At between 50 and 100 million tonnes of methane a year, rice agriculture is a big source of atmospheric methane, possibly the biggest of man-made methane sources. The warm, waterlogged soil of rice paddies provides ideal conditions for methanogenesis, and though some of the methane produced is usually oxidized by methanotrophs in the shallow overlying water, the vast majority is released into the atmosphere.

Rice is grown very widely and rates of methane emission may vary greatly between different areas. Differences in average temperature, water depth and the length of time that the rice paddy soil is waterlogged can all result in big regional variations. However, methane emission from worldwide rice agriculture has been well studied in recent years and fairly reliable estimates of global emissions now exist. Emissions from rice paddies can vary hugely during the course of a year.

On average, the rice paddy soil is 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 methane.

Human Impact

Clearly, humans are directly responsible for the world's paddy fields and so also for their methane emissions. The expansion of the human population has necessitated increased rice production and so methane emission from this source. There are, though, strategies which may lessen our impact via this greenhouse gas source as outlined below.

Potential for control

With an increasing world population, reductions in rice agriculture remain largely untenable as on methane emission 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 big reductions on methane emission without any loss in yeild. Additionally, there is the great potential for improved varieties of rice, able 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.