Importance of indirect nitrous oxide emissions at the field, farm and catchment scale

Abstract

Direct and indirect nitrous oxide (N 2O) emissions and leaching losses from an intensively managed grazed pasture in the Ythan catchment, Aberdeenshire, UK, were measured and compared over a 17-month period. Simultaneous measurements of farm-wide leaching losses of N 2O were also made and catchment-wide fluxes were estimated from existing N leaching data. The relative importance of direct and indirect N 2O fluxes at the field, farm and catchment scale was then assessed. At the field scale we found that direct N 2O emissions were low (1.2 kg N ha −1 year −1, 0.6% of N input) with indirect N 2O emissions via drainage waters comprising a significant proportion (25%) of total N 2O emissions. At the whole-farm scale, the N 2O-N emission factor (0.003) for leached NO 3-N (EF 5-g) was in line with the IPCC's recent downward revision. At the catchment scale, a direct N 2O flux of 1.9 kg N ha −1 year −1 and an indirect flux of 0.06 kg N 2O-N ha −1 year −1 were estimated. This study lends further support to the recent downward revision of the IPCC emission factor for N 2O arising from leached N in surface and ground waters (EF 5-g) and highlights the need for multiple point sampling to ensure that the importance of indirect N 2O losses via drainage waters is not misrepresented at the farm and catchment scales.