What goes in must come out, a truism that now may be applied to global river networks.
Human-caused nitrogen loading to river networks is a potentially important source of nitrous oxide emission to the atmosphere. Nitrous oxide is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction.
It happens via a microbial process called denitrification, which converts nitrogen to nitrous oxide and an inert gas called dinitrogen.
When summed across the globe, scientists report this week in the journal Proceedings of the National Academy of Sciences (PNAS), river and stream networks are the source of at least 10 percent of human-caused nitrous oxide emissions to the atmosphere.
That's three times the amount estimated by the Intergovernmental Panel on Climate Change (IPCC).
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," says Jake Beaulieu of the University of Notre Dame and the U.S. Environmental Protection Agency in Cincinnati, Ohio, and lead author of the PNAS paper.
"Much of this nitrogen is transported into river and stream networks," he says, "where it may be converted to nitrous oxide, a potent greenhouse gas, via the activity of microbes."
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.
"This multi-site experiment clearly establishes streams and rivers as important sources of nitrous oxide," says Henry Gholz, program director in NSF's Division of Environmental Biology, which funded the research.
"This is especially the case for those drain
|Contact: Cheryl Dybas|
National Science Foundation