Biogeochemistry of Methane Exchange between Natural Wetlands and the Atmosphere

This review examines the interactions among physical, chemical, and biological factors responsible for methane (CH4) emission from natural wetlands. Methane is a chemically and radiatively important atmospheric trace gas. Emission from wetlands is a significant component of the atmospheric CH4 budget, releasing 145 Tg CH4 annually to the atmosphere, or about 25% of total emissions from all anthropogenic and natural sources. Wetlands are characterized by a subsurface, anaerobic zone of CH4 production by methanogenic bacteria and an surficial, aerobic zone of CH4 oxidation by methanotrophic bacteria. Wetlands transfer CH4 to the atmosphere by diffusion, ebullition, and by transport through arenchymous vascular plants. However, about 20 to 40% of the CH4 produced in anaerobic wetland soils is oxidized in the rhizosphere and in surficial oxic layers during diffusive transport to the soil surface. Rates of CH4 emission in wetlands are commonly 100 mg m-2 day-1, and represent the net effect of production and consumption. Water table position, temperature, and plant community composition are important ecosystemlevel controls on wetland CH4 emission.