17 Dec 2018

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Nitrous oxide emissions from rice farms may speed up global warming: study

WASHINGTON, Sept. 10 (Xinhua) — A study published on Monday in the Proceedings of the National Academy of Sciences showed that rice farming across the world could be responsible for up to twice the level of climate impact that scientists previously estimated.

Intermittently flooded rice farms can emit 30 to 45 times more nitrous oxide as compared to the maximum from continuously flooded farms that predominantly emit methane, according to the new study.

A global analysis released by Environmental Defense Fund (EDF) on Monday showed that methane and nitrous oxide emissions from rice farms could have the same long-term warming impact as about 600 coal plants. In the short-term, this warming impact could be as much as 1,200 average-sized coal power plants because nitrous oxide lasts many more decades in the atmosphere than methane.

The authors also found an inverse correlation between methane and nitrous oxide emissions from rice farming. Water and organic matter management techniques that reduce methane emissions can increase nitrous oxide emissions.

This is crucial because nitrous oxide is a long-lived greenhouse gas that traps several times more heat in the atmosphere than methane over both 20 and 100-year time frames.

“The full climate impact of rice farming has been significantly underestimated because up to this point, nitrous dioxide emissions from intermittently flooded farms have not been included,” said Kritee Kritee, senior scientist at EDF and the lead author of the paper.

“Increasing pressure on limited water resources under a changing climate could make additional rice farming regions look to intermittent flooding to address water limitations and concerns about methane emissions,” said Kritee.

According to her, the water management on rice farms needs to be calibrated to balance water use concerns with the climate impacts of both methane and nitrous oxide emissions.

Rice is a critical source of nutrition for the world’s rapidly growing population, providing more calories to humans than any other food. But growing rice is also resource-intensive: rice cultivation covers 11 percent of the earth’s arable land, consumes one-third of irrigation water.

Current climate mitigation strategies for rice production focus on reducing methane emissions by alternate wetting and drying, or intermittent flooding.

Most rice producing countries, including the Unites States and the world’s biggest producers of rice like China, India, Indonesia, Vietnam and Bangladesh do not report nitrous oxide emissions associated with rice production as part of their national greenhouse gas inventories submitted to the United Nations.

The investigators measured greenhouse gas emissions from rice farms across southern India and found that nitrous oxide emissions from rice can contribute up to 99 percent of the total climate impact of rice cultivation at a variety of intermittently flooded farms.

These emissions contributed substantially to global warming pollution, far more than the estimate of 10 percent previously suggested by multiple global rice research organizations, according to the study.

Also, they found that carefully chosen farming techniques at individual farms reduced net greenhouse gas emissions from rice cultivation by as much as 90 percent by integrating shallow flooding with co-management of nitrogen and organic matter.

If all irrigated rice farmers only used the proposed shallow flooding instead of continuous or intense forms of intermittent flooding, estimates showed that the rice farms with irrigation had the potential to reduce their global climate impact by 60 percent.

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