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

Abstract

Abstract There is considerable uncertainty in the estimates of indirect N 2 O emissions as defined by the Intergovernmental Panel on Climate Change's (IPCC) methodology. Direct measurements of N 2 O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N 2 O budget. The objectives of this research were to measure the N 2 O fluxes from a spring‐fed river, relate these fluxes to the dissolved N 2 O concentrations and NO 3 ‐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 μL N 2 O L −1 . River NO 3 ‐N and dissolved N 2 O concentrations ranged from 2.5 to 5.3 mg L −1 and 0.4 to 1.9 μg N 2 O‐N L −1 , respectively, with N 2 O saturation reaching 404%. Floating headspace chambers were used to sample N 2 O fluxes. N 2 O‐N fluxes were significantly related to dissolved N 2 O‐N concentrations ( r 2 =0.31) but not to NO 3 ‐N concentrations. The N 2 O‐N fluxes ranged from 38 to 501 μg m −2 h −1 , averaging 171 μg m −2 h −1 (±SD 85) overall. The measured N 2 O‐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 N 2 O present in the groundwater that fed the river.