Friend and Foe: Nitrogen Pollution’s Little-Known Environmental and Human Health Threats

Article ID: 579863

Released: 25-Aug-2011 1:00 PM EDT

Source Newsroom: American Chemical Society (ACS)

  • Credit: Credit: iStock

    The use of nitrogen-based fertilizers, which has doubled in the past century, contributes to a rapidly growing environmental threat.

DENVER, Aug. 28, 2011 — Billions of people owe their lives to nitrogen fertilizers — a pillar of the fabled Green Revolution in agriculture that averted global famine in the 20th century — but few are aware that nitrogen pollution from fertilizers and other sources has become a major environmental problem that threatens human health and welfare in multiple ways, a scientist said here today.

“It’s been said that nitrogen pollution is the biggest environmental disaster that nobody has heard of,” Alan Townsend, Ph.D., observed at the 242nd National Meeting & Exposition of the American Chemical Society (ACS), being held here this week. Townsend, an authority on how human activity has changed the natural cycling of nitrogen to create a friend-turned-foe dilemma, called for greater public awareness of nitrogen pollution and concerted global action to control it. He spoke at a symposium on the topic, which included almost a dozen reports (abstracts of each presentation appear below) by other experts.

“Awareness has grown, but nitrogen pollution remains such a little-recognized environmental problem because it lacks the visibility of other kinds of pollution,” Townsend explained. “People can see an oil slick on the ocean, but hundreds of tons of nitrogen spill invisibly into the soil, water and air every day from farms, smokestacks and automobile tailpipes. But the impact is there — unhealthy air, unsafe drinking water, dead zones in the ocean, degraded ecosystems and implications for climate change. But people don’t see the nitrogen spilling out, so it is difficult to connect the problems to their source.”

Townsend described the scope and the intensification of the nitrogen pollution problem as “startling.” He noted that nitrogen inputs to the terrestrial environment have doubled worldwide during the past century. This increase is due largely to the invention and widespread use of synthetic fertilizer, which has revolutionized agriculture and boosted the food supply.

The concern focuses on so-called “reactive” nitrogen. Air contains about 78 percent nitrogen. But this nitrogen is unreactive or “inert,” and plants can’t use the gas as a nutrient. In 1909, chemist Fritz Haber developed a way to transform this unreactive gas into ammonia, the active ingredient of synthetic fertilizer. By 2005, human activity was producing about 400 billion pounds of reactive nitrogen each year.

“A single atom of reactive nitrogen can contribute to air pollution, climate change, ecosystem degradation and several human health concerns,” Townsend said. He is an ecology and evolutionary biology professor at the University of Colorado at Boulder. Damage to the ecosystem — a biological community interacting with its nonliving environment — includes water pollution and reduced biological diversity, including the loss of certain plant species.

Though the full extent is currently unknown, nitrogen pollution can impact human health. Reactive nitrogen is a key contributor to air pollution, including the formation of ground-level ozone, which is a well-known health risk. Recent estimates suggest that nitrogen-related air pollution costs the U.S. well over $10 billion per year in both health costs and reduced crop growth. And though less well studied, high nitrogen levels in water can cause a variety of health concerns, ranging from the effects of drinking water nitrate to the potential to alter the risks of several human diseases.

Increased nitrogen levels also have implications for climate change, Townsend noted. Excess nitrogen can affect the rate of climate change in multiple and opposing ways. One the one hand, it leads to more warming via the greenhouse gas nitrous oxide, but on the other hand, it can reduce warming by fueling extra plant growth and by forming substances called reflective aerosols in the atmosphere, the scientists noted.

“The net effect of these processes remains uncertain, but appears to result in minor cooling presently,” Townsend said. However, he noted that excess nitrogen also has large and clear consequences for some worrisome impacts of a changing climate, notably air and water pollution.

“Climate change is expected to worsen each of these problems worldwide, but reduction of nitrogen pollution could go a long way toward lessening such climate-driven risks,” he added.

“We’re just now starting to recognize the scope of the problem,” said Townsend. “But the good news is that there are many opportunities for us to lessen the problems. These include ways in which chemists can help, ranging from the development of new technologies to reduce nitrogen’s impact to new measurement technologies and techniques that can better diagnose the problems we face with nitrogen.”

He outlined several possible solutions to the problem. They include continued and greater support for technologies that remove or reduce reactive nitrogen formation during fossil fuel burning and incentives that can encourage farmers to be more efficient with their fertilizer use. The latter could include subsidies that reward the application of environmental practices that reduce nitrogen levels, he said.

Several other solutions exist for improving the efficiency of agricultural nitrogen use, Townsend added. "In many ways, we already know how to do it —the problems are largely about finding the political and cultural means to implement these new practices," he said.

The National Science Foundation and the David & Lucile Packard Foundation provided funding for Townsend’s research.

