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June 5, 2009 — More and more, 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, such as the atmosphere, soil or water, to the living,
whether plants or animals. Scientists have determined that humans are
disrupting the nitrogen cycle by altering the amount of nitrogen that
is 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. But 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 paper published
June 5 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 that 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
"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."
Steig said the work also addresses a long-standing question about
changes in lake chemistry in remote regions. "Sediment cores in Arctic
lakes show that there have been significant 20th-century declines in
the nitrogen isotopic composition of organic nitrogen," Steig said.
"It's been unclear whether these are due to changes in the lake
biogeochemistry or to the direct effect of changes in the isotopic
composition of the incoming nitrate from the atmosphere. Our study
makes it clear that it is primarily the latter."
The group now wants to determine the ratio of nitrogen-14 and
nitrogen-15 isotopes for individual sources of nitric oxides, including
lightning, biomass burning, bacterial "fixing" of nitrogen, and fossil
fuel burning. The goal would be to pinpoint sources of nitrogen
overloading, whether natural or human-caused.
"For example in Narragansett Bay, we could distinguish between
nitrogen caused by sewage overflows or vehicular pollution, power
plants, fertilizers, or other sources and know how to attack the
problem," Hastings said.
Even more, the researchers want to quantify changes in the natural
sources of nitric oxides and see whether climate change is influencing
The task is complicated, however, because nitrogen, when cycling
through the atmosphere or deposited on land or in water, is subject to
influences that can alter the isotopic ratios, thus masking the source.
So, the scientists will need to tease out the extent of those
alterations to trace the isotopic signatures of nitric oxide sources
The research was funded by the National Science Foundation's Office
of Polar Programs and the Joint Institute for the Study of the
Atmosphere and the Ocean (JISAO).
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