Nitrogen enrichment and the emission of nitrous oxide from streams

Nitrous oxide (N 2 O) is a potent greenhouse gas produced during nitrogen cycling. Global nitrogen enrichment has resulted in increased atmospheric N 2 O concentrations due in large part to increased soil emissions. There is also a potentially important flux from streams, rivers and estuaries; although measurements of these emissions are sparse, and role of aquatic ecosystems in global N 2 O budgets remains highly uncertain. Using the longest‐term measurements of N 2 O fluxes from streams to date, we found annual fluxes from 14 sites in five streams of south‐central Ontario, Canada varied widely–from net uptake of 3.2 ± 0.2 (standard deviation) μ mol N 2 O m −2 d −1 to net release of 776 ± 61 μ mol N 2 O m −2 d −1 . N 2 O consumption was associated with very low nitrate concentrations (<2.7 μ M). Mean annual (log‐transformed) N 2 O emissions from our study streams (across sites and years) were positively related to nitrate concentrations ( r 2 = 0.59).This nitrate‐N 2 O relationship can be generalized across all 20 streams (in Canada, Japan, Mexico, and the midwestern United States) for which published data now exist and could provide a new basis for the IPCC to calculate agricultural emissions from streams. In addition to predicting annual emissions, we present the first measurements of N 2 O concentrations under ice in streams. Nitrate was a strong predictor of N 2 O % saturation during periods of ice cover ( r 2 = 0.89), when gas exchange is negligible. Given the small surface area of streams within a catchment and the fact our measured areal fluxes are comparable to reported fluxes from agricultural soils, this suggests streams are a small regional N 2 O source.