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Researchers have identified a group of soil-dwelling microbes that help make Asian rice-farming one of the world's largest sources of a major greenhouse gas.

Methane is the most important greenhouse gas, after carbon dioxide. Although more carbon dioxide is emitted, methane is thought to be 20 times more effective at trapping heat in the atmosphere.

Much of the methane produced by human activity originates in rice fields, 90 per cent of which are in Asia.

In a study published today (11 August) in Science, Yahai Lu, of the China Agricultural University, and Ralf Conrad, of the German Max-Planck-Institute for Terrestrial Microbiology, identified a group of microbes that play a big role in producing these emissions.

The microbes absorb carbon released by the plants' roots and use it to make methane, which is then released into the soil and emitted from there into the atmosphere.

Until now, nobody knew which specific microbes were responsible for this process.

Lu and Conrad exposed rice plants to radioactively labelled carbon dioxide, which the plants absorbed through their leaves, before releasing some of it from their roots into the soil.

By looking at which methane-producing organisms in the soil also contained radioactively labelled carbon, the researchers identified the "central importance" of a group of microbes known as Rice Cluster I in global methane production.

Peter Cox, a climate change expert at the UK Centre for Ecology and Hydrology (CEH), says the information is interesting because his own research indicates that methane emissions will double by 2100.

"Once scientists know which organisms are involved in a particular process, they can focus right down on them and design experiments to work out how important they are," says Andrew Whiteley, head of molecular microbial ecology at CEH.

"If we find they are globally important, we need to understand what factors cause these organisms to produce more or less methane."

Currently, human activities release 300 to 400 million tonnes of methane each year. Their contribution to global warming is equivalent to one-third of that made by man-made carbon dioxide emissions.

Rice fields emit between 50 and 100 million tonnes of methane each year.

Link to full paper in Science

Reference: Science 309, 1088 (2005)

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