Global Nitrogen: Cycling out of Control

Scott Fields

Environ Health Perspect. 2004;112(10) 

In This Article

A Natural History of Nitrogen

Everything that lives needs nitrogen. But most atoms of nitrogen--which represents 78% of the atmosphere--are bound tightly in pairs as N2. Most organisms can't break the powerful triple bond of the N2 molecule's two atoms. For plants to grow and animals to thrive, they need the element in a reactive fixed form that is bonded to carbon, hydrogen, or oxygen, most often as organic nitrogen compounds (such as amino acids), ammonium (NH4), or nitrate (NO3). Animals get their reactive nitrogen from eating plants and other animals somewhere along the food chain. And plants get reactive nitrogen from the soil or water.

Lightning accounts for some naturally occurring reactive nitrogen--worldwide each year, lightning fixes an estimated 3-10 teragrams (Tg), the usual measurement unit for discussing the global nitrogen cycle. The energy that lightning generates converts oxygen and nitrogen to nitric oxide (NO), which oxidizes to nitrogen dioxide (NO2), then to nitric acid (HNO3). Within days the HNO3 is carried to the ground in rain, snow, hail, or other atmospheric deposition. This source of reactive nitrogen is important to areas in which nitrogen-fixing plants are scarce.

Lightning is responsible for fixing a portion of the Earth's naturally occuring reactive nitrogen, which is important for the soil in areas with few nitrogen-fixing plants. image credit: Photodisc

Most naturally occurring reactive nitrogen comes from nitrogen fixation by bacteria, including cyanobacteria and specialized bacteria such as those in the genus Rhizobium, which most often live symbiotically in plants such as peas, beans, and alfalfa. According to a literature review published in the April 2003 issue of BioScience by Galloway and colleagues, experts believe natural, nonagricultural organisms fix 100-300 Tg of nitrogen per year on the land surfaces of the Earth, although most estimates tend toward the lower end.

Farmers eventually learned to increase the levels of reactive nitrogen in their soil using plants that have nitrogen-fixing bacterial symbionts, but their resources were limited: at the beginning of the twentieth century they could rotate with nitrogen-fixing crops such as legumes, or add naturally occurring fertilizers such as manure, guano, and nitrate mineral deposits mined in Chile. At this point, according to the BioScience review, humans were producing about 15 Tg of reactive nitrogen per year.

Around this time, however, German scientists Fritz Haber and Carl Bosch developed a way to convert nonreactive atmospheric nitrogen to ammonia, the reactive compound that forms the base of nitrogen fertilizer. Currently, the Haber-Bosch process is used to produce about 100 Tg of reactive nitrogen per year worldwide, most of which is used to produce nitrogen fertilizer. Food grown with this fertilizer feeds some 2 billion people, estimates Vaclav Smil, a professor of geography at the University of Manitoba, writing in the July 1997 issue of Scientific American.

The past 15 years have seen a huge explosion in the amount of reactive nitrogen that humans have produced and injected into the environment, according to a report on relationships between the global nitrogen cycle and human health in volume 1, issue 5 (2003) of Frontiers in Ecology and the Environment by Alan Townsend, an assistant professor of ecology and evolutionary biology at the University of Colorado, and colleagues. Human production of reactive nitrogen is currently estimated to be about 170 Tg per year, write Galloway and colleagues in the BioScience review, and the global use of nitrogen fertilizers is increasing by about 15 Tg per year. The ratio of anthropogenic to natural reactive nitrogen creation is likely to increase with population increases, Galloway says. More mouths to feed will require both more reactive nitrogen fertilizers in the ground and the clearing of unspoiled, nitrogen-fixing lands to make farmland.