Study leader Alan Townsend of CU-Boulder's Institute for Arctic and Alpine Research said changes in the global nitrogen cycle, while beneficial in increasing crop growth, appear to pose a growing health risk. Roughly half of the inorganic nitrogen ever used on the planet has occurred in the past 15 years.
An obvious, positive aspect of using nitrogen as a fertilizer has been a huge increase in food production in poor nations, reducing hunger and malnutrition, he said. Although nitrogen is the most abundant of Earth's atmospheric gases, it must be converted to chemically usable forms like nitrate or ammonium. In the absence of humans, this happens during lightning strikes or more commonly through microbes.
"The major global changes in the nitrogen cycle have occurred because humans now convert more nitrogen to such usable forms than all natural processes combined," he said. "The synthesis of nitrogen fertilizers accounts for most of this change. But the overuse of nitrogen fertilizers can lead to a number of problems, including air and water pollution."
So far, most nitrogen studies have focused on problems such as losses in biodiversity, increased acid rain and changes in coastal ocean ecology that include oxygen-poor "dead zones" like those seen in the Gulf of Mexico.
However, excess nitrogen also can be a health concern for humans in many ways, including respiratory ailments, heart disease and several cancers, said Townsend, who also is an assistant professor in CU-Boulder's ecology and evolutionary biology department.
"Ecological feedbacks to excess nitrogen can inhibit crop growth, increase allergenic pollen production and potentially affect the dynamics of several vec