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. This new global emission estimate is startling.

—Henry Gholz, program director in NSF’s Division of Environmental Biology, which funded the research

Nitrous oxide is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. It is produced via a microbial process called denitrification, which converts nitrogen to nitrous oxide and an inert gas called dinitrogen. Rates of nitrous oxide production via denitrification in small streams increase with nitrate concentrations.

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.

—Jake Beaulieu of the University of Notre Dame and the US EPA, and lead author of the PNAS paper

Beaulieu and co-authors measured nitrous oxide production rates from denitrification in 72 streams draining multiple land-use types across the United States. Their work was part of a broader cross-site study of nitrogen processing in streams.

Atmospheric nitrous oxide concentration has increased by some 20% over the past century, and continues to rise at a rate of about 0.2 to 0.3% per year. The global warming potential of nitrous oxide is 300-fold greater than carbon dioxide; nitrous oxide accounts for some 6% of human-induced climate change, scientists estimate.

They believe that nitrous oxide is the leading human-caused threat to the atmospheric ozone layer, which protects Earth from harmful ultraviolet radiation from the Sun.

Researchers had estimated that denitrification in river networks might be a globally important source of human-derived nitrous oxide, but the process had been poorly understood, says Beaulieu, and estimates varied widely.

While more than 99% of denitrified nitrogen in streams is converted to the inert gas dinitrogen rather than nitrous oxide, river networks are still leading sources of global nitrous oxide emissions, according to the new results. The findings, the authors hope, will lead to more effective mitigation strategies.

Other authors of the paper are: Jennifer Tank of the University of Notre Dame; Stephen Hamilton of Michigan State University; Wilfred Wollheim of the University of New Hampshire; Robert Hall of the University of Wyoming; Patrick Mulholland of Oak Ridge National Laboratory and the University of Tennessee; Bruce Peterson of Marine Biological Laboratory in Woods Hole, Mass.; Linda Ashkenas of Oregon State University; Lee Cooper of the Chesapeake Biological Laboratory in Solomons, Md.; Clifford Dahm of the University of New Mexico; Walter Dodds of Kansas State University; Nancy Grimm of Arizona State University; Sherri Johnson of the US Forest Service in Corvallis, Ore.; William McDowell of the University of New Hampshire; Geoffe Poole of Montana State University; HM Valett of Virginia Polytechnic Institute and State University; Clay Arango of Central Washington University; Melody Bernot of Ball State University; Amy Burgin of Wright State University; Chelsea Crenshaw of the University of New Mexico; Ashley Helton of the University of Georgia; Laura Johnson of Indiana University; Jonathan O’Brien of the University of Canterbury in Christchurch, New Zealand; Jody Potter of the University of New Hampshire; Richard Sheibley of the University of Notre Dame and the US Geological Survey in Tacoma, Washington; Daniel Sobota of Washington State University; and Suzanne Thomas of the Marine Biological Laboratory in Woods Hole, Mass.

Resources

• Jake J. Beaulieu et al. (2010) Nitrous oxide emission from denitrification in stream and river networks. PNAS doi: 10.1073/pnas.1011464108