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
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.
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
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.