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Global Nitrogen Cycle

Contact: Alan Townsend

Jim Scott

University of Colorado at Boulder

Despite greatly increasing food production for humans, the growing use of nitrogen as a nutrient is affecting people's health far beyond just the benefits of growing more crops, according to a new University of Colorado at Boulder-led study.

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 vector-borne diseases, including West Nile virus, malaria and cholera," the researchers wrote. A paper on the subject appeared in the June 2 issue of Frontiers in Ecology.

The project was funded by the Andrew Mellon Foundation.

Co-authors on the paper are from Cornell, Harvard and Princeton universities, the Woods Hole Marine Biological Laboratory, the National Center for Atmospheric Research in Boulder and the Institute for Agriculture and Trade Policy in Minneapolis. Other co-authors are from the University of North Carolina at Wilmington, the New England School of Acupuncture in Watertown, Mass., and Visteon Corp. in Sterling Heights, Mich.

"On the bright side, there are solutions to these problems," said Townsend. "Too much fertilizer is being used in developed countries, while in some impoverished countries, additional fertilizer is needed. This is something that can be changed."

In the United States, for example, fertilizer-intensive crops are common and more fertilizer than is needed for maximum crop yields often is used. Reducing fertilizer also would lessen crop pollution to our waterways and air, he said.

In addition, the use of fertilizer in modern industrial nations is not optimized for the production of the healthiest food, Townsend said. Crops like corn largely become food for domestic animals, leading to further nitrogen losses to the environment, disparities in world food distribution and a growing tendency for unhealthy diets even in wealthy nations, the researchers concluded.

In the United States, more than half of the grain produced is fed to animals, and corn is used much more widely as a sweetener than for human consumption. Meat consumption by humans has doubled worldwide since 1960, and excess meat consumption has been linked to numerous health issues, including heart disease.

In addition, increased nitrogen pouring into the world's oceans can cause algal blooms that can harm fish, shellfish and humans. On land, ozone, a major pollutant produced with high amounts of nitrogen oxides, causes numerous health problems as well as billions of dollars of crop damage, according to the research team.

"We believe the greatest net health benefits come from using nitrogen at moderate levels," said Townsend. "Making and using it at higher levels does not lead to parallel increases in benefits, but does greatly exacerbate environmental and health problems."

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