Determining the nitrous oxide transfer velocity and emission factor of an agricultural drain

There have been few studies examining indirect nitrous oxide (N2O) emissions (EN2O) associated with nitrogen (N) leaching from agricultural soils. For agricultural drainage water, EN2O equals the water’s N2O concentration in excess of the atmospheric value multiplied by the N2O transfer velocity (VN2O). Using this equation, tracers and chamber methods, EN2O and VN2O were measured from an agricultural drain. Estimates of VN2O were made by measuring water speed and depth using the relationship developed by O’Connor and Dobbins [1958. Mechanisms of reaeration in natural streams. Transactions of the American Society of Civil Engineers. 123:641–684]. The measurements and estimates were not significantly different and VN2O averaged 5 m d−1. Alternatively, for the method developed by the Intergovernmental Panel on Climate Change, EN2O equals the mass of N flowing in the water multiplied by an emission factor (EF). By additional measurements of the drain’s width, the water’s flow rate and nitrate (NO−3 ) concentration, the estimated EF was 1.2 × 10−4 kg N2O − N kg−1 NO−3 − N.