Reducing Methane Emissions From Rice

Agricultural Research, May, 2000 by Tara Weaver-Missick

Rice, one of the most versatile foods, is the foundation of many dishes, including rice pudding, rice cakes, beans and rice, and fried rice. So, it's no wonder that world rice production is 384 million tons. Rice is the primary food for about 50 percent of the world's population. It may also have a major impact on global warming by contributing to the emission of an important greenhouse gas: methane.

"Rice is a plant that grows best in wet soil, with its roots flooded," says L. Hartwell Allen. "But flooded rice crops emit substantial amounts of methane to the atmosphere, especially when fresh organic matter--like plant residues--is added back to the soil." Allen is a soil scientist in ARS' Crop Genetics and Environmental Research Unit in Gainesville, Florida.

Methane is a gas that contributes to the greenhouse effect, having a 20-fold greater global warming potential than carbon dioxide ([CO.sub.2]). Some studies show that up to 20 percent of global methane emissions come from flooded rice fields.

For the past few years, Allen and colleague Jeff T. Baker, now with ARS' Remote Sensing and Modeling Laboratory in Beltsville, Maryland, have studied the effects of global change on flooded rice and found that rice could stand a little drying out.

In a recent study, they simulated potential global change conditions--increased drought and rising [CO.sub.2]--by growing rice plants in special outdoor chambers. The studies showed that rice yields drop when the plants are grown during short, 2-week droughts occurring when plants flower. However, when the researchers doubled [CO.sub.2] levels by injecting the gas into the chambers, the plants maintained yield while using less water and enduring a longer drought period.

Allen and Baker also recently discovered that periodically draining the soil to aerate roots with atmospheric oxygen drastically decreases methane emissions. "This may be an easy on-farm practice that would help manage methane emissions," says Allen.

"Our research shows that reducing methane emissions from rice fields is important in helping to reduce or prevent the contribution of rice to global warming," notes Allen. "Since the United States produces only a small fraction of the world's rice, this water management practice needs to be tested and applied more internationally, especially in Asia."

L. Hartwell Allen is in the USDA-ARS Crop Genetics and Environmental Research Unit, Bldg. 350, Agronomy Physiology Laboratory, Gainesville, FL 32611; phone (352) 392-6180, fax (352) 392-6139, e-mail lhajr@gnv.ifas.ufi.edu.