Do soil microarthropods influence microbial biomass and activity in spruce forest litter

The effects of grazing microarthropods on microbial biomass, protease and urease activity, inorganic N-content of litter, anaerobic N-mineralization and pH were assessed regularly in microcosms containing 80 and 130 g of spruce litter and a mixed microarthropod fauna in two experiments with a duration of 84 and 105 days, respectively. Enzyme activities remained at a constant level after initial microbial adaptation. Depending on the method of analysis, microbial biomass declined during the whole incubation period (SIR) or remained at a constant level after an initial decrease (CFE). SIR measures the response of the glucose-responsive microbial biomass only, so it is possible that the glucose metabolizing biomass declined while the total biomass did not. Values of total inorganic N multiplied. NH4+ increased slightly whereas almost all mineralized N was nitrified immediately. High nitrification rates are unusual in spruce litter under natural conditions and are a consequence of litter pretreatment (sieving). No influence of microarthropods was detected other than a significant increase of NH4+ at the end of the second experiment. The lack of faunal influence may be explained by the suitable conditions for microbial growth in the microcosms. In a milieu characterised by high humidity and small-scale heterogeneity of the litter, microorganisms are more likely to be limited by competition than by microarthropod grazing. Our results support the hypothesis that microarthropods may exert an indirect influence only facultatively under unfavourable conditions for the microorganisms.