Addition of soluble metal salts to soil in laboratory tests often results in negative effects of elevated metal concentrations on soil microbial processes while these negative effects are some- times not seen in field contaminated soils with similar total metal concentrations. This discrepancy is due to different available metal concentrations in soil. But also changes in soil microbial populations can change the measured response in field contaminated soils. The combination of both different available metal concentrations and changes in microbial population (adapta- tion or acclimatization) can explain the absence of toxic effects in the field. The aim of this investigation is to ifentify the role of both processes. Nitrification is chosen as microbial process and Zn as contaminant. Finally, this should result in a better extrapolation of short-term laboratory test to long-term field contaminations.