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

Authors


Dr Tim Clough, fax +64 3 325 3607, e-mail: Clought@lincoln.ac.nz

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 h period, over a 180 m 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 to 140 μg m−2 h−1 while fluxes predicted using the dissolved N2O concentration ranged from 13 to 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 6 × 10−4% 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.

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