SOURCES AND SINKS OF GREENHOUSE GASES IN THE SOIL-PLANT ENVIRONMENT

The paper is concerned mainly with nitrous oxide, methane and carbon dioxide, which account for more than 70% of predicted greenhouse warming. All three have significant sources in the soil-plant environment and principal sinks in the atmosphere or the oceans. The emphasis is on methodological problems associated with measuring source and sink strengths, but the biogeochemistry of individual gases and problems of scaling to longer times and larger areas are addressed also. Nitrous oxide accounts for some 6% of predicted greenhouse warming. Its atmospheric concentration is 315 ppbv and is increasing at 0.25% per year. The principal sink appears to be destruction through photochemical processes in the stratosphere. The main causes of the N2O increase are thought to be biomass burning, fossil fuel combustion processes, and what now seem to be substantial emissions from soils associated with increased nitrogen inputs, irrigation and tropical land clearing. Uncertainty about the strengths of the soil sources is due largely to our reliance on enclosure techniques for flux measurement, and the lack of appropriate scaling procedures. Methane now accounts for 18% of anticipated greenhouse warming. Its atmospheric concentration is 1.7 ppmv and is increasing at 1% per year. Its greenhouse effect seems likely to increase over the next 50 years. The biggest sink appears to be oxidation in the atmosphere, but some oxidation occurs in soils as well. The main sources are rice fields, wetlands, biomass burning, ruminants, land fills, natural gas production, and coal mining. As for N2O, there is much uncertainty about individual source strengths and there are urgent needs for better measurement and scaling techniques. Increased CO2 concentrations account for 49% of the greenhouse effect. The present atmospheric CO2 concentration is 350 ppmv, increasing at 0.4% per year. Over 80% of the increase is due to fossil fuels, and the rest to deforestation and biomass burning. Atmospheric fluxes of CO2 can be measured much more precisely than those of N2O and CH4, by micrometeorological techniques, but the scaling problem still remains. The largest known sink for CO2 is the oceans, but recent calculations point to a large 'missing' sink for CO2, which may be as yet unidentified sequestering processes in terrestrial ecosystems.