ble fields of the Lovinkhoeve farm, growth-limiting and non-limiting c onditions for protozoans alternate throughout the year and this may al so hold for their food dependence (bacteria). For bacterivorous nemato des it is assumed that Rhabditidae depend on patchily distributed, tem porally high densities of bacteria and have to deal more frequently wi th food densities below their threshold for reproduction than the Ceph alobidae. In microcosm experiments with sterilised soil amended with c rop residues, and reinoculated selectively with microorganisms, protoz oans, bacterivorous nematodes and a nematophagous fungus, it was shown that protozoans and nematodes substantially affected the pattern of C and N mineralisation and that effects on C and N did not always run i n parallel.