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.
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.