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Improving rice yields may reduce emissions of methane, the second most important greenhouse gas after carbon dioxide, according to new research.

A study carried out at the International Rice Research Institute (IRRI) in the Philippines has shown that rice paddies — a major source of the gas — produce less methane when yields are high. Attempts to boost yields, for example by optimising the timing of fertiliser application, could therefore reduce methane emissions.

The researchers believe that the amount of methane produced is greater when yields are low because carbon that would otherwise be stored in the grain is deposited in the soil, where it is used as fuel by methane-producing bacteria.

One consequence of this finding is that the development of varieties of rice that continue to store carbon, even after the grain is full, may also help reduce methane emissions.

Rice production currently accounts for about 10 to 16 per cent of global methane emissions. Previous studies had indicated that more methane is released in the wet season, when yields are lower, than in the dry season. Tests showed that this could not be explained by seasonal temperature differences.

The team of researchers from IRRI, together with scientists from the Wageningen University in the Netherlands and the Fraunhofer Institute for Atmospheric Environmental Research in Germany, have now found that it is differences in yield that cause variations in methane emissions.

They experimentally reduced yield by removing flowers from rice plants — and found that this resulted in increased methane emissions.

This may have implications for ways of cutting greenhouse gas emissions, the researchers say. "Because reducing [human-induced methane] emission may be a major way to minimise global warming over the next several decades, rice agriculture may well contribute to such an approach," they say in this week's journal The Proceedings of the National Academy of Sciences.

The concentration of methane in the atmosphere has more than doubled over the last 200 years, and has increased by about 50 per cent in the last 40 years. The gas is estimated to be responsible for about 15 per cent of climate change caused by human activities.

But Rhoda Lantin, a researcher at IRRI who worked on the study, says that the finding that high yields reduce methane emissions will make little difference to poor farmers.

"In the developing world, rice farmers are in general concerned with maintaining or increasing yields," she says. "Farmers must be persuaded to adopt technologies while avoiding any unnecessary burden to them. If the findings of this study are transferred to farmers through timing fertiliser application [and] developing varieties that tiller late, then the double goal of increasing yield and reducing emissions would be served."

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Photo credit: IRRI