The effects of long-term N fertilizer management history and field drainage on soil microbial biomass N and C and its activity in the surface soil of a poorly-drained pasture soil in S.W. England were examined periodically for 1 year. The immediate effects of changes in N fertilizer inputs were also determined. In general, there were few differences in the trends of biomass C and N with time, nor of drainage, withdrawal of fertilizer N from a previously-fertilized soil, or addition of fertilizer N to a previously-unfertilized soil. However, soil microbial biomass N and C were substantially greater in the previously-unfertilized than in the fertilized treatment. Measurements of ATP, enzyme activity and respiration made on one occasion indicated, in contrast to the microbial biomass status, significant effects of both the short- and long-term treatments. There was also a substantial difference in the numbers of culturable bacteria with over 4 times less being present in soils without previous N application, again in contrast to total biomass contents: cfu of fungi were not influenced by any of the treatments. Important differences in microbial activity in response to short-term management changes were therefore demonstrated which may have an effect on N cycling in grassland soils. There is a need to understand the factors controlling these soil microbial characteristics and to identify and quantify differences in microbial community structure, in order to make greater progress in increasing the efficiency of N utilization. © 1995.