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.