Agricultural scientist Pratap Bhattacharyya may have found a remarkable piece of evidence that absolves swathes of paddy fields stretching over millions of hectares of a climate crime. On the contrary, he believes that rice is doing its bit for the environment.
A study by Bhattacharyya and his colleagues at the Cuttack-based Central Rice Research Institute (CRRI) shows that lowland rice cultivation does not release carbon into the atmosphere. Instead, it captures atmospheric carbon and stores it in soil, and thus acts as a carbon sink.
“We have shown probably for the first time that the tropical lowland submerged ecosystem is a net carbon sink, not a carbon source,” Bhattacharyya told KnowHow from the University of Georgia in the US, where he has gone for training in sophisticated systems for measuring greenhouse gas (GHG) emissions.
The process of capturing carbon and storing it — called carbon sequestration — is considered beneficial as it rids the atmosphere of noxious gases to an extent. A system that “fixes” carbon dioxide is called a carbon sink whereas that emits carbon dioxide is a carbon source. Thick forests are a good example of a carbon sink.
The findings — reported a fortnight ago in the journal Agriculture, Ecosystems and Environment — may have major implications for the way tropical countries’ contribution to climate change is calculated.
The climate villain among major staple crops, paddy in standing water is blamed for emitting methane — a potent gas which is 21 times more heat trapping than carbon dioxide. Ninety per cent of flooded paddy cultivation is in Asian countries, with China and India accounting for the lion’s share.
According to the latest assessment report released by the Intergovernmental Panel on Climate Change (IPCC) earlier this month, flooded paddy fields in tropical countries account for up to 11 per cent of man-made global methane emissions, estimated to be anywhere between 493 and 723 million tonnes of carbon dioxide equivalent a year.
While cultivating rice, farmers apply organic and green manure, which leads to a greater carbon stock. This is true for any cropland. But methane is released because of anaerobic conditions (when oxygen is not readily available), caused by the standing water in which paddy grows, says N.H. Ravindranath, professor at the Indian Institute of Science in Bangalore, and a contributor to the latest IPCC report.
As much as 16 million hectares in the states of Andhra Pradesh, Orissa, Bihar, West Bengal and Assam — nearly 40 per cent of India’s total rice cultivation area — are under lowland rice cultivation.
But an experiment carried out by the CRRI researchers using sophisticated instruments capable of measuring greenhouse gas emissions at millisecond levels has shown that the net carbon inflow into lowland rice systems is more than carbon that gets out (as methane).
The CRRI scientists calculated the carbon budget of rice cultivation for two seasons in 2012 — during the wet monsoon months and in the dry season after the monsoon in an experimental plot of two hectares.
According to the calculations made by the scientists using the advanced eddy covariance technique — capable of gauging even the minutest quantities of gases exchanged between the soil and the atmosphere — each hectare of land during the wet season received 6.15 mega grams (Mg) of carbon (6150 kg) as input and lost 5.24 Mg of carbon (5240 kg). The carbon removed includes carbon contained in the crop as well as that which escaped as methane into the atmosphere.
This means that one hectare of lowland ecosystem has the potential to store as much as 0.91 Mg of carbon (910 kg), says Bhattacharyya. Similarly in the dry season, carbon left behind after the harvest was found to be 0.59 Mg (590 kg).
“This clearly shows that flooded paddy fields do not contribute to climate change. On the contrary, they help in mitigating it,” CRRI scientist A.K. Nayak, who is a co-author of the paper, asserts.
If this is proven to be true, it could emerge as a major contribution from the Indian agricultural scientific community towards fixing climate responsibility. On an earlier occasion, too, through meticulously planned scientific experiments, Indian scientists had showed to the world that methane emissions from tropical rice fields were one-tenth of what was then projected. Based on these studies, the IPCC had in fact revised the methane emission estimates for tropical countries in its subsequent assessment reports.
“In the eighties, the methane emission figures for tropical countries were extrapolated from experiments conducted in Japan, a temperate country. Through experiments, our scientists showed that carbon stocks in tropical soil are substantially less than in temperate soil and a correction was made subsequently,” Nayak says.
Notwithstanding its significance, the findings have failed to elicit an independent critique from the Indian Council of Agricultural Research (ICAR), the custodian of agricultural research in India and of which CRRI is a part. Himanshu Pathak, ICAR’s coordinator on climate change matters, refused to comment on the work citing bureaucratic reasons. ICAR director general S. Ayyappan did not reply to a mail on the matter.