Greenhouse gases from rice fields can be twice as high as expected

The way in which irrigated rice paddies are managed around the world, with flood cycles followed by dry spells, can lead to greenhouse gas pollution that is twice as high as expected, researchers said on Monday. .

Given that rice is a staple food for at least half of the world's seven billion people, the way it is managed has significant effects on global warming, the report said in the paper. Proceedings of the National Academy of Sciences, peer reviewed American journal.

For the study, researchers at the nonprofit Environmental Defense Fund took a closer look at emissions of nitrous oxide, a long-lived atmospheric pollutant, more potent than methane or carbon dioxide.

N2O increases when rice fields are allowed to dry before being re-wetted again.

This process, called intermittent flooding, occurs when the water falls below ground level several times a year.

It is used by some rice farmers – how much is unknown – because it reduces methane, another important greenhouse gas emitted by rice fields.

"When soils are frequently wet and dried, they often become ideal environments for microbes that produce nitrous oxide," said senior author, Kritee Kritee, an experienced scientist with the EDF.

"On the other hand, methane is produced by microbes that need to submerge soils in water," she told AFP by email.

It is widely accepted that "almost all irrigated farms in the world are constantly flooded and that continuously flooded farms do not produce large amounts of nitrous oxide," she added.

But it is not true that all farms are continually flooded.

This is why Kritee says that "the total impact of rice growing on the climate has been considerably underestimated".

200 coal power plants

According to the authors, the amount of unrecorded global emissions of N2O from rice could be as high as the annual climate pollution of about 200 coal plants.

In India alone, where the study was conducted on five intermittently flooded rice paddies, nitrous oxide emissions "could be 30 to 45 times higher than those reported in case of failure." flooding continues, "said the researchers.

Overall, they calculated that nitrous oxide per hectare (2.5 acres) was three times higher than ever reported by research on previously intermittently flooded farms.

"When this new information is extrapolated around the world and incorporated into methane emission estimates, the net climate impact of methane and nitrous oxide could be twice as high as previous estimates," Kritee said. .

Experts say that a better way would be for all irrigated rice farmers to gradually flood their fields, which means that the water level remains five to seven centimeters off the ground.

"This flooding regime produces the least amount of methane and nitrous oxide," said Kritee.

At present N2O from rice is simply not widely monitored and is excluded from greenhouse gas inventories reported at the United Nations by major rice producing countries, of which China and India.

But as water becomes scarce throughout the world, many rice farmers may consider wet and dry cycles without knowing the danger they pose to the planet.

To avoid this, scientists need better monitoring and reporting on N2O around the world, said EDF.

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More information:
Kritee el al., "Nitrous oxide high fluxes from rice indicate the need to manage water for short- and long-term climate impacts" PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1809276115