By altering the amount of nitrogen stored in the biosphere, a new research has found that humans are disrupting the nitrogen cycle.
The chief culprit for disruption in the nitrogen cycle 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.
Now, 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 the research, the group traced the source of nitrates to nitric oxides released through fossil fuel burning that parallels the beginning of the Industrial Revolution.
The group also revealed 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 research paper's lead author. "So, we've added this new source - and not just a little bit of it, but a lot of it," she added.
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 are the nitric oxide sources, because we've introduced this whole new source - and that's fossil fuels burning," Hastings said.
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.