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 (N₂O) 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 N₂O production. Studies in temperate stream systems have also shown diurnal changes in stream chemistry. The objectives of this study were to measure N₂O fluxes and dissolved N₂O concentrations from a spring-fed temperate river to determine if diurnal cycles were occurring. The study was performed during a 72 hour period, over a 180m reach, using headspace chamber methodology. Significant diurnal cycles were observed in radiation, river temperature and chemistry including dissolved N₂O-N concentrations. These data were used to further assess the IPCC methodology and experimental methodology used. River NO₃-N and N₂O-N concentrations averaged 3.0 mg L⁻¹ and 1.6 µg L⁻¹ respectively, with N₂O saturation reaching a maximum of 664%. The N₂O-N fluxes, measured using chamber methodology, ranged from 52-140 µg m⁻² h⁻¹ while fluxes predicted using the dissolved N₂O concentration ranged from 13-25 µg m⁻² h⁻¹. The headspace chamber methodology may have enhanced the measured N₂O flux and this is discussed. Diurnal cycles in N₂O % saturation were not large enough to influence downstream N transfer or N form with variability in measured N₂O fluxes greater and more significant than diurnal variability in N₂O % saturation. The measured N₂O fluxes, extrapolated over the study reach area, represented only 6x10⁻⁴ percent of the NO₃-N that passed through the study reach over a 72 h period. This is only 0.1% of the IPCC calculated flux.