Nitrous oxide emission from agricultural drainage waters

Uncertainty about the amounts of the greenhouse gas nitrous oxide (N-2 O), which arise from N leaching from agricultural soils, predominantly as nitrate (NO3 (-) ), is large. To date, the bulk of studies of N-2 O in aquatic systems have relied upon measurement of dissolved N-2 O 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 N-2 O concentrations in agricultural drainage water with novel measurement of net N-2 O emission from the same drainage system, we found that dissolved N-2 O in agricultural drainage water was very rapidly lost to the atmosphere, while dissolved NO3 (-) in the same water was conserved. Consequently, the N-2 O 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 N-2 O emission from the drainage water confirmed the disappearance of N-2 O 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 N-2 O 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 N-2 O 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 N-2 O actually released in the ditches gave values for EF5-g (the IPCC's emission factor for N-2 O 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 N-2 O 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.