Study: Small Streams Are a Source of Greenhouse Gas

December 21, 2010

Reducing fertilizer use will reduce nitrous oxide emissions from streams and rivers, which contributes to climate change and ozone destruction, according to a paper co-written by University of Wyoming Professor Robert Hall.

Published Monday in the Proceedings of the National Academy of Sciences, the article "Nitrous oxide emission from denitrification in stream and river networks," describes nitrous oxide (N2O) as "a potent greenhouse gas . Nitrous oxide is the leading human-caused threat to the atmospheric ozone layer which protects the earth from harmful ultraviolet radiation from the sun."

The report was written by 27 stream ecologists across the United States, led by Jake Beaulieu from the University of Notre Dame and the Environmental Protection Agency.

They found that the amount of nitrous oxide produced in streams is related to human activities that release nitrogen into the environment, such as fertilizer use and sewage discharges.

A microbial process called denitrification may be a globally significant source of N2O, especially in streams polluted with large amounts of nitrate from fertilizer and urban runoff, says Hall, a professor in the UW Department of Zoology and Physiology. The only way to get rid of this nitrate-nitrogen is via the natural process of denitrification, where bacteria convert biologically reactive nitrate (NO3) to both N2O and nearly inert N2 gas.

The researchers found that more than 99 percent of denitrified nitrogen in streams is converted to the inert N2 rather than N2O. Despite this small percentage, streams and rivers are the source of at least 10 percent of global N2O emissions -- three times the amount estimated by the Intergovernmental Panel on Climate Change.

Hall points out that nine of the 72 streams studied in the project were in the Jackson area, but that nitrogen concentrations and N2O emissions in the Wyoming streams were very low.

"The bottom line is that streams produce very little N2O relative to N2, but globally rivers and streams are a large source for N2O emissions," he says.

"Runoff from agricultural and urban watersheds has increased the availability of nitrogen in streams and rivers, greatly increasing nitrous oxide production rates," Beaulieu said. "This research shows that river networks play an important role in how human nitrogen use affects climate change and ozone loss."

"We suggest that reductions in N2O emissions from stream and river networks can be achieved by decreasing nitrogen inputs to watersheds (e.g., reduced agricultural fertilizer use)," the paper concluded.

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