Indirect Nitrous Oxide Emission Factors for Agricultural Field Drains and Headwater Streams

Agriculture is a major source of nitrous oxide (N₂O) emissions, a potent greenhouse gas. While direct N₂O emissions from soils have been widely investigated, indirect N₂O emissions from nitrogen (N) enriched surface water and groundwater bodies are poorly understood. In this contribution, indirect N₂O emissions from subsurface agricultural field drains and headwater streams were monitored over a two-year period (2013-2015) in an intensive arable catchment in eastern England. Indirect N₂O emission factors for groundwater (EF_5g) and surface runoff (EF_5r) were calculated for both field drain and streamwater samples, respectively, using two approaches: the N₂O-N/NO₃-N ratio and the IPCC (2006) methodology. Mean EF_5g values derived from the N₂O-N/NO₃-N ratio were 0.0012 for field drains and 0.0003 for streamwater. Using the IPCC (2006) methodology, the mean EF_5g values were 0.0011 for field drains and 0.0001 for streamwater. Thus, EF values derived from both methods were below the current IPCC (2006) default value of 0.0025 and a downward revision to 0.0012 for EF_5g and 0.0002 for EF_5r is recommended. Such revision would halve current estimates of N₂O emissions associated with nitrogen leaching and runoff from agriculture for both the UK and globally.