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Grad student studies methane production in rice fields

hen 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.

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Photo 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.

Modeling methane

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 variable.

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 production

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 ways.

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. Why?

“We don’t know yet,” Mr. Xu said. “But empirically, you see a significant increase in methane transport capacity as the temperature rises.”

Other factors

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 nice science.”

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 warming.

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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