An intense precipitation event causes a temperate forested drainage network to shift from N 2 O source to sink

Abstract

Nitrous oxide (N 2 O) evasion from streams and rivers is a significant, yet highly uncertain, flux in nitrogen cycle models. Most global estimates of lotic N 2 O 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 N 2 O 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 p N 2 O and N 2 O evasion from eight nested streams and rivers and detects an abrupt change in N 2 O dynamics associated with an intense rainstorm. This rainstorm, and the associated hydrologic flood event, pushed forested reaches across the watershed from consistent N 2 O sources to prolonged N 2 O 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 ( ) for in‐stream processing, eliminating the possibility that ‐availability limited N 2 O production, and post‐storm N 2 O‐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 N 2 O 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 N 2 O dynamics illustrates the potential importance of storm events to control lotic N 2 O production and emissions.

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