REGULATION OF DECOMPOSER COMMUNITY STRUCTURE AND DECOMPOSITION PROCESSES IN HERBICIDE STRESSED HUMUS SOIL

Regulation of soil decomposer community structure and ecosystem proces ses, such as nutrient cycling, under herbicide stress was studied in a microcosm experiment. For the experiment, coniferous forest soil was defaunated and put into the microcosms. In the microcosms two differen t food webs including microbes, nematodes, tardigrades and oribatid mi tes, either with or without predatory mesostigmatid mites, were recons tructed. Half of the microcosms were stressed with a herbicide (active ingredient was terbuthylazine). During the 57 weeks incubation commun ity structure of decomposers and nitrogen mineralisation were studied at five destructive samplings and two water irrigations. Soil respirat ion was measured weekly starting at week 26. Mesostigmatid mites regul ated densities of some prey species and hence they had an effect on th e community structure of microbivores. A trophic cascade from predator s to microbes took place both in unstressed and stressed soils: microb ial activity decreased in the presence of predators. Predation effect was observed more clearly in the unstressed soil although predators ma intained their populations longer in the herbicide stressed soil. Pred ators had no significant effects on N mineralisation while herbicide i ncreased it. Oribatids were reduced by the herbicide at the later phas e of the experiment. It can be concluded that decomposer food webs and decomposition in the soil can partly be top-down controlled. Due to a high degree of omnivory it was impossible to determine precisely the trophic structure of the food web. Herbicide contamination altered the community regulation and ecosystem processes via direct toxicity and by affecting trophic interactions. Although the application of food we b analysis in risk assessment procedures has been proved to be problem atic, there is a clear need for system level studies because of the ch emical-induced indirect effects on the food webs. (C) 1997 Elsevier Sc ience B.V.