Soil Processes and Greenhouse Effect

Soil can be a major source or sink of principal greenhouse gases such as CO 2 , CH 4 , and N 2 O (Bouwman, 1980; Lal et al., 1995a ; b). The relative contribution to the greenhouse effect is estimated at about 50% by CO 2 , 18% by CH 4 , 6% by N 2 O, 14% by CFCs, and 13% by other gases ( Boyle and Ardill, 1989 ). Carbon dioxide presently constitutes about 355 parts per million by volume (ppmv) in the atmosphere, and is increasing at 0.5% per annum ( Adger and Brown, 1994 ; IPCC, 1995 ). Principal C pools comprise: biota or biomass C estimated at 550 to 830 Pg (Pg = petagram = 10 15 ), soil organic carbon at 1500 to 2000 Pg, atmosphere at 720 Pg, fossil fuel reserves at 6000 Pg, and oceans at 38,000 Pg. These pools interact with one another through fluxes and gaseous exchange. The IPCC (1995) estimate fossil fuel emissions at 5.4 Pg C/yr, emissions from deforestation at 1.6 Pg C/yr, increase in atmospheric CO 2 concentration at 3.2 Pg C/yr, and absorption by the oceans at 2.0 Pg C/yr. It is estimated that about 40% of anthropogenic emissions (by fossil fuel emissions and land use change or deforestation) is absorbed by the atmosphere. The present rate of increase in the atmospheric concentration of CO 2 is less than half of that expected. There is, therefore, an unknown sink (most likely terrestrial) at about 1.8 Pg C/yr ( Tans et al., 1990 ; Watson et al., 1992 ; Taylor, 1993 ; Brown et al., 1992 ; Houghton, 1993 ). World soils, especially those in the northern latitudes and highly productive soils under arable, pastoral and silvicultural land uses may be important sinks ( Jenkinson, 1971 ; Duxbury and Mosier, 1993 ; Tate, 1992 ). This missing or unknown sink is of critical importance in identifying policy issues for carbon sequestration within terrestrial ecosystems.