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ABSTRACTS:Nitrogen and the Human EndeavorAlan R Townsend, Professor, University of Colorado at Boulder, Department of Environmental Studies, INSTAAR, 1560 30th Street, Boulder, CO, 80303, United States, 303-492-6865, 303-492-6388,

Modern human society relies upon extraordinary disruptions to several global biogeochemical cycles. Transformation of one such cycle – nitrogen (N) – was a pillar of the Green Revolution and remains necessary to ensure future food security, but also creates multiple negative environmental and socio-economic consequences. Both the scale and pace of human changes to the N cycle are startling: in two human generations, we have more than doubled “reactive” N inputs to the terrestrial biosphere. Many of nitrogen's harmful effects are a product of its diverse chemical nature; a single atom of human-created reactive nitrogen can contribute to air pollution, climate change, ecosystem degradation and several human health concerns as it passes among multiple redox states. This talk will review human changes to the global N cycle and their major consequences, and then present examples of how society can pursue a more sustainable relationship with this essential element.

Anthropogenic impacts recorded in the isotopes of atmospheric nitrateMeredith Hastings, Brown University, Dept of Geological Sciences/Environmental Change Initiative, 324 Brook Street, Box 1846, Providence, RI, 029012, United States, 401-863-3658, 

Atmospheric nitrate is the result of nitrogen oxide emissions and reactions between NOx and major oxidants such as ozone and hydroxy compounds. Sources of NOx include fossil fuels combustion, biomass burning, biogenic processes in soils, lightning and stratospheric oxidation of nitrous oxide. With significant variability in latitude and altitude, distinguishing source contributions, particularly from the natural sources of NOx,is a challenge. The isotopic record of nitrate in an ice core from Greenland, reveals a clear change in the δ15N of nitrate that reflects changes in the sources of nitrate. The δ15N of nitrate decreases from a pre-industrial value of +11‰ (vs. air) to significantly lower values in the last decade (~ -1‰). This decrease is highly correlated with fossil fuel emissions estimates since 1750, but may also reflect soils emissions related to agricultural changes over this time period. The ability to trace the sources of NOx, in the modern environment and over time, has implications for studies of acid deposition, air quality, eutrophication, biogeochemistry and paleoclimate.

Poorly known roles of iron and other metals in the nitrogen cycleEric A Davidson, The Woods Hole Research Center, 149 Woods Hole Road, Falmouth, MA, 02540-1644, United States, 15084441532, 15084448132,

The nitrogen cycle includes numerous processes that alter the oxidation state of the N atom, from -3 in ammonia to +5 in nitrate. Iron and other metals are known to play roles in some of the enzymatic reactions of the N cycle. Abiotic reactions involving iron are thought to affect nitrate in aquatic systems, but their role in soil processes is less well understood. The fate of nitrate from atmospheric deposition onto forest soils remains a mystery. Abiotic reduction of nitrate by ferrous iron is thought to be thermodynamically unfavorable under the acid conditions of forest soils, and yet there is evidence for abiotic consumption of nitrate. Another mystery is reduction of atmospheric nitrous oxide in well drained soils, which is unlikely to be due to classical biological denitrification, with a tantalizing hint that it is more common in iron-rich soils.

Fertilizer management effects on nitrous and nitric oxide emissions from cropping systems of the upper Midwest U.S.Rodney T Venterea, USDA-ARS, Soil and Water Management Research, 1991 Upper Buford Circle, St. Paul, MN, 55108, United States, 612-6244-7842,

Increases in atmospheric N2O concentrations, driven largely by application of nitrogen fertilizers, continue at a steady rate with important effects on climate forcing and stratospheric ozone depletion. Fertilizer-derived NO emissions influence local and regional atmospheric chemistry and air quality. Corn production consumes more than 40% of all N fertilizers applied to crops in the U.S. and represents the largest single source of fertilizer-derived soil N2O and NO emissions relative to other crops. While reduction in N fertilizer application rates may be an effective means of reducing these emissions, this may come at the cost of decreased yields. We have been examining the use of alternative practices that could reduce emissions without necessarily reducing N inputs or crop yields, such as modification of fertilizer source, placement or timing, and alteration of tillage and other management regimes. We will present a summary of our recent studies in this regard and discuss the challenges in developing management recommendations for effective mitigation of N emissions and enhancement of N use efficiency.

Understanding the role of the nitrogen cycle in the climate and earth systemsElisabeth A Holland, Senior Scientist, Dr., National Center for Atmospheric Research, Atmospheric Chemistry Division, 3090 Center Green Dr., Boulder, CO, 80301, United States, 303-497-1433,

The bio-atmospheric exchange of reactive nitrogen has increased three- to five-fold since 1850 as a result of intensified fossil fuel use and agriculture. The changing nitrogen cycle is poised at the crossroads of climate change, air and water quality, agriculture, ecosystem function and sustainability making it a truly global issue. The carbon cycle has been in the limelight of public attention. Yet, the nitrogen cycle is central to the atmospheric concentrations of four of the top five greenhouse gases, carbon dioxide, methane, tropospheric ozone and nitrous oxide, and to the concentration of atmospheric aerosols that provide radiative cooling offsets. State of the art coupled carbon, nitrogen and climate models have been developed in the last few years enabling a more complete evaluation of the climate impact of the nitrogen cycle. As global citizens, do we need to think about our nitrogen footprint AND our carbon footprint?

