Global Nitrogen: Cycling out of Control

Scott Fields

Environ Health Perspect. 2004;112(10) 

In This Article

Abstract and Introduction

The nitrogen cycle is the basic element cycle that has been the most drastically altered by humankind--and the one that could have the greatest impact on global warming, water quality, and human health. Since the early 1900s discovery of a new chemical process to cheaply produce reactive nitrogen for industrial uses, mankind's production of these compounds has exploded by more than 10 times. Although nitrogen fertilizer makes it possible for more of the world's hungry to be fed, the unchecked production of reactive nitrogen compounds also harshly affects biodiversity, global warming, water quality, and other aspects of human health. Scientists are looking for ways to rein in this off-kilter cycle in a way that preserves the benefits offered by reactive nitrogen.

Like the Earth's water, nitrogen compounds cycle through the air, aquatic systems, and soil. But unlike water, these compounds are being injected into the environment in ever increasing quantities. In doing so, we are altering the global nitrogen cycle, causing possible grave impacts on biodiversity, global warming, water quality, human health, and even the rate of population growth in developing nations.

In a world surrounded by nitrogen, you would think there's always been plenty to go around and that perhaps a little more wouldn't matter. But having enough of the right kind of nitrogen--reactive nitrogen that has been "fixed," or converted from the nonreactive N2 form--determines such fundamentals of life as the extent of plant growth, which in turn determines to a large extent the dynamics of the world's food supply. During the twentieth century, mankind has produced increasingly more reactive nitrogen, intentionally as fertilizer and unintentionally as a by-product of combusting fossil fuels.

Food grown with nitrogen fertilizers feeds an estimated 2 billion people worldwide. Areas including Asia are becoming increasingly dependent on such fertilizers, to the detriment of the environment. image credit: Flat Earth

Although carbon dioxide may get more press, "the nitrogen cycle has been altered more than any other basic element cycle," says John Aber, vice president for research and public service at the University of New Hampshire. And now, he says, humans are adding more reactive nitrogen to the global nitrogen cycle than all other sources combined. Yet, reactive nitrogen is hardly all bad. The use of nitrogen fertilizer is critical to feeding the world's hungry, say researchers including University of Virginia environmental sciences professor James Galloway. The question, then, is how do we manage nitrogen responsibly?