Reactive Nitrogen in the United States: An Analysis of Inputs, Flows, Consequences, and Management Options

This original SAB study analyzes sources and fate of reactive nitrogen in the environment, and provides advice to the EPA on integrated nitrogen research and control strategies.

Sources of Reactive Nitrogen

Nitrogen gas in the air is an abundant, inert form of nitrogen that is transformed by nitrogen-fixing microbes into reactive forms of nitrogen that are taken up by algae, plants and other producers at the base of the food web. Human activities (primarily production and use of nitrogen fertilizers, nitrogen-fixing legume crops, and burning of fossil fuels) introduce five times more reactive nitrogen into the U.S. environment than natural sources.

Environmental Effects

The overload of reactive nitrogen causes a range of effects as it cycles in the atmosphere, on land, and in water bodies. This sequence of effects is called the “nitrogen cascade.” Reactive nitrogen provides essential benefits as a fertilizer for food production. However, most of this nitrogen is not taken up by crops and is lost to the environment where it can contribute to the impacts noted above. Nitrogen oxides from burning of fossil fuels for transportation and power generation contribute to formation of smog, particulate matter and acid rain, and then can go on to contribute to over-fertilization of unmanaged forests and grasslands, coastal eutrophication, greenhouse effect and stratospheric ozone depletion.

Management Implications

The SAB recommends (1) the use of the nitrogen cycle as an essential framework to address the environmental loading of reactive nitrogen; (2) an integrated cross-media approach to more effectively manage reactive nitrogen; (3) and monitoring and research to support management of reactive nitrogen. The SAB suggests that a 25 percent reduction of excess reactive nitrogen can be achieved with existing technology in the near term. The SAB also emphasizes that this decrease alone will not solve the problems of excess reactive N in the environment.