Effects of Zn pollution on a keystone species of forest soils, the enchytraeid Cognettia sphagnetorum, and its consequent indirect effects on microbial biomass and activity were studied in a microcosm experiment using experimentally contaminated humic soil. Microbial growth and decomposition were enhanced in the presence of C. sphagnetorum. At high Zn concentrations (< 2,393 mg/kg dry soil), populations of C. sphagnetorum went extinct, resulting in negative indirect effects on microbial activity as measured by soil respiration. Results indicate that pollution may affect species interactions in a soil food web and indirectly affect ecosystem processes such as decomposition rate. Effects of pollution on keystone organisms may radically alter soil ecosystem functioning and should be taken into account during risk-assessment procedures.