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Nitrous oxide emission from agricultural drainage waters

Uncertainty about the amounts of the greenhouse gas nitrous oxide (N2O), which arise from N leaching from agricultural soils, predominantly as nitrate (NO3–), is large. To date, the bulk of studies of N2O in aquatic systems have relied upon measurement of dissolved N2O concentrations at wide spatial intervals (of the order of km) down a stream, river or estuary. When we combined a fine‐scale (m) assessment of N2O concentrations in agricultural drainage water with novel measurement of net N2O emission from the same drainage system, we found that dissolved N2O in agricultural drainage water was very rapidly lost to the atmosphere, while dissolved NO3– in the same water was conserved. Consequently, the N2O emission factor (as a proportion of the nitrate‐N present, the IPCC's ‘EF5’) fell by a factor of more than 5 within only 100 m. Direct measurement of N2O emission from the drainage water confirmed the disappearance of N2O as being due to emission from water to the atmosphere, rather than in situ consumption by denitrification. Our findings indicate that making widely spaced measurements of dissolved N2O concentration and/or emissions from the water surface will not take account of this much more dynamic behaviour over short distances. Realistic assessment of the ‘indirect’ agricultural emissions of N2O from leached N will necessitate much more intensive sampling of the whole drainage system, from ditch to stream to river to estuary, accompanied by measurements of in‐stream production. The quantities of N2O actually released in the ditches gave values for EF5‐g (the IPCC's emission factor for N2O from surface drainage and groundwaters) of between 0.02 and 0.03%, compared with the IPCC value of 1.5%. For the latter to be realistic, the quantity of N2O required to be formed after the initial entry of water into the drainage system would need to exceed the initial load by the order of 50‐fold.