Determinants of Nitrous Oxide Emission from Agricultural Drainage Waters

Emissions of the powerful greenhouse gas nitrous oxide (N2O) from agricultural drainage waters are poorly quantified and its determinants are not fully understood. Nitrous oxide formation in agricultural soils is known to increase in response to N fertiliser application, but the response of N2O in field drainage waters is unknown. This investigation combined an intensive study of the direct flux of N2O from the surface of a fertilised barley field with measurement of dissolved N2O and nitrate (NO3) concentrations in the same field's drainage waters. Dissolved N2O in drainage waters showed a clear response to field N fertilisation, following an identical pattern to direct N2O flux from the field surface. The range in N2O concentrations between individual field drains sampled on the same day was large, indicating considerable spatial variability exists at the farm scale. A consistent pattern of very rapid outgassing of the dissolved N2O in open drainage ditches was accentuated at a weir, where increased turbulence led to a clear drop in dissolved N2O concentration. This study underlines the need for carefully planned sampling campaigns wherever whole farm or catchment N2O emission budgets are attempted. It adds weight to the argument for the downward revision of the IPCC emission factor (EF5-g) for NO3 in drainage waters