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 ¹⁵N-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 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.