Global Nitrogen: Cycling out of Control

Scott Fields

Environ Health Perspect. 2004;112(10) 

In This Article

Human Sources of Reactive Nitrogen

Where does all this human-generated reactive nitrogen come from? The largest contributor is nitrogen fertilizer. As of 2000, about 100 Tg of reactive nitrogen were released each year from nitrogen fertilizer spread on farmlands around the world, according to the BioScience review. As modern farming methods have been increasingly adopted, so has the rate at which nitrogen is being fixed, with much of the increase coming in developing countries, according to Townsend and colleagues in Frontiers in Ecology and Environment. In their BioScience review, Galloway and colleagues write that widespread cultivation of nitrogen-fixing crops such as legumes has added another approximately 40 Tg of reactive nitrogen.

Global Population & Reactive Nitrogen Trends

Burning of biomass--the use of wood for fuel and the clearing of forests and grasslands for agriculture--converts another 40 Tg or so. Draining wetlands allows organic material in the soil to oxidize, and clearing land of vegetation for crops can free reactive nitrogen from soils. These sources contribute about 10 and 20 Tg, respectively, according to an article in the Spring 1997 Issues in Ecology by a team led by Peter Vitousek, a professor of population and resource studies at Stanford University.

Fossil fuel combustion also contributes to the reactive nitrogen load. "It's not just agriculture that's changing the nitrogen cycle," says Michael Mallin, a research professor at the University of North Carolina at Wilmington's Center for Marine Science. "Urbanization is doing it in a big way. Cities are full of cars. Cars release nitrogen oxides [NOx; the collective term for NO and NO2]. It goes up into the air and comes down as somebody else's problem." By fixing atmospheric nitrogen and releasing reactive nitrogen that otherwise would be sequestered indefinitely in fuels, fossil fuel combustion contributes about 20 Tg of reactive nitrogen globally each year.

Very few parts of the Earth now lack their own regional sources of reactive nitrogen pollution, says David Tilman, a professor of ecology at the University of Minnesota. "Agricultural expansion has really taken over the whole world," he says. "The rates of fertilization per hectare--the nitrogen added per hectare--are not that different. Not just among the seven or eight most industrialized nations, but even among nations that are not industrial giants, the agricultural side has really pursued nitrogen fertilization." Galloway adds that nitrogen pollution is distributed globally not just by wind and water but also by ship and truck: "International commerce is a major way of shipping reactive nitrogen around the world," he says.

As a result, Galloway says, there are significant sources of polluting reactive nitrogen in just about any corner of the Earth, with the unfortunate exception of much of Africa, which although spared much direct nitrogen pollution, is also deprived of the sorely needed fertilizer. Currently Asia, Europe, and North America account for almost 90% of human-generated reactive nitrogen, Galloway says. European countries such as the Netherlands (where long-term nitrogen fertilizer use and many concentrated animal farms have created perhaps the world's most nitrogen-saturated area) and Germany have long shown the effects of nitrogen pollution. In the Netherlands, for example, extreme reactive nitrogen levels have changed the Dutch countryside's characteristic heathlands to grasslands. But over the next 50 years, Galloway says, the developing world's growing dependence on nitrogen fertilizers, rising population densities, and adoption of gasoline-powered vehicles are all likely to result in increases in nitrogen-related environmental and human health impacts.