Nitrogen is an essential, fundamental building block for life. Without an adequate supply of nitrogen, crops do not thrive and fail to reach their maximum production potential. In many ecosystems, nitrogen is the limiting element for growth. However, when present in excess, reactive nitrogen causes a range of negative environmental effects, poses risks to human health, and consequently can have negative economic and social consequences. While synthetic nitrogen fertilizers increase crop production, unintended releases of reactive nitrogen to the broader environment through agricultural runoff can contaminate groundwater, rivers and estuaries. Another major source of reactive nitrogen is the combustion of fossil fuels, which creates nitrogen oxides (NOX) in the atmosphere.

In the last 150 years, the annual inputs of reactive nitrogen from agricultural, industrial and transportation sources to the earth’s atmosphere, soils, and water bodies have increased by more than a factor of ten and now exceed the annual rate of production of reactive nitrogen from natural sources. In the air, excess reactive nitrogen contributes to higher levels of ozone in the lower atmosphere, causing respiratory ailments and damaging vegetation. From the atmosphere, nitrogen falls to the surface, generating a series of effects — corrosion of buildings, bridges and other human-made structures, acidification of soils and water bodies, and inadvertent fertilization of trees and grasslands, creating unnatural growth rates, nutrient imbalances, and decreasing biodiversity. Leaching out of the soils, reactive nitrogen can make groundwater and surface water unfit for human consumption. Reactive nitrogen also promotes eutrophication in coastal ecosystems, which deteriorates fish habitat and aquatic biodiversity. Eventually, most reactive nitrogen is converted back to harmless nitrogen gas by soil and aquatic bacteria called “denitrifiers”, but a portion is converted to nitrous oxide which contributes to both the greenhouse effect and to stratospheric ozone depletion. Thus once introduced into the environment, nitrogen cascades from one negative environmental impact to another.

While an excess of reactive nitrogen exists in industrialized countries, too little nitrogen is available in many developing countries, thus limiting the ability of local agriculture to meet the basic challenge of producing enough food to sustain the population and to contribute to economic development. Insufficient nitrogen and other agricultural nutrient inputs can also lead to land degradation, soil erosion, desertification and their attendant long-term environmental and economic consequences. Our challenge is to manage reactive nitrogen wisely, so that it supports productive agriculture where needed, while causing minimal harm to native ecosystems and human health through air and water pollution. This challenge will be magnified in the future as both population and per-capita use of resources increases.

The complex environmental, human health and economic issues surrounding reactive nitrogen (whether in excess or in deficiency) require monitoring, research and assessment of their effects, as well as broader information sharing to inform the design of specific policy responses at local, national and regional levels. Experience shows that well designed policy instruments can play a major role in rectifying reactive nitrogen imbalances and their resulting effects. However, developing effective policies is not a simple matter because the effects of reactive nitrogen are not limited to a single medium — air or soil or water — and a policy to remedy one issue may inadvertently aggravate another. Thus, a comprehensive and coherent understanding of the issue, and of the status of related policy initiatives, is an initial requirement for any action. A lack of technical capacity, economic incentives, and informational resources hampers effective policy and action on this issue. Successfully resolving the challenges created by the excesses of reactive nitrogen requires an integrated approach across policy sectors. This integration must be guided by further assessment, monitoring and analysis of the behavior and impacts of reactive nitrogen in the environment and by experience already gained in developing and implementing policies to reduce the impacts of reactive nitrogen.

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