Methane maker: method gets to root of gas from rice paddies.

In experiments using a rare isotope of carbon, scientists have singled out microorganisms that appear to be largely responsible for natural emissions of the greenhouse gas methane from rice paddies. The finding may lead to methods to trim this major source of planet-warming gas.

In Earth's atmosphere, natural and industrial greenhouse gases permit sunlight to reach the planet but block heat from escaping to space. Molecule for molecule, methane traps more than 20 times as much heat as carbon dioxide does, but methane is much less prevalent in the atmosphere than carbon dioxide is and so contributes less to the current problem. However, in the past 200 years, methane's atmospheric concentrations have doubled, outpacing increases in atmospheric carbon dioxide.

Microbes living in the oxygen-poor soils of rice paddies are responsible for as much as 25 percent of global methane emissions, says Ralf Conrad of the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany. Those microorganisms produce methane as they consume dead plants and organic substances that ooze from the roots of live plants. The material that the microbes break down contains carbon atoms that the plants took up from carbon dioxide in the atmosphere. Conrad and Yahai Lu of China Agricultural University of Beijing took advantage of that carbon pathway in laboratory experiments designed to identify the methane producers.

Lu and Conrad sealed rice plants in airtight chambers that isolated the soil and roots from gases circulating around the leaves and steins. Seven times each day, the researchers exposed the upper plant to high concentrations of carbon dioxide enriched with atoms of the rare isotope carbon-13. After a week, the researchers found unusually large amounts of carbon-13 in the RNA of a group of microbes previously dubbed Rice Cluster I archaeans but normal amounts in other soil microbes. Scientists had previously observed carbon-13-rich methane emissions from soil within hours of rice-plant exposure to carbon dioxide tagged with carbon-13.

Although the team hasn't yet grown Rice Cluster I archaeans without other soil microbes, the new experiments indicate that Rice Cluster I species play an important role in methane production, the researchers say in the Aug. 12 Science.

Scientists don't know much about the Rice Cluster I group, says Jennifer Y. King, a bio-geochemist at the University of Minnesota in St. Paul. However, the new findings are "an important step to identify how plants influence methane emissions," she says.

Knowing which microbes in a rice paddy produce methane may enable scientists to develop ways to mitigate those emissions, says William S. Reeburgh, a geochemist at the University of California, Irvine. Future research on Rice Cluster I archaeans, Reeburgh says, may reveal practical ways to interfere with a paddy's methane production.