4 em september 2015 awma.org

by Christian Hogrefe

Christian Hogrefe is with the Atmospheric Modeling and Analysis Division of the National Exposure Research Laboratory at the U.S. Environmental Protection Agency. Christian is Chair of the Journal ’s Editorial Review Board and a long-time member of EM ’s Editorial Advisory Committee.

em • cover story

ReactiveNitrogen

and possiblemanagementapproaches

A detailed look atCHAIN REACTION :

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Committee.

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september 2015 em 5awma.org

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ANNIVERSARY

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ANNNNIVERIVERIVERIVERSARY

As reactive nitrogen

moves through

environmental

systems, it can

change the

behavior of these

systems and,

thereby, also affect

human health and

well-being.

Nitrogen is one of the building blocks of life, yet excessive amounts in the envi-ronment can cause problems in various ecosystems. Abundant in the atmosphere

as dinitrogen (N2), nitrogen needs to be combined with other elements such as hydrogen or oxygen to form compounds collectively known as reac-tive nitrogen before it can be used by most living organisms. This process is referred to as fi xation. Natural pathways for creating reactive nitrogen are biological fi xation by algae and bacteria hosted by leguminous crops such as soybeans and alfalfa, as well as the production of nitrogen oxides from lightning. Man-made pathways include the pro-duction of synthetic fertilizer through the Haber-Bosch process and the combustion of fossil fuels.

The advent of synthetic fertilizer production in the early 20th century has increased food secu-rity around the globe and has helped support a growing global population. However, increasing use of synthetic fertilizer, as well an increase in livestock, have also had adverse effects on aquatic ecosystems through run-off and leaching that can lead to eutrophication of waterbodies. The burning of fossil fuels has led to changes in atmo-spheric chemistry and composition which, in turn, can affect concentrations of harmful pollutants such as ozone, alter the radiative balance of the atmosphere, contribute to changes in stratospheric ozone levels, and lead to acidifi cation of terrestrial and aquatic ecosystems through deposition.

A single molecule of reactive nitrogen, cre-ated through natural or man-made processes, can cycle through various environmental sys-tems—the atmosphere, terrestrial ecosystem, and aquatic ecosystems—where it can be transformed or temporarily stored. As it moves through these systems, it can change the behavior of these sys-tems and, thereby, also affect human health and well-being. This movement through the different environmental systems following the initial cre-ation of reactive nitrogen is often referred to as nitrogen cascade. Since the starting point of this cascade often is related to human activity, a part of the solution to address imbalances in the reac-tive nitrogen cycle may include choices about our lifestyle, most notably by reconsidering the dietary choices we make and the amount of energy we use in our daily lives.

The six articles in this issue examine various aspects of reactive nitrogen and potential manage-ment approaches, both from a North American and European perspective.

In the fi rst article, Galloway et al. provide an over-view of both the successes and the challenges in managing reactive nitrogen pollution in the United States and discuss potential pathways going forward. Rotz and Leytem focus on reactive nitrogen emissions from agricultural operations, summarizing the various types of emissions, their relative contributions, and potential mitigation approaches. Winiwarter et al. discuss manage-ment approaches and institutional mechanisms in Europe, both within multi-country frameworks, as well as initiatives taken in individual countries. These articles on management and mitigation aspects of reactive nitrogen pollution are followed by two articles on the impacts of nitrogen pollu-tion. Pardo et al. provide an overview of the var-ious impacts of nitrogen pollution on terrestrial ecosystems in the United States, while Erisman et al. discuss trends in nitrogen deposition over Europe and the impacts on ecosystems. Finally, Bash et al. discuss reactive nitrogen from a mod-eling perspective by summarizing recent devel-opments, highlighting observational needs, and proposing future research directions. em

DISCLAIMER:Although this intro-duction and any feature articles in this issue involving U.S. Environmen-tal Protection Agency authors or co-authors have been reviewed and approved for publi-cation by the agency, they do not refl ect the views and poli-cies of the agency.

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