Effects of harmful chemicals on soil animal communities and decomposition

Abstract

Effects of harmful chemicals on soil decomposer commumt1es, decomposition processes and soil fertility were studied in laboratory microcosms containing coniferous forest soil. Pentachlorophenol (PCP) and triazine herbicide (active ingredient was terbuthylazine) were used as contaminants. Humus soil sorbed chemicals efficiently. Hence high concentrations of chemicals were needed before lethal effects on soil organisms were observed. Microbial biomass was reduced by PCP which led to reduced densities of animals at higher trophic levels due to lowered food resources. Patchy PCP contamination affected distribution of soil organisms via lowered preference of the contaminated patches. PCP contamination reduced average size of organisms, and lowered biodiversity and biomass of decomposer community. Fungivorous animals, enchytraeids and predatory mites were sensitive to PCP contamination while some bacterial-feeding animals were indifferent or they even increased their numbers in the contaminated soil. PCP lowered decomposition rate and altered nutrient cycling in the soil. PCP lowered primary production through direct toxicity and altered nutrient cycling. Herbicide stress reduced biomass and diversity of soil decomposers. Although the herbicide was toxic to the predatory mites in the toxicity tests, no lethal effects were observed in a long lasting experiment. However, herbicide indirectly affected microbial feeding animals via lowered hunting activity of stressed predators. Herbicide also altered nutrient cycling in the soil. Decomposer food webs in the heterotrophic microcosms appeared mainly to be bottom-up controlled although predators could occasionally affect some populations of their prey and have cascading effects down to the microbes and their activity. Results clearly showed that due to complex interactions between direct and indirect effects of harmful chemicals on the decomposers, system level monitoring are needed for proper ecological risk assessment in the soil.