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Streams a major source of nitrous oxide, an ozone-destroying greenhouse gas

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When journalists write about greenhouse gases that humans are releasing into the atmosphere, they usually reserve their headlines for carbon dioxide. This week, carbon dioxide shares top billing with nitrous oxide, which traps heat 300 times more effectively than carbon dioxide and is responsible for six percent of all human-induced climate change.

Just like carbon dioxide, nitrous oxide has been increasing in concentration in the Earth's atmosphere, rising 20 percent over the past century and between 0.2 and 0.3 percent every year.

Not only does nitrous oxide contribute to global warming, it destroys the ozone layer, which protects Earth from harmful ultraviolet radiation from the Sun, as well. In fact, now that chloroflurocarbons are no longer being produced, nitrous oxide may be the leading human-caused threat to the ozone layer.

In an article in this week's print edition of the Proceedings of the National Academy of Sciences (PDF), a team of scientists from more than a score of universities and government agencies, among them Michigan State University, has discovered that nitrous oxide escaping from the world's rivers and streams contributes at least ten percent of the annual human-caused nitrous oxide to the planet's atmosphere. This is three times the percentage that the Intergovernmental Panel on Climate Change (IPCC) estimated.

Humans do not substantially contribute to the increase of nitrous oxide by the direct production of the gas, which is used as an anaesthetic and a propellant for whipped cream. Instead, bacteria in the soil and water convert the nitrogen in fertilizers and sewage into nitrous oxide, a process called denitrification. Also, high-temperature burning of fossil fuels combines the nitrogen and oxygen in the air into oxides of nitrogen, including nitrous oxide.

In a press release from the National Science Foundation (NSF), which funded the research, lead author Jake Beaulieu of the University of Notre Dame and the U.S. Environmental Protection Agency in Cincinnati, Ohio, says "Human activities, including fossil fuel combustion and intensive agriculture, have increased the availability of nitrogen in the environment. Much of this nitrogen is transported into river and stream networks, where it may be converted to nitrous oxide, a potent greenhouse gas, via the activity of microbes."

In the same NSF press release, Henry Gholz, program director in NSF's Division of Environmental Biology, says, "This multi-site experiment clearly establishes streams and rivers as important sources of nitrous oxide. This is especially the case for those draining nitrogen-enriched urbanized and agricultural watersheds, highlighting the importance of managing nitrogen before it reaches open water."

Stephen Hamilton, a professor at MSU’s Kellogg Biological Station and one of the authors of the study, elaborated in a press release from MSU, "Even with more than 99 percent of denitrified nitrogen in streams and rivers being converted to the inert gas, dinitrogen, river networks still contribute to at least 10 percent of global anthropogenic nitrous oxide emissions."

As Gholz says, "This new global emission estimate is startling."

The study concluded that the best way to restrict the amount of nitrous oxide released from waterways would involve limiting the amount of nitrogen entering the environment. Reducing the use of agricultural fertilizer and other sources of nitrogen would be examples of strategies to achieve the goal of slowing down or even halting the increase of nitrous oxide in the atmosphere.

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