Predation on fungal and bacterial biomass in a soddy-podzolic soil amendedwith starch, wheat straw and alfalfa meal

The variation in bacterial, fungal and total microbial biomass and activity was studied together with the abundance of soil nematodes and microarthrop ods after the addition of substrates differing in nitrogen availability to a soddy-podzolic soil. The experiments were carried out in microcosms with native and defaunated soil to evaluate stimulatory and suppressive effects of the microfauna on soil micro-organisms. Predation by microfauna (nematod es) and mesofauna (microarthropods) reduced the microbial biomass and micro bial respiration by approximately 25% after addition of nitrogen rich alfal fa meal. When starch and wheat straw were supplied, the microbial biomass a nd activity were stimulated by up to 30% by grazing. Thus, the effect of pr edation on the microbiota depended on the composition of the available subs trates and available nitrogen seems to be an important factor controlling s timulation or suppression of soil micro-organisms by the soil fauna when fr esh organic compounds are accessible. The presence of soil fauna stimulated bacteria and, thus, reduced the fungal/bacterial ratio during the course o f decomposition. In contrast, the fungal/bacterial ratio declined due to de creasing fungal biomass in defaunated soil. (C) 2001 Elsevier Science B.V. All rights reserved.