Nitrous oxide in agricultural drainage waters following field fertilisation

Dissolved nitrous oxide (NO), nitrate (NO), and ammonium (NH) concentrations in an agricultural field drain were intensively measured over the period of field nitrogen (N) fertilisation and for several weeks thereafter. Supersaturations of dissolved NO were observed in field drain waters throughout the study. On entry to an open drainage ditch, concentrations of dissolved NO rapidly decreased and a total NO-N emission via this pathway of 13.2 g over the period of study (45 days) was calculated. This compared with a predicted emission of the order of 300 g, based on measured losses of NO and NH in the field drainage water, and the default IPCC emission factor of 0.01 kg NO-N per kg N entering rivers and estuaries. In contrast to widespread evidence of a clear relationship between the amount of N applied to agricultural land and subsequent direct NO emission from the soil surface, the relationship between the amount of NO in soil drainage waters and the amount of N applied was poor. We conclude that the complexity, both spatially and temporally, of the processes ultimately responsible for the amount of NO in agricultural drainage waters make a straightforward relationship between NO concentration and N application rate unlikely in all but the simplest of systems.