Measurement of field scale N2O emission fluxes from a wheat crop using micrometeorological techniques

Measurements of N₂O emission fluxes from a 3 ha field of winter wheat were measured using eddy covariance and relaxed eddy accumulation continuously over 10 days during April 1994. The measurements averaged fluxes over approximately 10⁵ m² of the field, which was fertilised with NH₄NO₃ at a rate of 43 kg N ha^-1 at the beginning of the measurements. The emission fluxes became detectable after the first heavy rainfall, which occured 4 days after fertiliser application. Emissions of N₂O increased rapidly during the day following the rain to a maximum of 280 ng N m^-2s^-1 and declined over the following week. During the period of significant emission fluxes, a clear diurnal cycle in N₂O emission was observed, with the daytime maximum coinciding with the soil temperature maximum at 12 cm depth. The temperature dependence of the N₂O emission was equivalent to an activation energy for N₂O production of 108 kJ mol^-1. The N₂O fluxes measured using relaxed eddy accumulation, averaged over 30 to 270 min, were in agreement with those of the eddy covariance system within 60%. The total emission of N₂O over the period of continuous measurement (10 days) was equivalent to about 10 kg N₂O-N, or 0.77% of the N fertiliser applied.