Comprehensive look at rice farming during entire crop season also points to soil content, field management as keys to understanding emissions
Irvine, Calif. -- A UC Irvine study has determined that the world's rice paddies emit a small but significant amount of methyl halide gases that contribute to stratospheric ozone depletion, suggesting that agricultural sources also play a role in this atmospheric phenomena.
In the first field study to understand methyl halide gas emissions from agricultural crops during an entire season, a team led by UCI Chancellor Ralph J. Cicerone, an internationally recognized researcher on stratospheric ozone depletion, and graduate student researcher Kelly R. Redeker has estimated the amounts of these gases contributed by rice farming. These findings will be published in the Nov. 3 issue of Science.
Methyl halide compounds include methyl chloride, methyl bromide and methyl iodide, which become reactive agents when released into the atmosphere. Bromine and chlorine, two products of these reactions, are directly involved with stratospheric ozone depletion. Iodine is believed to affect tropospheric ozone, a common air pollutant. The 1987 Montreal Protocol has restricted products such as chlorofluorocarbons (CFCs), halons and methyl bromides that contribute chlorine and bromine to the atmosphere and are believed to be primarily man-made. The goal of these restrictions is to reduce the human contribution to halogens in the atmosphere.
"As the major industrial sources of these halides increasingly are being regulated, it's now even more important to uncover their natural sources," Cicerone said. "We only know where half of the methyl chloride and two-thirds of the methyl bromide are coming from. This study is significant because it gives direct evidence that some of the unknown sources of these halides could very well be plant sources.
"Methyl bromide used as a soil fumigant, for example, is being banned because of concern over its effect on stratospheric ozone. Yet agricultural plants also release it into the air. So to understand the potential benefits of a ban on methyl bromide, we must learn about the sizes of natural sources."
After monitoring the methyl halide gases emitted from a rice paddy in Maxwell, Calif., over two planting seasons in 1998 and 1999, the UCI team calculated that worldwide rice farming contributes 1 percent of the methyl bromide and 5 percent of the methyl iodide to atmospheric totals.
"We were surprised at the scale of methyl iodide emissions," Redeker said. "We're still not sure how important a role methyl iodide plays in atmospheric chemistry, but we have found that it lingers over the fields during its maximum emission stage, meaning that it may have some impact on local environmental issues."
They also noted that unplanted flood fields emit as much methyl chloride as planted flooded fields, suggesting that global wetlands may be a notable natural source and worth further study.
The UCI team also found that the rate of methyl halide emissions is not constant, varying with stage of plant development in the growing season, soil halide amount and soil organic content. Emissions of methyl bromide, for instance, increased during tilling and appeared to peak during the reproductive stage of rice growth. Methyl iodide appears to have maximum emissions during the vegetative phase. However, methyl chloride counts were unaffected by stages of rice growth.
In addition, Cicerone took air samples from rice paddies in Texas and Japan in 1997 and 1998 during selected phases of plant growth and harvest, and in comparing data with the Maxwell study, found that halide and organic differences in various soils influence emission amounts.
The UCI team followed its Maxwell research by monitoring a planting season in rice paddies near Houston. In addition, Redeker is involved with greenhouse studies on the UCI campus, growing rice in different soil and water conditions to study the impact on methyl halide emissions. Eventually, Cicerone and his team would like to expand this research to rice-growing regions in Southeast Asia and to other plants.
Studying rice farming is important in understanding the natural sources of ozone-depleting gases. Rice is the world's largest crop and is the primary source of food for billions. As the human population grows, so will the need for increased rice cultivation. Currently, it is estimated that rice paddies cover 1 percent of the Earth's landmass. Rice paddies also are a significant contributor of methane, a greenhouse gas, which forms in soil pores and is passed into the air through the rice plant.
The study was conducted by researchers from UCI's departments of Earth System Science and Chemistry. Assisting Cicerone and Redeker were associate researchers Stanley Tyler and Nun-Yii Wang and graduate student researchers Jason C. Low and Andrew M. McMillan. Research funding came from the National Science Foundation and the U.S. Department of Agriculture.
Karen Newell Young
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