Studies of Nitrous Oxide Emission from a Grass Sward

Abstract The exchange of nitrous oxide (N 2 O) between soils and the atmosphere is believed to play an important part in controlling stratospheric ozone levels, but so far, very little information is available on rates of emission or absorption of N 2 O by soils in the field. The paper reports measurements of N 2 O exchange made over 5 months from winter to summer in an unfertilized, mown, grass sward. Ambient air was drawn continuously in an open flow system through the above‐ground portion of a steel cylinder driven into the ground. The enrichment or depletion of N 2 O in the air stream was measured with an infrared gas analyzer. Exchange rates as small as 1 ng N m −2 sec −1 could be measured. Nitrous oxide was always emitted from the sward, even at very low soil moisture contents. The emission showed a clear diurnal cycle in phase with the temperature of the surface soil; peak emission rates occurred in the afternoon, minima near sunrise. Additions of water as small as 5 mm increased emission rates markedly. Both results suggest that most of the N 2 O was produced close to the soil surface rather than at depth in the soil profile. Further, the production of N 2 O was accompanied by a net increase in soil nitrate. Emissions were largest in the spring. With the soil moist to less than field capacity, average daily emissions over periods of several days ranged from 0.6 to 2.5 mg N m −2 .