Tree species differ in their effect on soil development and nutrient cycling. Conversion of beech coppice to pine plantations call after soil physical and chemical properties, which ill turn may have significant impacts oil soil microbial biomass C and N (C(mic), N(mic)). The major objective of this study was to evaluate soil quality changes associated with the forest conversion in humid NW Turkey. Results from this study showed that levels of soil organic carbon (C(org)), total nitrogen (N(t)), moisture, C(mic), and N(mic) under beech coppice were consistently higher but levels of pH, CaCO(3) and EC were lower compared to pine plantation. Differences between the forest stands in C(mic) and N(mic) were mainly related to the size of the C(org) stores in soil and to tree species. In addition, high level of CaCO(3) is likely to reduce pools of soil organic C and possibly even microbial biomass C and N in pine forests. The average C(mic):N(mic) ratios were higher in soils under beech coppice than pine plantation, while C(mic):C(org) and N(mic):N(t) percentages were similar in both forest types. These results revealed the differences ill microbial community structure associated with different tree species and the complex interrelationships between microbial biomass, soil characteristics, litter quantity and quality. Copyright (C) 2008 John Wiley & Sons, Ltd.