THE ECOLOGY OF BACTERIVOROUS PROTOZOANS AND NEMATODES IN ARABLE SOIL

Protozoans and nematodes were studied as part of extensive investigations into the functioning of ecosystems in arable soil under low and high input management regimes. Bacterial-feeding soil protozoans and nematodes largely share the same niche in the soil ecosystem, the water film surrounding soil particles and filling soil pores. This paper presents general information on the ecology of protozoans and nematodes combined with various results from field observations and laboratory experiments. Biomass (B) dynamics of protozoans and nematodes in the arable fields of the Lovinkhoeve Experimental Farm and their estimated annual production rates (P) under winter wheat in 1990 amounted on average to 16 kg C ha(-1) and 105 kg C ha(-1) respectively (0-25 cm) for protozoans and to 0.330 kg C ha(-1) and 11.6 kg C ha(-1) for bacterivorous and omnivorous nematodes. Experiments are described on the dependence of protozoans and nematodes on soil moisture. It was demonstrated that considerable differences exist with respect to drought tolerance among different protozoan and nematode taxa: taxa such as the flagellate Spumella sp. and the bacterivorous Rhabditidae (Nematoda) proved to be more vulnerable to drought than the flagellate Cercomonas sp. and the bacterivorous Cephalobidae (Nematoda). It is assumed that in the arable fields of the Lovinkhoeve farm, growth-limiting and non-limiting conditions for protozoans alternate throughout the year and this may also hold for their food dependence (bacteria). For bacterivorous nematodes it is assumed that Rhabditidae depend on patchily distributed, temporally high densities of bacteria and have to deal more frequently with food densities below their threshold for reproduction than the Cephalobidae. In microcosm experiments with sterilised soil amended with crop residues, and reinoculated selectively with microorganisms, protozoans, 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 in parallel.