Rice cultivation

At 25-60 Tg CH4/year, rice agriculture is a substantial anthropogenic source of atmospheric CH4, possibly the biggest of all anthropogenic sources. The warm, waterlogged soil of rice paddies provides ideal conditions for methanogenesis and, though some of the CH4 produced is usually oxidized by methano-trophs in the shallow overlying water, much is released into the atmosphere.

Rice is grown very widely and rates of CH4 emissions may vary greatly between different areas. Differences in average temperature, water depth and the length of time during which the rice paddy soil is waterlogged can all result in big regional and seasonal variations. However, CH4 emissions from worldwide rice agriculture have been well studied in recent years and fairly reliable estimates of global emissions now exist.

On average, rice paddy soils are only fully waterlogged for about 4 months each year. For the rest of the time methanogenesis is generally much reduced and, where the soil dries out sufficiently, rice paddy soil can become a temporary sink for atmospheric CH4.

With an increasing world population, reductions in rice agriculture remain largely untenable as a CH4 emissions reduction strategy. However, through a more integrated approach to rice paddy irrigation and fertilizer application, substantial reductions remain possible. Many rice varieties can be grown under much drier conditions than those traditionally employed, with large reductions in CH4 emissions without any loss in yield. Additionally, there is the potential for improved varieties of rice to produce a much larger crop per area of rice paddy, and so allow for a cut in the area of rice paddies, without a cut in rice production. Finally, the addition of compounds such as ammonium sulphate, which favour activity of other microbial groups over that of the methanogens, has proved successful under some conditions.

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