Effects of benomyl and diazinon on isopod-mediated leaf litter decomposition in microcosms

Soil pollutants may exert ecotoxicological effects not only through affecti ng the performance of individual species, but also through changing ecologi cal interactions between species. In this study we analysed the effects of a fungicide (benomyl), expected to inhibit microflora, and an insecticide ( diazinon), expected to inhibit arthropod feeding. Carbon dioxide production , mineral nitrogen and dehydrogenase activity were measured in microcosms c ontaining a mixture of leaves from different tree species and a graded seri es of concentrations of the pesticides. After a pre-incubation period of tw o weeks isopods, Porcellionides pruinosus, were introduced to part of the m icrocosms to estimate the effect of the pesticides on microflora alone and on microflora plus isopods. Chemical analysis of pesticide residues reveale d first order degradation kinetics with half-lives of 32 days for benomyl carbendazim, and 8.9 days for diazinon. Degradation rates were, however, l ower at the higher dose rates. LC50 values for benomyl and diazinon towards P. pruinosus were respectively 21,400 mu g/g and 110 mu g/g in litter. Jud ged by the toxic unit approach, benomyl and diazinon acted synergistically in their toxicity to isopods (the toxic strength of the mixture was 0.17 un its). Isopods enhanced respiration and dehydrogenase activity in the litter . The effects of the pesticides on isopods in the microcosm could be predic ted well from earlier results obtained in single-species tests. Dehydrogena se and respiration were hardly affected by benomyl, but it decreased nitrif ication at high concentrations. Diazinon at 400 mu g/g reduced respiration, dehydrogenase and nitrification. The data illustrate that microcosms with detritivore-microbial interactions are a useful additional instrument in th e risk assessment of chemicals in soil.