Soil as a source or sink for atmospheric nitrous oxide

Abstract

INCREASED use of nitrogenous fertiliser has been thought to cause1–4 greater nitrous oxide (N2O) production by soils and natural waters. After transport to the stratosphere, this additional N2O may be converted to nitric oxide (NO) and deplete the ozone layer5. However, there is a diversity of opinion about the probable effects of increasing nitrogenous fertiliser use on the ozone layer6–8. The controversy cannot be settled until there is reliable information on (1) the rate of N2O production from both natural sources and fertilised soils; (2) the concentration of N2O in the atmosphere; and (3) the existence and capacity of sinks for N2O at the Earth's surface or in the troposphere9,10. Brice et al.11 have reported evidence for an appreciable but unidentified ground surface sink. We report here the results of investigations designed to identify the soil moisture conditions for which soil could act as that sink. We found, in both the laboratory and the field, that at soil moisture contents less than field capacity, N2O was emitted from soils; there was no evidence that soils in the field ever absorbed N2O. In laboratory studies some N2O was absorbed (as reported elsewhere12), but the phenomenon was transitory, probably resulting from a low oxygen content in the atmosphere above the soil. Nitrous oxide was eventually emitted again when the atmosphere was returned to normal. These results suggest that, in the field, soils are more likely to be sources than sinks of N2O for the atmosphere.