INFOTERRA: Shifts in rice farming practices in China reduce greenhouse gas methane
http://www.eurekalert.org/pub_releases/2002-12/nsfc-sir121902.php
Public release date: 19-Dec-2002
Contact: Krishna Ramanujan
kramanuj@pop900.gsfc.nasa.gov
301-286-3026
NASA/Goddard Space Flight Center--EOS Project Science Office
Shifts in rice farming practices in China reduce greenhouse gas methane
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.
For more information, please see:
http://www.gsfc.nasa.gov/topstory/2002/1204paddies.html
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