The growing role of farming and nitrous oxide in climate change

A farmer fertilizes his field in India, where consumption of nitrogen from fertilizer has shot up by 50 percent during the past 10 years.

By Carol Clark

Most people know nitrous oxide as laughing gas, used as a mild anesthetic for dental patients. What’s less well-known is that nitrous oxide is the leading cause of the depletion of the protective layer of ozone in the Earth’s atmosphere, and the third-largest greenhouse gas, after carbon dioxide and methane.

“Not many people know about the impact of nitrous oxide, and very few people are studying the nitrogen cycle,” says Eri Saikawa, an assistant professor in Emory’s Department of Environmental Studies.

Nitrous oxide is released naturally from the soil, as part of the process of microbes breaking down nitrogen. However, human activity, especially agriculture, has boosted the emission levels in recent decades. Livestock manure and fertilizers containing nitrates, ammonia or urea all generate nitrous oxide as they decompose.

“Nitrous oxide emissions stay in the atmosphere for 125 years, similar to carbon dioxide. So it’s very important that we take action now,” Saikawa says.

Saikawa, whose research is focused on emissions linked to air pollution, ozone depletion and global warming, will give an overview of her work on nitrous oxide as part of Environmental Studies’ fall lecture series. Her talk, “Laughing Gas: No Laughing Matter for Climate Change and the Environment,” is set for 4 pm on Monday, October 21 in the Math and Science Center, room N306.

Over-fertilization can degrade soil quality.

Until fairly recently, the United States was the main nitrogen consumer from fertilizers. Since 2000, however, U.S. consumption has declined about 9 percent, according to data from the International Fertilizer Industry Association.

Meanwhile, China’s nitrogen consumption from fertilizers has shot up 40 percent during the past 10 years, making it the number one consumer. And India has moved into the number two spot, with a 50 percent increase.

As the two most populous nations rapidly industrialize, they are also using more fertilizer, in an attempt to boost yields, Saikawa says. “Actually, over-fertilization wastes money and can sometimes degrade soil quality, while also creating more nitrous oxide emissions.”

In her previous position with MIT’s Center for Global Change Science, Saikawa developed a computer model, based on local soil temperatures and moisture content, to estimate global nitrous oxide emissions from natural sources in different regions of the world, from 1975 to 2008. The simulation was checked against the few available actual nitrous oxide measurements, including 25 locations in the Amazon, North and Central America, Asia, Africa and Europe.

The results, verifying the simulation model’s accuracy, were recently published in the journal Global Biogeochemical Cycles, and are highlighted in this month’s issue of Nature Geoscience.

Watch a data visualization of the findings, below:



“We wanted to see if we could reproduce natural soil emissions first,” Saikawa says. “Our next step is to include the agricultural components, so we can understand more about how much nitrous oxide is coming from the activities of people. We can then use the model to simulate possible future scenarios.”

Saikawa is continuing to collaborate with her former colleagues from MIT for the research into the impact of nitrous oxide on climate change and the stratosphere, which is funded by the National Oceanic and Atmospheric Administration (NOAA).

Her computer simulation revealed that El Niño weather events decrease nitrous oxide emissions in tropical South Asia, while the opposing weather pattern, La Niña, causes a spike. This variation is likely due to the change in the rainfall associated with El Niño and La Niña, and the fact that warm, wet soil boosts emission levels, Saikawa says.

The simulation also showed that in temperate regions, snow cover appears to have an effect on emissions.

“There are so many variables, and things that we don’t know about nitrous oxide emissions,” Saikawa says. “We have to get as many measurements as possible to refine and validate our model, and to determine if there are optimal agricultural practices and other ways to potentially minimize emissions. Without more knowledge, it’s difficult to make recommendations, or to regulate the emissions.”

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Photos: iStockphoto.com