Impact of agriculture on soil consumption of atmospheric CH4 and a comparison of CH4 and N2O flux in subarctic, temperate and tropical grasslands

Increasing concentrations of methane (CH4) in the atmosphere are projected 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. Approximately 40 Tg y(-1) of the atmospheric burden is oxidized in aerobic soils. Research efforts during the past several years have focused on quantifying CH4 sources while relatively less effort has been directed toward quantifying and understanding the soil sink for atmospheric CH4. Recent research has demonstrated that land use change, including agricultural use of native forest and grassland systems has decreased the soil sink for atmospheric methane. Some agricultural systems consume atmospheric CH4 at rates less than 10% of those found in comparable undisturbed soils. While it has been necessary to change land use practices over the past centuries to meet the required production of food and fiber, we need to recognize and account for impacts of land use change on the biogeochemical nutrient cycles in the biosphere. Changes that have ensued in these cycles have and will impact the atmospheric concentrations of CH4 and N2O. Since CH4 and N2O production and consumption are accomplished by a variety of soil microorganisms, the influence of changing agricultural, forest, and, demographic patterns has been large. Existing management and technological practices may already exist to limit the effect of land use change and agriculture on trace gas fluxes. It is therefore important to understand how management and land use affect trace gas fluxes and to observe the effect of new technology on them. This paper describes the role of aerobic soils in the global CH4 budget and the impact of agriculture on this soil CH4 sink. Examples from field studies made across subarctic, temperate and tropical climate gradients in grasslands are used to demonstrate the influence of nutrient cycle perturbations on the soil consumption of atmospheric CH4 and in increased N2O emissions.