Nitrogen enrichment and the emission of nitrous oxide from streams

Nitrous oxide (N2O) is a potent greenhouse gas produced during nitrogen cycling. Global nitrogen enrichment has resulted in increased atmospheric N2O 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 N2O budgets remains highly uncertain. Using the longest-term measurements of N2O 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) mu mol N2O m(-2) d(-1) to net release of 776 +/- 61 mmol N2O m(-2) d(-1). N2O consumption was associated with very low nitrate concentrations (<2.7 mu M). Mean annual (log-transformed) N2O emissions from our study streams (across sites and years) were positively related to nitrate concentrations (r(2) = 0.59). This nitrate-N2O 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 N2O concentrations under ice in streams. Nitrate was a strong predictor of N2O % 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 N2O source.