Variation in rates of decomposition can be explained in part by physical factors (temperature and moisture) and the chemistry and quantity of available substrate (C:N ratio and lignin content).  These factors however, do not explain variation in rates of decomposition where climate and substrate are held constant.  The objectives of our research were to: 1) measure the impacts of a long-term nitrogen (N) fertilization treatment on characteristics of the microbial community most likely to influence rates of decomposition and 2) determine if variation in the response of the microbial community to long-term N addition could explain previously observed patterns of litter decomposition across a range of sites varying in aboveground plant cover.  At eight sites, representing five different forested and grassland plant communities, we measured microbial biomass, community composition, and the activity of extracellular enzymes involved in lignin and cellulose decomposition.  Based on results from previous studies, we hypothesized that N fertilization would increase bacterial:fungal ratios, increase cellulose degrading enzyme activity, and decrease lignin degrading enzyme activity and that these measurements would correlate with rates of decomposition.  Results from mineral soil assays showed that in general, N had positive effects on the activity of several cellulose degrading enzymes, but that the response was not consistent across all sites and was not predictable based on plant community composition.  Contrary to our expectations, N also increased lignin degrading enzyme activity at several sites, but this response varied across different sampling dates.  This study is the first to measure N effects on microbial enzyme activity in litter and soil simultaneously at multiple sites that vary in aboveground plant composition.  Our results reveal unexplained and previously underappreciated variation in the response of microbial communities to long-term N addition, even among nearby sites with similar soils.