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[THIRUVANANTHAPURAM] Rice cultivation in flooded paddies, long vilified as a source of methane emissions that contribute to global warming, actually absorbs carbon rather emits it, says a new study by Indian scientists.

Like all plants, rice takes in carbon dioxide from the air to make food and releases some of it during respiration. Post-harvest, decaying rice stalks and organic matter in the paddies release carbon back to the soil.

When rice fields are flooded during cultivation, conditions of low oxygen availability are created favouring the growth of anaerobic bacteria that release carbon as methane.

According to the latest assessment report of the Intergovernmental Panel on Climate Change in April, flooded paddy fields in tropical countries account for up to 11 per cent of man-made global methane emissions — estimated at between 493 and 723 million tonnes of carbon dioxide equivalent a year.

However, a study published online in the journal Agriculture, Ecosystems and Environment on 1 May shows that carbon inputs in rice fields through photosynthesis, algal biomass and organic carbon added as manure exceed output, making paddies carbon sinks rather than carbon emitters.
Pratap Bhattacharyya, author of the study and senior scientist at the Central Rice Research Institute, Cuttack, and his colleagues measured the amount of carbon dioxide and methane produced by a two-hectare rice field over 2012—2013, including the wet (July-November) and dry (January-May) growing seasons.

Both carbon dioxide and methane are major greenhouse gases that contribute to global warming by trapping heat on the surface of the earth.  
Using a novel ‘eddy covariance’ approach that accurately measures carbon dioxide and methane emissions, the researchers calculated the net gain (or loss) of carbon. They then calculated carbon lost from the system — by harvest, dissolution in water, fire, erosion and bacterial production of methane — to calculate the ‘net carbon balance’.

“Although methane was a source of carbon loss, considering all the components of carbon balance, this ecosystem has a good potential to store a considerable amount of carbon”, Bhattacharya tells SciDev.Net.

Production of methane was found higher during the initial growth stages of the rice plant, when there was plenty of soil organic matter available for anaerobic bacteria to flourish and produce methane.

“Generally, during the growing season of a flooded rice field, there is a net carbon uptake because a lot of carbon dioxide is being absorbed by the plant through photosynthesis and only part of it is released to the atmosphere through respiration,” explains Ma. Carmelita Alberto, a researcher with the International Rice Research Institute, Manila.

“Further studies are needed to check if other rice varieties, and other modes of rice cultivation, also act as carbon sinks,” she adds.

> Link to abstract of the research paper
This article has been produced by SciDev.Net's South Asia desk.