ABSTRACT
The exchange of gases between the biosphere and the atmosphere has had a profound effect on the development of the Earth environment. Globally, vegetation (principally forests) releases and absorbs about 60 billion tonnes of carbon dioxide, CO2, annually and it is the perturbation of this exchange by additional anthropogenic emissions of about a tenth of this quantity that is the principal contribution to global warming-the “greenhouse effect.” Emissions of methane from natural wetlands, rice fields, and landfills, and of nitrous oxide from fertilized agricultural soils and the soils of tropical rainforests, add to global warming. A further contribution comes from tropospheric ozone, produced by reactions involving volatile organic compounds from natural vegetation, and NOx, which comes variously from combustion sources and from soils. Soil surfaces can, on the other hand, act as sinks for many pollutant gases in the atmosphere, through both physicochemical sorption and microbial oxidation. Improved methods of measurement of the fluxes of these gases have become of major environmental importance, both to determine flux levels and in order to improve understanding of the processes involved, to aid the prediction of future trends.
