Grad student studies methane production
in rice fields
it comes to greenhouse gases, carbon dioxide is the major celebrity.
And with good reason: carbon dioxide has the second highest
atmospheric concentration of all the greenhouse gases behind
water vapor and makes the largest contribution to global warming.
by Frank Wojciechowski
Graduate student Shangping Xu
is studying the methane emissions of rice fields.
Other gases that receive less publicity,
however, still have a large impact on global climate change.
Take, for example, methane, also known
as CH4. Although the amount of atmospheric methane is only
a tiny fraction of that of atmospheric carbon dioxide, scientists
estimate that methane accounts for a hefty 15 to 20 percent
of the radiative forcing.
Another interesting trait of methane is
that it breaks down in a brief eight years (unlike carbon
dioxide, which takes more than 100), meaning that efforts
to restrain the amount of methane making it into the atmosphere
would have measurable effects on climate change rather soon.
Shangping Xu, a graduate student
in the Department of Civil and Environmental Engineering
(CEE), is giving methane the attention it’s due. Methane
is produced and transported into the atmosphere through a
number of natural and anthropogenic processes and is highly
Mr. Xu is working with CEE Department
Chair Peter Jaffé and Professor Denise Mauzerall of
the Woodrow Wilson School of Public and International Affairs
to create a new model to better understand methane production
and guide scientists into creating ways to combat methane
emission. Mr. Xu is conducting this research through a PEI-STEP
grant. He is doing this project in addition to his Ph.D.
work on wetland chemistry.
Methane is created in anaerobic environments
and is naturally produced and emitted from wetlands and other
natural situations. Mother Nature, however, is not the predominate
generator of methane. Humans are. The decomposition of waste,
the burning of biomass, the extraction of fossil fuels, the
digestion of livestock, and rice cultivation combine to emit
more than twice the methane emitted by natural processes.
It is this last source that Mr. Xu is
focusing his research on. Most rice is grown in flooded rice
paddies, mainly because the floodwater has no adverse effects
on the rice plants but controls most weeds and pest insects.
The flood water creates an anaerobic environment just right
for methane production. Rice cultivation accounts for 17
percent of the anthropogenically produced methane.
Some changes in agricultural practices
may be effective in reducing methane production in flooded
rice paddies, but what works for one paddy won’t work
precisely the same for the paddy next door. Comprehensive
models are needed to address the methane problem effectively.
“It’s really hard to model
this,” Mr. Xu said, “because it’s very
variable over both space and time.”
Existing models usually focus on
only a few factors and are often not flexible enough to
be applied under various environmental and agriculture
conditions. Mr. Xu believes that the influence of individual
factors may vary from site to site, and thus a truly useful
model must incorporate the gamut of factors.
A second look
Further, Mr. Xu’s research has shown
that two factors may be more significant than they were given
credit for in previous studies. His model includes both temperature
and the growth of the rice plants, which he said have great
impacts upon methane production and emission in rice paddies.
“Rice plants and natural wetlands
share many traits and processes,” he said. “The
model would need some modifications (to accurately model
natural wetlands), but the basic ideas are the same.”
Being that wetlands and rice paddies combined
are responsible for more than 30 percent of global methane
production, this could be an extremely valuable model.
How does it work?
In a flooded rice paddy, methane production
starts with the degradation of organic materials in the soil.
The organics behave as electron donors
and react with compounds that are electron acceptors to create
new compounds. This process is called reduction.
Some of these reducing reactions create
methane, but the organics do not reduce compounds that create
methane until all the other possible electron acceptors have
already been reduced.
This methane then diffuses into the rice
plant through its roots. The methane travels up the roots
of the plant and into the aerenchyma, and is then emitted
into the air to begin its journey into the atmosphere.
The amount of methane that a rice plant
emits is called its transport capacity, and this is a vitally
important factor for scientists to understand.
Mr. Xu has included the growth dynamics
of rice plants as another important factor in his model.
The growth of rice plants affects a plant’s transport
capacity—and thus methane emission—in two main
First, as the rice plant grows, the root
mass increases. More roots mean more surface area to serve
as sites for methane transport.
Secondly, plants produce organic matter
that is released into the soil. Higher root mass provides
more surface area for the release of organics from the roots
into the soil. Since methane is produced through the degradation
of organic materials, this increases the transport capacity
of a plant once again.
Methane transport capacity of rice plants
and methane production rates are also affected by soil temperature.
The bacteria that mediate the reactions that produce methane
are more active in high temperatures.
Yet there is another, less explicable
way that temperature relates to methane emission. Experimental
data show that the methane transport capacity of the rice
plants is, itself, a function of temperature. Data show that
rice paddies emit more methane as the temperature increases.
“We don’t know yet,” Mr.
Xu said. “But empirically, you see a significant
increase in methane transport capacity as the temperature
Other factors included in the model are
vertical distribution of the rice roots, the ebullition of
methane directly from the soil, and the sequential use of
electron acceptors in the soil.
Mr. Xu has tested his model against 11
years of empirical data gathered from Chinese rice paddies.
Once Mr. Xu publishes his paper, he hopes to receive comments
from critics to help him do more tweaking.
“I’m pretty confident with
this model. I believe it will be a front-runner of the methane
emission models,” he said. “But in terms of the
details, other researchers may be able to point errors out
and help us improve the model. Sometimes progress is limited
by access to the experimental data. If people know about
our work, they might be willing to provide us more data,
or collaborate with us. I’m hoping they will.”
A requirement of the PEI-STEP grant is
that a policy report be written in conjunction with the research
paper. Once Mr. Xu completes the development of his model,
he will work more closely with Professor Mauzerall on the
questions of policy that arise from his research.
The report will mainly focus on rice cultivation
in China, Mr. Xu’s home country. In this paper, Mr.
Xu will examine a number of options for reducing methane
emissions from rice paddies and study the agricultural practice,
national policy, cost-benefit analysis, and other relevant
factors surrounding these options.
Methane emission mitigation is not the
only important aspect of agriculture, as Mr. Xu well knows.
“If you ask a farmer to switch cultivars,
use a fertilizer, or change any other practice, you have
to know how it will affect the labor, the cost, the yield
and the quality of the rice,” Mr. Xu said.
Mr. Xu said he is very excited about this
research, even though he has to squeeze it into his schedule
around his Ph.D. work.
“It’s fun,” he
said. “I never imagined when I began the project
that we’d come up with this model. It’s very
A briefing on Earth's greenhouse gasses
The Earth’s atmosphere is
composed of greenhouse gases that are responsible for trapping
heat energy from the sun and warming the Earth. This process
is called the greenhouse effect, and without it the Earth
would not be habitable.
The problem arising now is that this atmospheric
blanket is getting too thick.
The more greenhouse gas in the atmosphere,
the warmer the Earth becomes. Human activity causes huge
quantities of greenhouse gases to be emitted into the atmosphere,
thus raising the Earth’s global temperature and creating
more severe climate systems. This phenomenon is called global
Two primary human actions greatly affect
climate change: emissions and land-use changes.
Huge amounts of greenhouse gases are emitted
into the atmosphere through the combustion of fossil fuels
and various industrial and agricultural processes.
Plants breathe carbon dioxide (CO2) and
expel oxygen. Large-scale deforestation is thus a huge factor
in climate change, because without the trees to absorb the
CO2, the greenhouse gas makes its way into the atmosphere.
Much research now focuses on greenhouse
gas mitigation to curb global climate change.
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