Flux estimates from soil methanogenesis and methanotrophy: Landfills, rice paddies, natural wetlands and aerobic soils.

Present and future annual methane flux estimates out of landfills, rice paddies and natural wetlands, as well as the sorption capacity of aerobic soils for atmospheric methane, are assessed. The controlling factors and uncertainties with regard to soil methanogenesis and methanotrophy are also briefly discussed.The actual methane emission rate out of landfills is estimated at about 40 Tg yr(-1). Changes in waste generation, waste disposal and landfill management could have important consequences on future methane emissions from waste dumps. If all mitigating options can be achieved towards the year 2015, the CH4 emission rate could be reduced to 13 Tg yr(-1). Otherwise, the emission rate from landfills could increase to 63 Tg yr(-1) by the year 2025. Methane emission from rice paddies is estimated at 60 Tg yr(-1). The predicted increase of rice production between the years 1990 and 2025 could cause an increase of the CH4 emission rate to 78 Tg yr(-1) by the year 2025. When mitigating options are taken, the emission rate could be limited to 56 Tg yr(-1). The methane emission rate from natural wetlands is about 110 Tg yr(-1). Because changes in the expanse of natural wetland area are difficult to assess, it is assumed that methane emission from natural wetlands would remain constant during the next 100 years. Because of uncertainties with regard to large potential soil sink areas (e.g. savanna, tundra and desert), the global sorption capacity of aerobic soils for atmospheric methane is not completely clear. The actual estimate is 30 Tg yr(-1).In general, the net contribution of soils and landfills to atmospheric methane is estimated at 180 Tg yr(-1) (210 Tg yr(-1) emission, 30 Tg yr(-1) sorption). This is 36% of the global annual methane flux (500 Tg yr(-1)).