Hydrologic and Biogeochemical Drivers of Riparian Denitrification in an Agricultural Watershed

This study investigated drivers of denitrification and overall NO3- removal in an agricultural riparian area in central New York. Denitrification was measured using an in situ push-pull method with N-15-NO3- as a tracer during summer and fall 2011 at a pair of riparian sites characterized by different hydrologic regimes. Median denitrification rates were 1347 and 703 mu g N kg soil(-1) day(-1) for the two study sites. These rates are higher than those reported for other riparian areas, emphasizing the role of some riparian areas as hotspots of NO3- removal. N2O production was significantly higher at one site, demonstrating that riparian areas can be a greenhouse gas source under certain conditions. Denitrification was negatively correlated with groundwater flux, suggesting that slower flushing of water, and thus longer residence time, promotes denitrification. A mass balance of NO3- loss revealed that denitrification only accounted for 5-12% of total NO3- loss, and production of NH4+ indicated that dissimilatory NO3- reduction to NH4+ (DNRA) may be occurring at both sites. While both sites were characterized by high NO3- removal, differences in denitrification rates and NO3- removal processes demonstrate the need to improve our ability to capture spatial and process heterogeneity in landscape biogeochemical models.