Diurnal fluctuations of dissolved nitrous oxide (N2O) concentrations and estimates of N2O emissions from a spring‐fed river: implications for IPCC methodology

Abstract There is uncertainty in the estimates of indirect nitrous oxide (N 2 O) emissions as defined by the Intergovernmental Panel on Climate Change (IPCC). The uncertainty is due to the challenge and dearth of in situ measurements. Recent work in a subtropical stream system has shown the potential for diurnal variability to influence the downstream N transfer, N form, and estimates of in‐stream N 2 O production. Studies in temperate stream systems have also shown diurnal changes in stream chemistry. The objectives of this study were to measure N 2 O fluxes and dissolved N 2 O concentrations from a spring‐fed temperate river to determine if diurnal cycles were occurring. The study was performed during a 72 h period, over a 180 m reach, using headspace chamber methodology. Significant diurnal cycles were observed in radiation, river temperature and chemistry including dissolved N 2 O‐N concentrations. These data were used to further assess the IPCC methodology and experimental methodology used. River NO 3 ‐N and N 2 O‐N concentrations averaged 3.0 mg L −1 and 1.6 μg L −1 , respectively, with N 2 O saturation reaching a maximum of 664%. The N 2 O‐N fluxes, measured using chamber methodology, ranged from 52 to 140 μg m −2 h −1 while fluxes predicted using the dissolved N 2 O concentration ranged from 13 to 25 μg m −2 h −1 . The headspace chamber methodology may have enhanced the measured N 2 O flux and this is discussed. Diurnal cycles in N 2 O% saturation were not large enough to influence downstream N transfer or N form with variability in measured N 2 O fluxes greater and more significant than diurnal variability in N 2 O% saturation. The measured N 2 O fluxes, extrapolated over the study reach area, represented only 6 × 10 −4 % of the NO 3 ‐N that passed through the study reach over a 72 h period. This is only 0.1% of the IPCC calculated flux.