Nitrous Oxide Emissions From Cropped Fields

Abstract From mid‐May to mid‐September 1978, nitrous oxide (N 2 O) emissions from an irrigated corn ( Zea mays L.) field in northern Colorado totaled only 2.5 kg N ha −1 , and even smaller losses were measured from a nearby sugarbeet ( Beta vulgaris L.) field. Fluxes measured by a simple soil cover method compared favorably with micrometeorological estimates of vertical N 2 O flux density. About 30% of the N 2 O lost from the corn field was emitted during the 2 weeks following fertilization while NH 3 was being rapidly nitrified, and 59% was evolved during the week following the field's first irrigation, when restricted oxygen diffusion favored denitrification. Other occurrences of irrigation or precipitation exceeding 0.7 cm were also followed by rapid, though much smaller, increases in N 2 O emissions. The flux of N 2 O was not significantly correlated with soil nitrate concentration but was strongly correlated with soil water content and N 2 O concentration in the soil atmosphere, which always exceeded the ambient atmospheric concentration. We found no evidence that either site ever behaved as a sink for tropospheric N 2 O. Total N 2 O emissions from the corn field amounted to only 1.3% of the 200 kg NH 3 ‐N ha −1 applied to the crop, a much smaller fraction than has been used in models predicting the effect of agricultural fertilizers upon stratospheric ozone depletion.