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Nitrogen and Sulfur Deposition Effects

When air pollutants such as sulfur and nitrogen are deposited into ecosystems, they may cause acidification, or enrichment of soils (fertilization) and surface waters (eutrophication). Ecosystems in national parks can be vulnerable to the effects of airborne pollutant deposition because parks contain sensitive areas including high elevation lakes and streams, alpine meadows, sugar maple forests, and desert shrublands. The National Park Service (NPS) conducts and sponsors short-term research and monitoring studies designed to answer specific questions about the effects of air pollutants on natural resources, and the sources of air pollutants contributing to these impacts. Information gained from these high-quality scientific analyses are then provided to State and Federal air regulators to help determine where pollution reductions are most needed to maintain park ecosystems in a healthy condition.

  • Impacts
  • Critical Loads
  • Research
  • Monitoring
  • Resources

What are the impacts of nitrogen and sulfur?

River of Grass at Everglades NP, Florida, a wetland ecosystem sensitive to nutrient nitrogen.
Excess nitrogen runoff and atmospheric deposition can impact wetlands and estuaries.

Atmospheric nitrogen and sulfur deposition effects include changes in water chemistry that impact aquatic vegetation, invertebrate communities, amphibians, and fish. Deposition can also cause chemical changes in soils that alter soil microorganisms, plants, and trees. Plant species composition and abundance may change where nitrogen over-stimulates growth, favoring some plant species and inhibiting the growth of others. The deposition of nitrogen also contributes to nutrient enrichment in coastal and estuarine ecosystems, which can cause toxic algal blooms, fish kills, and loss of plant and animal diversity. Although nitrogen is an essential plant nutrient, excess nitrogen from atmospheric deposition can stress ecosystems. Excess nitrogen acts as fertilizer, favoring some plants and leaving others at a competitive disadvantage. This creates an imbalance in natural ecosystems, and over time may lead to shifts in the types of plant and animal species present, increases in insect and disease outbreaks, disruption of ecosystem processes (such as nutrient cycling), and changes in fire frequency.

Computer models have been used to identify ecosystems and resources in national parks at risk for acidification and excess nitrogen enrichment. Summary reports from acidification and nitrogen enrichment studies show park rankings of relative risk according to pollutant exposure, ecosystem sensitivity, and park protection mandates.

Air pollutants, including sulfur and nitrogen, can also impact essential ecosystem services such as air and water purification, decomposition and detoxification of waste materials, climate regulation, regeneration of soil fertility, production, and biodiversity maintenance. Ecosystem thresholds (Fenn et al. 2011 [pdf, 1.5 MB]) are increasingly being identified to indicate the amount of chemical and biological changes that ecosystems can tolerate from air pollutants.

What are critical loads?

Subalpine meadow ecosystem at North Cascades NP
Alpine ecosystems are very sensitive to excess nitrogen deposition.

“Critical load” is a term used to describe the amount of pollution that starts harmful changes in sensitive ecosystems. If deposition stays below these levels, ecosystems are usually protected from damage. Some parts of an ecosystem are more sensitive than others, therefore the effects of pollutant loading will differ within ecosystems. For example, critical loads for healthy surface waters capable of supporting fish are different than for healthy forests.

Research on critical loads of nitrogen that protect plants, soils, and surface waters in U.S. ecosystems (Pardo et al. 2011) and on critical loads that prevent acid rain impacts is extensive and highly relevant to national park resources. Critical loads science can be used to develop pollution reduction strategies that are specifically targeted to the areas and natural resources most affected by air pollutants.

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Why study nitrogen and sulfur deposition?

Desert ecoystem at Santa Monica Mountains NRA
Desert ecosystems can be changed by nitrogen deposition, which can stimulate growth of non-native grasses.

Research studies have shown that high elevation ecosystems in the Rocky Mountains, Cascades, Sierra Nevada, southern California, and the upland areas of the eastern U.S. are generally the most sensitive to atmospheric deposition. Soils in these areas have limited ability to neutralize acid deposition or to absorb excess nitrogen. Streams in Shenandoah and Great Smoky Mountains National Parks are experiencing chronic and episodic acidification that impacts brook trout fisheries. Nutrient poor ecosystems in parks, including some high elevation lakes, alpine tundra, coastal sage ecosystems, serpentine grasslands, and lichen communities are experiencing changes in plant species and soil nutrient cycling due to excess nitrogen deposition. For example, the health of aquatic and terrestrial ecosystems on the east side of Rocky Mountain National Park has declined due to atmospheric deposition of nitrogen (overview fact sheet [pdf, 214 KB]). In Joshua Tree National Park, airborne nitrogen deposition has had a fertilizing effect on non-native grasses allowing them to spread so much that they increase fire frequency in these ecosystems. Research findings on the effects of air pollution for each park are incorporated into park air quality summaries. more »

How are nitrogen and sulfur monitored?

High elevation lake at Denali National Park and Preserve
The National Park Service monitors air pollution in and near many national parks.

Nitrogen and sulfur compounds are monitored by the National Park Service and interagency partners in and near many national parks. The Interagency Monitoring of Protected Visual Environments (IMPROVE) program monitors sulfate and nitrate in the atmosphere and Wet deposition chemistry of rain and snow is tested through the National Atmospheric Deposition Program (NADP).

Learn more about nitrogen and sulfur deposition monitoring »

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Last Updated: January 03, 2017