The consequences are grave: Nitric oxide (NO) rises from farms, power plants and vehicles, for instance, in the upper Midwest and drifts toward New England forests where nitric acid (HNO3) in the rain leaches important plant nutrients like potassium, calcium and magnesium from the soil, Schlesinger says. Researchers at Hubbard Brook Experimental Forest in White Mountain National Forest, N.H., found evidence of this rain and reported that it may cause a reduction in cold or stress tolerance in some tree species including red spruce and sugar maple. Similarly, nitric oxide has been documented as rising from similar sources in Kentucky and Tennessee and drifting toward the Great Smoky Mountains, where some of the worst acid rain and forest decline has been observed, Schlesinger says. Acid in rain also liberates aluminum in the soil, which can be is poisonous to insects and fish if the metal enters stream runoff. And, excess available nitrogen in rain may promote some species of plants as it diminishes others. In fact, researchers at the University of Minnesota reported in 2008 that atmospheric nitrogen deposition reduced plant species numbers in the state's prairie grasslands by 17 percent.
In the U.S. there are neither comprehensive laws nor adequate monitoring devices for regulating atmospheric nitrogen emissions from livestock and farms. Europeans passed the Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-Level Ozone in 1999, a pact signed by 49 countries, but the U.S. has dragged its feet. Schlesinger thinks that national arguments over climate change have allowed the U.S. to ignore the nitrogen problem, which he predicts will be the next big environmental issue. "It's another example of humans upsetting global biogeochemical cycles with unintended consequences," he says. Since Gothenburg, Europe has decreased its nitrogen emissions by a third, whereas U.S. emissions remain flat. And the U.S. has increased its ammonia emissions, an atmospheric component of the nitrogen problem, by 27 percent from 1970 to 2005, according to a 2009 paper in Environmental Science & Technology.
Without intervention, the problem will likely worsen. With world population predicted to grow from 6.5 billion to nine billion by 2050, agriculture must feed more mouths, and that's probably going to require more nitrogen fertilizer, thereby resulting in more nitric acid rain and atmospheric pollution. The Integrated Nitrogen Committee's 247-page draft report discusses inputs, flows and management options for reactive nitrogen in the U.S. environment. It also discusses ways to monitor atmospheric emissions, currently the weak link in the nitrogen control picture.
It's clear that humans are adding nitrogen to Earth's surface. Researchers do not know yet where it all goes, "but we do know that increasing concentrations of nitrogen in unexpected places will cause significant environmental damage that we will all learn to regret," Schlesinger wrote in a 2009 report in Proceedings of the National Academy of Sciences.
Still, Aneja sees promise. Agriculture has adopted modern technologies and science to maximize productivity, but it has not yet been subjected to the same environmental regulations applied to other modern industries, he says. "The Integrated Nitrogen Committee report is an effort to develop more stringent measures," he adds, "and we're not ignoring the atmospheric contribution."