Methane Efflux from Wetland Rice Fields

Wetland rice fields release significant amounts of methane, an important greenhouse gas, into the atmosphere. The mixing ratio of methane in the troposphere is increasing more rapidly than any of the other long-lived radiatively active atmospheric trace gases. Methane has increased from its preindustrial level of about 700 ppbv to 1,720 ppbv at present (IPCC, 1995). The annual increase in tropospheric methane concentration results from a source/sink imbalance of about 37 Tg and a reduction in anthropogenic emissions of only 8% is sufficient to eliminate the annual increase (IPCC, 1995). The overall atmospheric balance between source and sinks is relatively well known while the intensity of individual sources is highly uncertain. The major sink is photochemical oxidation with hydroxyl radical in the troposphere. Upland soils have been recognized recently as a significant but much smaller sink. The total annual source strength is about 500 Tg. Isotopic measurements reveal that 70–80% of atmospheric methane is of biogenic origin. Estimates for most of the biological sources are very uncertain and vary by a factor of two or more. Therefore, a great deal of recent attention has been focused on understanding and quantifying the strength of different sources. Global methane emission from wetland rice fields is estimated to be 60 Tg yr-1 with a range of 20 to 100 Tg yr-1 (IPCC, 1995).