Sluggish ocean nitrogen budget during the last ice ageHaojia Ren, Princeton University, Department of Geosciences, Guyot Hall, Princeton, NJ, 08544, United States, 609-933-0094,

Using the “persulfate/denitrifier” strategy for analyzing the 15N/14N of nanomole quantities of organic N, we are exploring the paleoceanographic utility of the organic matter bound within the calcium carbonate shells of planktonic foraminifera. In deep sea sediment cores from the tropical Atlantic, we find that the 15N/14N of foraminifera-bound organic matter from the last ice age is higher than that from the current interglacial, indicating higher nitrate 15N/14N in the ice age Atlantic thermocline. This and species-specific differences are best explained by less N fixation in the Atlantic during the last ice age. The N fixation decrease was most likely in response to a previously recognized reduction in the ocean's loss rate of fixed N during ice ages. This ice age reduction in N fixation would have worked to balance the ocean N budget, at lower ice age rates of input and loss, and to curb ice age-to-interglacial change in the ocean's fixed N inventory.

Recent trends in reactive nitrogen: Air quality, deposition, and climate changeRobert W Pinder, US Environmental Protection Agency, Atmospheric Modeling and Analysis Division, Mail Drop E243-01, Research Triangle Park, NC, 27711, United States, 919-541-3731, UNITED STATES,

Reactive nitrogen emissions, including nitrogen oxides (NOx) and ammonia (NH3), are dangerous air pollutants that degrade human health, deteriorate ecosystem function and exacerbate climate change. After decades of persistently high levels, nitrogen oxide emissions have been decreasing over the past 10 years. This is largely due to more effective emission control systems on vehicles and power plants. The results of these emission reductions are evident in the satellite record, precipitation measurements and atmospheric concentrations. Since NH3 is largely an unintended byproduct of industrial agriculture, these emissions have not decreased in recent years. This talk will consider the impacts of these recent emission trends on air quality, nitrogen deposition and climate change.

Nitrogen deposition, carbon storage, and climateChristine L. Goodale, Cornell University, Ecology & Evolutionary Biology, E215 Corson Hall, Ithaca, NY, 14853, United States, 607-254-4211,

Human activities have greatly accelerated the emissions of reactive nitrogen to the atmosphere and the deposition of this N onto downwind ecosystems. This N deposition was long expected to enhance forest growth and C sequestration, but observational evidence of deposition-induced stimulation of regional forest C sequestration was lacking until recently. The response of forest soil C storage is even less well-understood and quantified, but observational evidence is accumulating to indicate that N deposition suppresses decomposition in many forest soils. We combine recent evidence of N-induced forest C sequestration to provide quantitative estimates of national- and global C sinks in trees and soils. These observations are now being used to develop and test local and global-scale models of C, N and climate.

Response of sensitive freshwater ecosystems to reactive nitrogen deposition in western North AmericaSarah A Spaulding, USGS, Fort Collins Science Center, 2150 Centre Ave., Building C, Fort Collins, CO, 80526, United States, 303-492-5158, Sarah.Spaulding@Colorado.EDU

We discuss the evidence for widespread, synchronous and threshold changes in sensitive freshwater ecosystems as the result of reactive nitrogen deposition in western NorthAmerica. Analyses of bulk N stable isotopes from multiple paleolimnological studies using lake sediments support observations of increased delivery of dissolved inorganic nitrogen with concurrent depletion of δ15N, a marker of anthropogenic sources. In nutrient poor aquatic ecosystems, including sensitive high elevation lakes, algae were historically limited by N. With increasing deposition of N, primary productivity has increased and algal species composition has shifted. In some cases, algal growth is no longer limited by N. As a result, these aquatic ecosystems are changing from oligotrophic (low nutrient) systems to mesotrophic (mid nutrient) systems. The eutrophication of lakes by deposition of atmospheric N is of great concern in the west, and there is evidence that many unproductive lakes are experiencing increased biological production due to atmospheric N.

Framework for action on nitrogen: Lessons from the California Nitrogen AssessmentThomas P Tomich, University of California, Davis, Agricultural Sustainability Institute, One Shields Ave, Davis, CA, 95616, United States, 530-752-2379,

This paper will report the results of the California Nitrogen Assessment, a major integrated scientific assessment which will be completed in June 2011. It will differentiate between what is scientifically well known and what is more uncertain about the causes and consequences of the flows of reactive nitrogen (Nr) into, out of and among major sectors in California focusing on agriculture. This paper will also consider a comprehensive range of agricultural practices and policy options that could more effectively balance the benefits of Nr with its costs for California, the most agriculturally productive, biologically diverse and populated state in the U.S.A.


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