Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions

Thanks largely to the early 20th century invention of synthetically manufactured nitrogen (N) fertilizers, the growing human population is, on average, better nourished now than ever before in human history. About 40 to 60% of the current human population depends upon crops grown with synthetic nitrogen fertilizer. Unfortunately, this impressive advance in agricultural productivity and human nutrition has come at a high price of environmental degradation and human health risks from pollution. A large fraction of nitrogen fertilizer applied to cropland - often over half - is not used by the crops and is lost to air, water, and downstream and downwind habitats, polluting landscapes and waterscapes. At the same time, energy, transportation, and industrial sectors also emit nitrogen pollution into the air through increasing use of fossil fuels. In 1997, the first Issue in Ecology described the magnitude, causes, and consequences of these human alterations of the nitrogen cycle, documenting how humans have more than doubled the amount of reactive nitrogen (see Glossary for definitions) annually in circulation in the terrestrial biosphere. Several of these trends have continued along with increasing numbers of people, including improving human diets in the developing world, increasing global use of fertilizers, increasing atmospheric concentrations of the potent greenhouse gas nitrous oxide, and increasing eutrophication of aquatic and terrestrial habitats. Fifteen years later, we now ask: "Has scientific awareness of the growing problems of nitrogen pollution fostered progress in finding solutions?".