Comparison of measured and EF5-r-derived N2O fluxes from a spring-fed river

There is considerable uncertainty in the estimates of indirect N2O emissions as defined by the Intergovernmental Panel on Climate Change's (IPCC) methodology. Direct measurements of N2O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N2O budget. The objectives of this research were to measure the N2O fluxes from a spring-fed river, relate these fluxes to the dissolved N2O concentrations and NO3-N loading of the river, and to try to define the indirect emission factor (EF5-r) for the river. Gas bubble ebullition was observed at the river source with bubbles containing 7.9 mu L N2O L-1. River NO3-N and dissolved N2O concentrations ranged from 2.5 to 5.3 mg L-1 and 0.4 to 1.9 mu g N2O-N L-1, respectively, with N2O saturation reaching 404%. Floating headspace chambers were used to sample N2O fluxes. N2O-N fluxes were significantly related to dissolved N2O-N concentrations (r(2)=0.31) but not to NO3-N concentrations. The N2O-N fluxes ranged from 38 to 501 mu g m(-2) h(-1), averaging 171 mu g m(-2) h(-1) (+/- SD 85) overall. The measured N2O-N fluxes equated to an EF5-r of only 6.6% of that calculated using the IPCC methodology, and this itself was considered to be an overestimate because of the degassing of antecedent dissolved N2O present in the groundwater that fed the river.