North American beavers (Castor canadensis), which were functionally extinct from the Northeast in 1900, began to rebound at remarkable rates in the latter half of the 20^th century. The return of beavers and the ponds and dams they create are reshaping headwater stream networks to a complex of ponds, wetlands, and connecting streams. Beaver ponds are likely to increase the denitrification and methanogenesis potential of fluvial systems due to the creation of extended retention times and increased storage of labile carbon. Three RI beaver ponds with a range in age (approximately 5 to 25 years) and size (530 to 80,000 m²) were studied through core mesocosm experiments to examine denitrification and soil nitrogen (N) transformations of the pond soil.  We obtained a nitrogen mass balance from pond sediment mesocosms amended with 50 atom % ¹⁵N-nitrate. We measured: 1) denitrification rates and mass of N denitrified; 2) changes in the mass of dissolved inorganic nitrogen (DIN) from soil mineralization; 3) changes in soil microbial biomass N; and 4) changes in the mass of total soil C and N. We hypothesized that seasonality would play a role in how nitrate-N is being transformed. An increase in labile carbon in the fall from increased leaf litter may lead to increased rates of denitrification and uptake of N by microbial biomass. Larger ponds, which can trap more sediment, are likely to have higher rates of N retention and transformation. Preliminary results show DIN transformations average 2.09 mg m^-2 hr^-1. Microbial biomass N results are comparable to published wetland biomass N estimates at 6.4 mg NH₄-N kg^-1 dry soil. Results from this study will lead to a better understanding of N cycling in beaver ponds and can be used in watershed modeling of N sinks.