Changes to farming practices in rice paddies in China may have led to a decrease in methane emissions, and an observed decline in the rate that methane has entered the Earth's atmosphere over the last 20 years, a NASA-funded study finds.
Changsheng Li, a professor of natural resources in the University of New Hampshire's Institute for the Study of Earth, Oceans, and Space, and lead author of the study, notes that in the early 1980s Chinese farmers began draining their paddies midway through the rice growing season when they learned that replacing a strategy of continuous flooding would in fact increase their yields and save water. As an unintended consequence of this shift, less methane was emitted out of rice paddies.
Methane is 21 times more potent as a greenhouse gas than carbon dioxide (CO2) over 100 years. At the same time, since 1750, methane concentrations in the atmosphere have more than doubled, though the rate of increase has slowed during the 1980-90s.
"There are three major greenhouse gases emitted from agricultural lands-carbon dioxide, methane and nitrous oxide," said Li. "Methane has a much greater warming potential than CO2, but at the same time, methane is very sensitive to management practices." Currently, about 8 percent of global methane emissions come from the world's rice paddies.
In an effort to reduce water use, farmers in China found that if they drained the soils, they could get higher yields. That's because draining stimulates rice root development, and also accelerates decomposition of organic matter in the soil to produce more inorganic nitrogen, an important fertilizer. Methane is produced by soil microbes in paddy soils under anaerobic conditions, or in the absence of air or free oxygen. Midseason drainage aerates the soil again, and hence interrupts methane production.
Li and his colleagues recorded reductions in methane caused by draining practices at several experimental sites in China and the U.S. At the same time, they observed that the amounts of methane reduction varied greatly in space and time due to complex interactions among many factors.
The researchers spent more than 10 years developing a biogeochemical model, called the Denitrification-Decomposition (DNDC) model, which would handle all the major factors relating to methane emissions from rice paddies. These factors included weather, soil properties, crop types and rotations, tillage, fertilizer and manure use, and water management. The model was employed in the study to scale up the observed impacts of water management from the local sites to larger regional scales. Remotely sensed data from the NASA/U.S. Geological Survey Landsat Thematic Mapper (TM) satellite were utilized to locate the geographic distributions and quantify the acreage of all the rice fields in China. A Geographic Information System database amended with this Landsat data was constructed to support the model runs at the national scale and to predict methane emissions from all rice fields in the country.
The researchers adopted 1990 as a mean representative year as they had detailed, reliable data for that year, and then ran the model with two water management scenarios to cover the changes in farming practices from 1980 to 2000. The two scenarios included continuous flooding over each season, and draining of paddy water three times over the course of each season.
When the two model runs were compared, the researchers found that methane emissions from China's paddy fields were reduced over that time period by about 40 percent, or by 5 million metric tons per year-an amount roughly equivalent to the decrease in the rate of growth of total global methane emissions.
"The modeled decline in methane emissions in China is consistent with the slowing of the growth rate of atmospheric methane during the same period," Li said. "Still, more work will be needed to further verify the relationship demonstrated in this study with limited data points."
Demand for rice in Asia is projected to increase by 70 percent over the next 30 years, and agriculture currently accounts for about 86 percent of total water consumption in Asia, according to a recent report from the International Rice Research Institute. Changes to management practices like this will be more important and likely in the future as the world's water resources become increasingly limited, Li said.
"Just like the Chinese farmers did, if farmers around the world change management practices, we can increase yields, save water and reduce methane as a greenhouse gas," Li said. "That's a win-win situation."
The study, which appears in the print version of Geophysical Research Letters in late December, was funded by NASA through grants from the multi-agency Terrestrial Ecosystems and Global Change Program, and also NASA's Earth Science Enterprise.
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