Soil as a source or sink for atmospheric nitrous oxide

Abstract

INCREASED use of nitrogenous fertiliser has been thought to cause^1–4 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 layer^6–8. 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 troposphere^9,10. 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.