Predation on fungal and bacterial biomass in a soddy-podzolic soil amended with 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 microarthropods 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 (nematodes) and mesofauna (microarthropods) reduced the microbial biomass and microbial respiration by approximately 25% after addition of nitrogen rich alfalfa meal. When starch and wheat straw were supplied, the microbial biomass and activity were stimulated by up to 30% by grazing. Thus, the effect of predation on the microbiota depended on the composition of the available substrates and available nitrogen seems to be an important factor controlling stimulation or suppression of soil micro-organisms by the soil fauna when fresh organic compounds are accessible. The presence of soil fauna stimulated bacteria and, thus, reduced the fungal/bacterial ratio during the course of decomposition. In contrast, the fungal/bacterial ratio declined due to decreasing fungal biomass in defaunated soil.