Carbon and nitrogen fluxes in decomposing leaf litter with microbial-detritivore interactions: model simulations compared to microcosm ecotoxicity tests

A model for the interactions between pollutants, isopods and microflora in the decomposition of leaf litter in microcosms was designed. The model describes carbon and nitrogen flows through four pools: litter, available nutrients for microorganisms, store in microorganisms and structural microbial biomass. The influence of isopods on microbial activities was modelled as the conversion of organically bound, unavailable nitrogen to inorganic nitrogen, available for microbial growth. Effects of a pollutant that inhibits maximum microbial growth rate and of a pollutant that inhibits isopod consumption rate were simulated for three different C:N ratios. The model predicts that pollutants inhibiting microbial growth rate influence decomposition negatively at low C:N ratios whereas pollutants inhibiting isopod consumption affect decomposition mainly at high C:N ratios. Comparison of simulation results with experimental data from microcosm experiments using cadmium, fentin and fluoride, demonstrated that the model describes carbon and nitrogen fluxes most adequately when no pollutants are present. When either microflora or isopods were affected by toxicants the model underestimated the observed effects on decomposition processes.