THE SIGNIFICANCE OF AGRICULTURAL SOURCES OF GREENHOUSE GASES

The impact of development of land for agriculture and agricultural pro duction practices on emissions of greenhouse gases is reviewed and eva luated within the context of anthropogenic radiative forcing of climat e. Combined, these activities are estimated to contribute about 25%, 6 5%, and 90% of total anthropogenic emissions of CO2, CH4, and N2O, res pectively. Agriculture is also a significant contributor to global emi ssions of NH3, CO, and NO. Over the last 150 y, cumulative emissions o f CO2 associated with land clearing for agriculture are comparable to those from combustion of fossil fuel, but the latter is the major sour ce of CO2 at present and is projected to become more dominant in the f uture. Ruminant animals, rice paddies, and biomass burning are princip al agricultural sources of CH4, and oxidation of CH4 by aerobic soils has been reduced by perturbations to natural N cycles. Agricultural so urces of N2O have probably been substantially underestimated due to in complete analysis of increased N flows in the environment, especially via NH3 volatilization from animal manures, leaching of NO3-, and incr eased use of biological N fixation. The contribution of agriculture to radiative forcing of climate is analyzed using data from the Intergov ernmental Panel on Climate Change (IPCC)(base case) and cases where th e global warming potential of CH4, and agricultural emissions of N2O a re doubled. With these scenarios, agriculture, including land clearing , is estimated to contribute between 28-33% of the radiative forcing c reated over the next 100yr by 1990 anthropogenic emissions of CO2, CH4 , and N2O. Analyses of the sources of agriculturally generated radiati ve climate forcing show that 80% is associated with tropical agricultu re and that two-thirds comes from non-soil sources of greenhouse gases . The importance of agriculture to radiative forcing created by differ ent countries varies widely and is illustrated by comparisons between the USA, India, and Brazil. Some caveats to these analyses include ina dequate evaluations of the net greenhouse effects of agroecosystems, u ncertainties in global fluxes of greenhouse gases, and incomplete unde rstanding of tropospheric chemical processes. Extension of the analyti cal approach to projected future emissions of greenhouse gases (IPCC m oderate growth scenario) indicates that agriculture will become a less important source of radiative forcing in the future. Technological ap proaches to mitigation of agricultural sources of greenhouse gases wil l probably focus on CH4 and N2O because emissions of CO2 are essential ly associated with the socio-political issue of tropical deforestation . Available technologies include dietary supplements to reduce CH4 pro duction by ruminant animals and various means of improving fertilizer N management to reduce N2O emissions. Increased storage of C in soil o rganic matter is not considered to be viable because of slow accretion rates and misconceptions about losses of soil organic matter from agr icultural soils.