Global Nitrogen: Cycling out of Control

Scott Fields

Environ Health Perspect. 2004;112(10) 

In This Article

A Vicious Cycle?

"The nitrogen cycle has changed on a global scale to a remarkable extent, but the rate at which that plays out locally is hugely variable," says Townsend. "There are major hot spots at all of the industrialized nations of the world. We're seeing incredible increases [in reactive nitrogen use/production and resulting pollution] in the United States, much of Europe, and much of Asia and China now. There are areas there, for example, that are seeing deposition from the atmosphere that is ten times or more what it was prior to human activity."

Some of this reactive nitrogen is, of course, put to good use, Townsend says. Nitrogen fertilizers can take credit for reductions in starvation and malnutrition in many parts of the world, especially in Asia in the last decade. In fact, Smil writes in the March 2002 issue of the Swedish journal Ambio that "for at least a third of humanity in the world's most populous countries the use of [nitrogen] fertilizers makes the difference between malnutrition and adequate diet."

Nitrogen fertilizers make the difference between an adequate diet and malnutrition for much of the world population, but an excess of reactive nitrogen compounds in the air, water, and soil wreaks havoc on fragile ecosystems. image credit: Photodisc

But as nitrogen levels continue to rise, Townsend says, the net health effects become increasingly negative. Furthermore, says Galloway, reactive nitrogen can not only impact many different ecosystems, but a single atom also can make mischief repeatedly, unlike most better recognized pollutants. "If you put a molecule of NOx in the atmosphere from fossil fuel combustion or a molecule of ammonium on an agricultural field as a fertilizer," he explains, "you have a whole series, or cascade, of effects that goes from acid rain to particle formation in the atmosphere, decreasing visibility and causing impacts on human health, acid rain, soil and stream acidification, coastal eutrophication, decreasing biodiversity, human health issues in groundwater, and nitrous oxide [N2O] emissions to the atmosphere, which impact the greenhouse effect and stratospheric ozone."