Reactive nitrogen: Important successes but major hurdles remain
Published in Sustainability

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Poster: River Fyrisån outside Uppsala. Photo: Sara Cousins

Successfully reduced emission of reactive nitrogen from transport and energy production systems has contributed to cleaner air and reduced the global emission from fossil fuel combustion to the environment, but the still increasing human use of anthropogenic nitrogen compounds in food production and agriculture leads to cascading ecosystem effects and poses a major environmental challenge for the 21st century.

James N. Galloway

Sidman P. Poole Professor, Environmental Sciences Department, University of Virginia, Charlottesville, Virginia, USA

Ellis B. Cowling

University Emeritus Professor-at-Large, North Carolina State University, Raleigh, North Carolina, USA

Behind the paper: Reflections on 200 years of nitrogen, 20 years later

In March 2002, a special issue of Ambio was published containing 18 overarching papers from the Second International Nitrogen Conference convened in Potomac, Maryland in October 2001. It is now our privilege, nearly 20 years after the conference and publication of the Ambio special issue, to offer a few reflections on some of the major points during the conference and in the special issue.

The increasing interest in nitrogen over the past two decades is a result of increased conversion of non-reactive N2 into chemically, biologically and radiatively active compounds (Nr, defined as all N molecules except N2) and the subsequent negative impacts on human and environment health. In 1990, 140 Tg Nr were created by human activities and in 2020, we estimate that ~240 Tg N will be created.

Most of this increase is caused by the increased use of N fertilizers and increased cultivation of legume to help satisfy the increased demand for food for a growing global population and an increasing preference for animal protein. On a positive note, globally NOx emissions from fossil fuel combustion reached their maximum in ~2000 and have significantly decreased from then on, due to air pollution legislation and increased use of renewable energy sources. This is especially true for North America and Europe.  There are two underpinning reasons for this success—NOx is produced by accident and emitted at point sources.   The first means it is not needed; the second means that it is easier to control.

The bad news is that the source of most of the Nr created by humans is used for food production.    It is both needed (to feed people), and hard to control (there are no point sources).  It begins with the fact that ~85% of the N used to produce food is lost to the environment during the food production process. Following food consumption, the remainder is injected into the environment as human waste because the vast majority of human waste does not undergo advanced wastewater treatment, which enhances denitrification. Once in the environment, Nr contributes to numerous environmental negative impacts connected in time and space by the N Cascade. Simply stated, once the triple bond within the N2 molecules is broken, the resulting Nr will cascade through environmental reservoirs contributing to negative impacts and will stay active until either converted to N2 or stored in a long-term reservoir (e.g., long-living forest trees and ocean sediments).

The list of environmental issues and public health associated with the nitrogen cascade have not changed much in the last two decades, but their extent has. For example, groundwater is increasingly contaminated with nitrate. Hypoxic coastal waters have increased in number and geographical area. The troposphere and stratosphere are increasingly burdened by N2O. In the troposphere N2O is a greenhouse gas that contributes to global warming; in the stratosphere N2O is the primary destroyer of ozone.

There are still major hurdles to overcome on the food production and consumption sides. In this connection there are two key words: people and protein. By 2050, the world population of people is estimated to be ~9.8 billion, up from 7.8 billion in 2020. With respect to protein, on average the per-capita protein consumption will likely increase, especially since 99% of the population growth from 2020 to 2050 is projected to occur in Asia and Africa which currently have low protein consumption rates now relative to North America and Europe.

James N. Galloway


Ellis B. Cowling


Original article

Galloway, J.N., and Cowling E.B. 2002. Reactive nitrogen and the world: 200 Years of change. Ambio 31: 64-71


All articles in Ambio's 50th Anniversary Collection: Eutrophication


Tedengren, M. 2021. Eutrophication and the disrupted nitrogen cycle. 50th Anniversary Collection. Theme: Eutrophication. Ambio. Volume 50.

Behind the paper

Caraco, N.F. 2021. A tribute to tributaries: River studies elucidate links between human activity and nutrient export across a broad range of watersheds. 50th Anniversary Collection: Eutrophication. Ambio. Volume 50.

Elmgren, R. 2021. Assessing human effects on the Baltic Sea ecosystem. 50th Anniversary Collection: Eutrophication. Ambio. Volume 50.

Galloway, J.N., and Cowling E.B. 2021. Reflections on 200 years of nitrogen, 20 years later. 50th Anniversary Collection: Eutrophication. Ambio. Volume 50.

Hall, S.J., K.A. Lohse, and P.A. Matson. 2021. Globalization of nitrogen deposition and ecosystem response: A twenty-year perspective. 50th Anniversary Collection: Eutrophication. Ambio. Volume 50. 


Bonsdorff, E. 2021. Eutrophication – early warning signals, ecosystem-level and societal responses, and ways forward. 50th Anniversary Collection: Eutrophication. Ambio. Volume 50. 

Melillo, J.M. 2021. Disruption of the global nitrogen cycle: A grand challenge for the 21st century. 50th Anniversary Collection: Eutrophication. Ambio. Volume 50. 

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