Indirect nitrous oxide emissions from surface water bodies in a lowland arable catchment: A significant contribution to agricultural greenhouse gas budgets?

In the UK agriculture is by far the largest source of nitrous oxide (N2O) emissions. Direct N2O emissions as a result of nitrogen (N) application to soils have been well documented in the UK, whereas indirect emissions produced in surface waters and groundwaters from leached N are much less understood with limited data to support IPCC emission factors. Indirect emissions were studied in surface waters in the Upper Thurne, a lowland drained arable catchment in eastern England. All surface waters were found to have dissolved N2O concentrations above that expected if in equilibrium with ambient concentrations, demonstrating all surface waters were acting as a source of N2O. The drainage channels represented 86% of the total indirect N2O flux, followed by wetland areas, 11%, and the river, 3%. The dense drainage network was found to have the highest dissolved N2O concentrations of all the water bodies studied with a combined N2O flux of 16 kg N2O–N per day in March 2007. Such indirect fluxes are comparable to direct fluxes per hectare and represent a significant proportion of the total N2O flux for this catchment. Separate emission factors were established for the three different surface water types within the same catchment, suggesting that the one emission factor used in the Intergovernmental Panel on Climate Change (IPCC) methodology for predicting all indirect N2O emissions is inappropriate.