Global models have indicated agriculturally impacted rivers and streams may be important sources of the greenhouse gas nitrous oxide (N2O). However, there is significant uncertainty in N2O budgets. Isotopic characterization can be used to help constrain N2O budgets. We present the first published measurements of the isotopic character of N2O emitted from low (2–4) order streams. Isotopic character of N2O varied seasonally, among streams, and over diel periods. On an annual basis, δ18O of emitted N2O (+47.4 to +51.4‰; relative to VSMOW) was higher than previously reported for larger rivers, but δ15N of emitted N2O (−16.2 to +2.4‰ among streams; relative to atmospheric N2) was similar to that of past studies. On an annual basis, all streams emitted N2O with lower δ15N than tropospheric N2O. Given these streams have elevated nitrate concentrations which are associated with enhanced N2O fluxes, this supports the hypothesis that streams are contributing to the accumulation of 15N-depleted N2O in the troposphere.
Diel (24‐h) cycling of dissolved O 2 (DO) in rivers is well documented, but evidence for coupled diel changes in DO and nitrogen cycling has only been demonstrated in hypereutrophic systems where DO approaches zero at night. Here, we show diel changes in N 2 O and DO concentration at several sites across a trophic gradient. Nitrous oxide concentration increased at night at all but one site in spring and summer, even when gas exchange was rapid and minimum water column DO was well above hypoxic conditions. Diel N 2 O curves were not mirror images of DO curves and were not symmetrical about the mean. Although inter‐ and intrasite variation was high, N 2 O peaked around the time of lowest DO at most of the sites. These results suggest that N 2 O must be measured several times per diel period to characterize curve shape and timing. Nitrous oxide concentration was not significantly correlated with NO 3 − concentration, contrary to studies in agricultural streams and to the current United Nations Intergovernmental Panel for Climate Change protocols for N 2 O emission estimation. The strong negative correlation between N 2 O concentration and daily minimum DO concentration suggested that N 2 O production was limited by DO. This is consistent with N 2 O produced by nitrite reduction. The ubiquity of diel N 2 O cycling suggests that most DO and N 2 O sampling strategies used in rivers are insufficient to capture natural variability. Ecosystem‐level effects of microbial processes, such as denitrification, are sensitive to small changes in redox conditions in the water column even in low‐nutrient oxic rivers, suggesting diel cycling of redox‐sensitive compounds may exist in many aquatic systems.
Stable isotopes (15N and 18O) of the greenhouse gas N2O provide information about the sources and processes leading to N2O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N2O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the values at typical concentrations in aquatic ecosystems differ significantly from both the source of N2O and the N2O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N2O, N2O source, and the emitted N2O. If the N2O production rate or isotopic ratios vary, then the N2O concentration and isotopic ratios may vary or be constant, not necessarily concomitantly, depending on the synchronicity of production rate and source isotopic ratios. Thus prima facie interpretation of patterns in dissolved N2O concentrations and isotopic ratios is difficult. The dynamic model may be used to correctly interpret diel field data and allows for the estimation of the gas exchange coefficient, N2O production rate, and the production-weighted values of the N2O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N2O flux to the atmosphere to be assessed.
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