Diazotrophs, microorganisms that fix atmospheric N2 into NH4, fill a crucial role in the nitrogen cycle providing the dominant natural source of fixed N in the biosphere. Model systems for N-fixation in soil are generally based on cultivated and symbiotic diazotrophs. However many of the diazotrophs in soils belong to groups that have yet to be cultivated in isolation. In soils, the non-cultivated free-living diazotrophs may play an important role for N fixation. In this project we sought to characterize the community structure of the free-living diazotrophs in soil and to determine whether changes in diazotroph community composition had functional consequences for Nfixation in soils. We examined diazotroph community, soil characteristics, and Nfixation rates over time in a long-term agricultural experiment located at the William H. Miner Institute in Chazy, Clinton County, New York. The results of this project show that agricultural management practices fundamentally altered the structure of the microbial community and that these changes in turn impacted the rate of N-fixation that was observed. The association between diazotroph diversity and N-fixation rates was not a simple function of richness but rather seemed influenced by short term temporal changes in community structure. N-fixation rates varied significantly over time and these changes were more strongly correlated to temporal variation in diazotroph community composition than to temporal variation in soil characteristics. The results suggest that controls on N-fixation are dynamic and are responsive to the composition of the diazotrophic community. Since the microbial community is itself responsive to land management practices there results significant interactions between land management practices and temporal variation in N-fixation rates.