A lot of gas

An Indian scientists has challenged the estimates of methane -- a greenhouse gas -- produced from paddy fields

 
Last Updated: Saturday 04 July 2015

-- (Credit: DRR)RICE staple diet of a large part of the world, was not entirely benign. Paddy fields, existing predominantly in tropical developing countries that account for about 90 per cent of globsl rice production, have been blamed for producing vast quantities of global-warming methane. But now, the former director of the Delhi-based Indian Agricultural Research Institute, Suresh K Sinha, has challenged the estimates of methane emissions from rice fields made by the Intergovernmental Panel on Climate Change (IPCC), an international group of scientists studying climate change.

Sinha's estimates are about 15 times less than the IPCC's figures. His conclusions are based on controlled laboratory studies of methane produced by submerged soils, and on an analysis of the mechanisms by which methane is produced in paddy fields. Says Sinha, "I believe that the (IPCC) figure will be further reduced to a more realistic and scientifically correct figure."

Sinha contends that the original IPCC estimates were highly exaggerated as they were based on extrapolations from a few studies of limited scope, done in temperate climates. The estimates, he says, overlook the biological and agricultural aspects of methane production in paddy fields. Their average annual methane emissions was originally estimated at 110 million tonnes in 1990, but further studies gave a figure of 60 million tonnes in 1992, which was revised to 37 million tonnes in the latest IPCC report released late last year.

Sinha contends that the studies which formed the basis for the IPCC estimates failed to adequately emphasise the role the soil plays in methane production. The rice plant does not by itself produce any methane: the gas is released when certain bacteria degrade organic matter -- the source of carbon for methane which is made up of carbon and hydrogen molecules -- present in the soil. However, the rice plant roots leach certain organic acids and sugars into the soil which bacteria can convert into methane.

Sinha ran a series of experiments to determine the relationship between organic matter and the amount of methane produced. Using glucose as a carbon source, he simulated conditions that would occur in submerged soils on which rice is grown. He found that the microorganisms in the soil converted the glucose to methane at an efficiency of 10 and 11 per cent at 30 and 35 0 C respectively.

Based on this efficiency value, Sinha challenges the IPCC 1990 estimate of 110 million tonnes. He says that to produce that amount of methane, some 1,100 million tonnes of glucose or glucose equivalent would have to be degraded at the 10 per cent efficiency, or 1,000 million tonnes of glucose at 11 per cent efficiency.

Sinha calculated that the global annual production of 527 million tonnes of rice would come from some 1,300 million tonnes of biomass. This quantity of rice crop would release through its roots some 130 million tonnes of glucose equivalent which, when acted upon by bacteria, would yield 11 to 13 million tonnes of methane from the paddy fields. He stresses that the upper limit for methane production can be no more than 13 million tonnes, and any estimates that "ignore the biological aspects of methane production are nothing but an artefact of methodology".

In another study, Sinha has used a well-established relationship between rice biomass and methane production and come up with an even lower estimate for methane produced from paddy fields around the world. Based on these calculations, which take into account several factors that affect methane production like fertilisers, day length and the period of growth, Sinha estimates that methane production from the world's paddy fields could be as low as 7.08 million tonnes.

Sinha says that the findings of the 2 studies and the observed atmospheric concentration of methane together suggest that scientists may have overlooked an important source of methane, and that efforts are necessary to identify it.

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