Paddy fields produce ozone-depleting gases

According to the study carried out through 1998 and 1999, worldwide rice farming contributes 1% of the total methyl bromide and 5% of the methyl iodide emissions to the atmosphere. However, unplanted flood fields emit as much methyl chloride as planted flooded fields, suggesting that wetlands may also be a notable natural source.

“As the major industrial sources of these halides increasingly are being regulated, it’s now even more important to uncover their natural sources,” said the head of the research team, UCI Chancellor Ralph J Cicerone. “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,” said Cicerone. “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.”

This is the first field study designed to understand the emissions of methyl halide gas from agricultural crops during an entire season, and has used rice as the world’s most common crop, the primary source of food for billions of people, and currently covering 1% of the Earth’s surface. Methyl chloride and methyl bromide are directly involved in stratospheric ozone depletion, and methyl iodide is thought to affect tropospheric ozone.

“We were surprised at the scale of methyl iodide emissions,” said graduate student researcher on the project, Kelly R Redeker. “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.”

The researchers also found that the methyl halide emissions were not constant, but varied with the stage of the plants’ growth, and the organic content and halide content of the soil. Emissions of methyl bromide increased during tilling, and appeared to peak during the reproductive stage of rice growth, whilst the maximum emissions of methyl iodide occurred during the vegetative phase. Methyl chloride emissions were unaffected by the growth stage of the plants.

Eventually, Cicerone and his team would like to expand the project to study the rice-growing regions of Southeast Asia, and other plant species.

The research was funded by the National Science Foundation, and the US Department of Agriculture. The findings have been published in the 3 November issue of the journal Science.