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11 June 2009

New evidence on disruption of nitrogen cycle

Scientists are getting a better grip on the nitrogen cycle. They are learning about sources of nitrogen and how this element changes as it loops from the nonliving to the living.

Scientists have determined humans are disrupting the nitrogen cycle by altering the amount of nitrogen stored in the biosphere.

The chief culprit is fossil fuel combustion, which releases nitric oxides into the air that combine with other elements to form smog and acid rain.

However, it has been difficult to know precisely the extent to which such emissions have altered the nitrogen balance.

Researchers from Brown University and the University of Washington have found a new way to make the link. The scientists show that comparing nitrogen isotopes in their deposited form, nitrates, can reveal the sources of atmospheric nitric oxide.

In a recently published paper in Science, the group traces the source of nitrates to nitric oxides released through fossil fuel burning that parallels the beginning of the Industrial Revolution. The group also reveals the greatest change in nitrogen isotope ratios occurred between 1950 and 1980, following a rapid increase in fossil fuel emissions.

“What we find is there has been this significant change to the nitrogen cycle over the past 300 years,” said Meredith Hastings, assistant professor of geological sciences at Brown and the paper’s lead author. “So we’ve added this new source—and not just a little bit of it, but a lot of it.”

To make the link, Hastings, with Julia Jarvis and Eric Steig from the Department of Earth and Space Sciences at the University of Washington, examined at high resolution for the first time two isotopes of nitrogen found in nitrates in a Greenland ice core. The core, 100 meters long and taken at the peak of the Greenland ice cap in June 2006, contains a record of nitrates from about 1718 to 2006, according to the group.

Tests showed the ratio of the nitrogen-15 isotope to the more common nitrogen-14 isotope had changed from pre-industrial times to the present.

“The only way I can explain the trend over time,” Hastings said, “are the nitric oxide sources, because we’ve introduced this whole new source—and that’s fossil fuels burning.”

For related information, go to www.isa.org/environment.