IMPACT OF AGRICULTURE ON SOIL CONSUMPTION OF ATMOSPHERIC CH4 AND A COMPARISON OF CH4 AND N2O FLUX IN SUB-ARCTIC, TEMPERATE AND TROPICAL GRASSLANDS

Increasing concentrations of methane (CH4) in the atmosphere are proje cted to account for about 25% of the net radiative forcing. Biospheric emissions of CH4 to the atmosphere total approximately 400 Tg C y(-1) . An estimated 300 Tg of CH4-C y(-1) is oxidized in the atmosphere by hydroxyl radicals while about 40 Tg y(-1) remains in the atmosphere. A pproximately 40 Tg y(-1) of the atmospheric burden is oxidized in aero bic soils. Research efforts during the past several years have focused on quantifying CH4 sources while relatively less effort has been dire cted toward quantifying and understanding the soil sink for atmospheri c CH4. Recent research has demonstrated that land use change, includin g agricultural use of native forest and grassland systems has decrease d the soil sink for atmospheric methane. Some agricultural systems con sume atmospheric CH4 at rates less than 10% of those found in comparab le undisturbed soils. While it has been necessary to change land use p ractices over the past centuries to meet the required production of fo od and fiber, we need to recognize and account for impacts of land use change on the biogeochemical nutrient cycles in the biosphere. Change s that have ensued in these cycles have and will impact the atmospheri c concentrations of CH4 and N2O. Since CH4 and N2O production and cons umption are accomplished by a variety of soil microorganisms, the infl uence of changing agricultural, forest, and, demographic patterns has been large. Existing management and technological practices may alread y exist to limit the effect of land use change and agriculture on trac e gas fluxes. It is therefore important to understand how management a nd land use affect trace gas fluxes and to observe the effect of new t echnology on them. This paper describes the role of aerobic soils in t he global CH4 budget and the impact of agriculture on this soil CH4 si nk. Examples from field studies made across subarctic, temperate and t ropical climate gradients in grasslands are used to demonstrate the in fluence of nutrient cycle perturbations on the soil consumption of atm ospheric CH4 and in increased N2O emissions.