Hydrologic and Biogeochemical Drivers of Riparian Denitrification in an Agricultural Watershed

McPhillips, Lauren E.; Groffman, Peter M.; Goodale, Christine L.; Walter, M Todd

Hydrologic and Biogeochemical Drivers of Riparian Denitrification in an Agricultural Watershed

2015

McPhillips, Lauren E. | Groffman, Peter M. | Goodale, Christine L. | Walter, M Todd

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 μg 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.

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