But global warming could raise greenhouse gases produced by paddies elsewhere.
Simple changes to farming practices in China have slashed the amount of methane released from rice fields, researchers say. Paddy fields account for around 20% of human-related emissions of methane — a potent greenhouse gas.
Farmers normally flood rice fields throughout the growing season, meaning that methane is produced by microbes underwater as they help to decay any flooded organic matter.
By studying experimental rice plots and real farmland, Chris Butenhoff and Aslam Khalil, physicists from Portland State University in Portland, Oregon, together with Xiong Zhenqin, an ecologist at Nanjing Agricultural University in China's Jiangsu province, and their colleagues set out to identify the different factors that affect this process.
The team found that draining paddy fields in the middle of the rice-growing season — a practice that most Chinese farmers have adopted since the 1980s because it increases rice yields and saves water — stopped most of the methane release from the field. The team presented their results on 13 August at a meeting on climate science convened at a Beijing hotel by the US Department of Energy and China's Ministry of Science and Technology.
Estimating regional methane emissions from paddies is notoriously tricky because of the lack of good models and the difficulty in obtaining local geographical information, says Raymond McCord, an ecologist at the Oak Ridge National Laboratory in Tennessee, who was not involved in the research. "This is a very important study," McCord adds.
In addition to the methane-reducing effect of draining water, the researchers found that paddies released more methane when rice straws from the previous growing season were added to flooded fields as a source of organic fertilizer — a common rice-farming practice — and when soil temperature increased.
Based on these findings, the researchers came up with a computer model to calculate methane emission from rice fields that also incorporated other information, such as the location and size of the cultivated area and the length of the rice-growing season.
By combining detailed inventories of agriculture practice throughout China with satellite data indicating the presence of straws in rice fields, the team was able to estimate that paddies across China release 5.1 million tonnes of methane a year — nearly a 70% reduction from the 1980 levels previously estimated1 by Changsheng Li, a biogeochemist at the University of New Hampshire in Durham.
"This is largely due to the shift towards [the practice of] mid-season drainage in the 1980s," says Butenhoff. Only 1% of Chinese farmers drained their paddies halfway during the rice-growing season in 1980, but around 80% of them have been routinely using this approach since 2000.
The catch, however, is that the practice of mid-season drainage has increased the release of nitrous oxide, another potent greenhouse gas, by about 20,000 tonnes for the same period, says Xiong. But overall, the reduction in greenhouse gases is equivalent to 270 million tonnes of carbon dioxide, she adds.
Using a similar approach to get a global estimate, but based on much less comprehensive information than that available from China, the researchers calculated that paddies worldwide release about 50 million tonnes of methane. Notably, India is responsible for nearly a third of the estimated global methane emissions — this may be due to the high temperatures in that country's rice-growing regions, the large cultivation areas and the practice of continuously flooding paddies, says Butenhoff.
Off the chart
With global temperatures rising as a result of climate change, the emission of methane — which traps about 25 times more of the Sun's heat than carbon dioxide — will play a greater part in the global carbon budget than it does now, says Butenhoff. Based on the temperature-increase projections of the Intergovernmental Panel on Climate Change, he and his colleagues estimate that methane emission from paddies in China could go up by a further 2.5 million tonnes by 2100. "This is a massive amount," he says. "We can't even get an estimate for India because the [methane] emission would be so high it's off the chart."
Given that most farmers outside China continuously flood their rice fields, the researchers say that simply shifting towards the practice of mid-season drainage could significantly reduce global paddy-derived methane emissions.
At the moment, most of the methane-monitoring network lies outside the areas where rice is grown. "We need more local information, including methane flux, to get a better global estimate of methane emission [from paddies]," says Butenhoff.
The study has important implications for climate change and mitigation efforts, says McCord. Methane is an attractive greenhouse gas to target for emission reduction, but the lack of an accurate estimate of the baseline level challenges the use of paddy emissions in carbon trading, he says.
Li, C. et al. Geophys. Res. Lett. 29, doi:10.1029/2002GL015370 (2002).
About this article
A two-dimensional transient analytical solution for a ponded ditch drainage system under the influence of source/sink
Journal of Hydrology (2018)
Mitigating methane emission from paddy soil with rice-straw biochar amendment under projected climate change
Scientific Reports (2016)
Hydraulics of a Partially Penetrating Ditch Drainage System in a Layered Soil Receiving Water from a Ponded Field
Journal of Irrigation and Drainage Engineering (2015)
C and N accumulations in soil aggregates determine nitrous oxide emissions from cover crop treated rice paddy soils during fallow season
Science of The Total Environment (2014)