A microcosm experiment was carried out to quantify the effects of organisms at various trophic levels on C and N mineralization after the addition of crop residues to arable soil. The effects of the bacterivorous nematodes Rhabditis sp. and Acrobeloides bfitschlii and of the nematophagous fungi Arthrobotrys oligospora und Drechmeria coniospora on soil respiration and N mineralization were measured over 6 months at 20 ~ In the presence of nematodes, C mineralization was increased during the first month and subsequently reduced; N mineralization was increased during the first 2 months and then reduced. The results support the assumption that nematodes influence C mineralization mainly indirectly by affecting bacterial activity, and N mineralization mainly directly by mineralizing bacterial biomass. A, oligospora contributed directly to C mineralization. The effect of both fungi on N mineralization was indirect and resulted from the reduction in the numbers of nematodes. The results showed that the effects of nematodes and nematophagous fungi and the mechanisms behind the effects may vary strongly in time, and are correlated with the type of organic matter decomposed. Key words Microcosm. Arable soil Bacteria Bacterivorous nematodes. Nematophagous fungi 9 Carbon and nitrogen 9 Mineralization Work was carried out at the