Short-term variations in abiotic and biotic environmental conditions modify microbial metabolism. These effects were studied in an arable soil during the oat straw decomposition. Cumulative CO2 evolution was significantly higher when soil was subjected weekly to drying-rewetting cycles (DR) or had been initially fertilised with nitrogen (+N, as NH4NO3) in comparison to samples incubated at constant conditions (CC). The microbial metabolic quotient (respiration rate per unit microbial C) of DR and +N exceeded CC by approximately 100-150%, indicating that variable water availability and improved N availability induced a higher activity level of the microbial biomass. In addition, the metabolic-responsive microbial communities, calculated by the ratio of microbial C estimated by substrate-induced respiration to fumigation-extraction technique, increased from 1.0 to approximately 1.3 after 7 weeks for DR and +N, but remained essentially at 1.0 for CC and control. A high abundance of soil nematodes significantly affected respiration rate, metabolic-responsive biomass and metabolic quotient during the early stages of decomposition. We concluded that short-term variation in environmental conditions promoted decomposition processes by influencing the physiology of soil microorganisms and increasing the metabolic-responsive biomass. Such a stimulated microbial metabolism seems to be unspecific due to stressing environmental conditions. The role of the interacting fauna such as nematodes varied during decomposition. (C) 2002 Elsevier Science Ltd. All rights reserved.
