|
SHIFTS
IN RICE FARMING PRACTICES IN CHINA REDUCE GREENHOUSE GAS METHANE
 |
|
 |
|
Image
1
|
| |
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.
|
|
 |
|
Image
2
|
| |
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.
|
|
 |
|
Image
3
|
| |
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.
|
|
 |
|
Image
4
|
| |
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.
Back
to Top
|
