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An intense precipitation event causes a temperate forested drainage network to shift from N2O source to sink

Peter A. Raymond and 8 other contributors

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    Abstract

    Nitrous oxide (N2O) evasion from streams and rivers is a significant, yet highly uncertain, flux in nitrogen cycle models. Most global estimates of lotic N2O emission assume that evasion rates are proportional to inorganic nitrogen inputs to a stream or river. However, many field studies do not detect relationships between lotic N2O evasion and dissolved nitrogen concentration, highlighting the need for better understanding of process-based controls on this flux. This study reports 4-yr time series of pN(2)O and N2O evasion from eight nested streams and rivers and detects an abrupt change in N2O dynamics associated with an intense rainstorm. This rainstorm, and the associated hydrologic flood event, pushed forested reaches across the watershed from consistent N2O sources to prolonged N2O sinks. We attribute this shift to disturbance of incomplete denitrification in the stream network and surrounding watershed, although alternate hypotheses are also discussed. There was continued availability of nitrate (NO3-) for in-stream processing, eliminating the possibility that NO3--availability limited N2O production, and post-storm N2O-to-nitrate ratios were lower than pre-storm ratios suggesting that the large storm affected in-situ nitrogen processing rates. The sustained period of post-storm N2O undersaturation resulted in net negative evasion for five of the eight study sites in 2018, which mitigated emissions over the 4-yr study. This nonlinear response in N2O dynamics illustrates the potential importance of storm events to control lotic N2O production and emissions.