Regulation of decomposer community structure and decomposition processes in herbicide stressed humus soil

Abstract

Regulation of soil decomposer community structure and ecosystem processes, 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 different food webs including microbes, nematodes, tardigrades and oribatid mites, either with or without predatory mesostigmatid mites, were reconstructed. Half of the microcosms were stressed with a herbicide (active ingredient was terbuthylazine). During the 57 weeks incubation community structure of decomposers and nitrogen mineralisation were studied at five destructive samplings and two water irrigations. Soil respiration was measured weekly starting at week 26. Mesostigmatid mites regulated densities of some prey species and hence they had an effect on the community structure of microbivores. A trophic cascade from predators to microbes took place both in unstressed and stressed soils: microbial activity decreased in the presence of predators. Predation effect was observed more clearly in the unstressed soil although predators maintained their populations longer in the herbicide stressed soil. Predators had no significant effects on N mineralisation while herbicide increased it. Oribatids were reduced by the herbicide at the later phase 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 web analysis in risk assessment procedures has been proved to be problematic, there is a clear need for system level studies because of the chemical-induced indirect effects on the food webs.