Wed, Sep 12, 2018 - Page 5 News List

Paddy pollution worse than thought


The way some irrigated rice paddies are managed worldwide, with cycles of flooding followed by dry periods, might lead to twice the planet-warming greenhouse gas pollution as previously thought, researchers said on Monday.

Since rice is a major staple for at least half the world’s 7 billion people, the way it is managed has significant effects on the Earth’s warming climate, researchers said in a report in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.

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

Nitrous oxide rises when rice fields are allowed to dry before being wetted again.

This process, called intermittent flooding, happens when water falls below the soil level several times per year. It is used by some rice farmers — how many is unclear — because it reduces methane, another major greenhouse gas emitted by rice paddies.

“When the soils are frequently wetted and dried, they repeatedly become ideal environments for microbes that produce nitrous oxide,” said lead author Kritee Kritee, a senior scientist at the fund.

“Methane, on the other hand, is produced by microbes that require soils to be submerged in water,” she told reporters in an e-mail.

It is widely assumed that “almost all irrigated farms in the world are continuously flooded and it is a fact that continuously flooded farms do not produce significant amounts of nitrous oxide,” she added.

However, it is not true that all farms are continuously flooded.

“[The] full climate impact of rice farming has been significantly underestimated,” Kritee said.

The amount of unaccounted-for nitrous oxide global emissions from rice might be as high as the annual climate pollution from about 200 coal power plants, the researchers said.

In India alone, where the study took place across five intermittently flooded rice fields, nitrous oxide emissions “could be 30 to 45 times higher than reported under continuous flooding,” they said.

Overall, they calculated that nitrous oxide per hectare was three times greater than ever reported by previous research on intermittently flooded farms.

“When this new information is extrapolated across the world and embedded into estimates of methane emissions, the net climate impact from both methane and nitrous oxide could be two times higher than previous estimates,” Kritee said.

Experts have said a better way would be for all irrigated rice farmers to shallowly flood their fields, meaning the water level stays within 5cm to 7cm of the soil level.

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

Nitrous oxide casued by rice-growing is not currently being tracked on a broad scale and is left out of greenhouse gas inventories reported to the UN by major rice-producing countries, including China and India.

However, as water becomes scarcer around the globe, many rice farmers might look to wet-and-dry cycles as a solution, not knowing the danger they are posing to the planet.

To avoid that, scientists need better tracking and reporting of nitrous oxide worldwide, the fund said.

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