Nitrogen & the National Parks

After being closed for fourteen days during the recent government shutdown, America’s national parks are now open. While that is good for those who love the outdoors, a new paper published in Atmospheric Chemistry and Physics suggests that all may not be well in our parks. Using a chemical transport model, the authors suggest that in 64% of our national parks, human activity is resulting in nitrogen deposition levels that exceed critical thresholds.

At first blush, high levels of nitrogen deposition may not sound like a scourge in the national park system. It actually sounds quite mundane. But excessive nitrogen can have a corrosive effect on ecosystems. For example, high levels of nitrogen run-off are partially responsible for the growth of the largest dead-zone in the history of the Gulf of Mexico. In national parks, the deposition of excessive amounts of nitrogen threatens the healthy development of native ecosystems by “decreas[ing] biodiversity, disrupt[ing] soil nutrient cycling and caus[ing] acidification of waters.” Ironically, these threats are occurring in areas set aside to protect the natural environment.

The authors of the recent study utilize the GEOS-Chem continental simulation of North America to model how pollutants emitted in one place, measured by the EPA’s National Emissions Inventory, spread across the United States. The model provides estimates of the wet and dry deposition of nitrogen across the United States. Ground-truthing based on observed nitrogen deposition amounts at 29 national parks found a strong correlation between the results of the model and actual amounts. In general, the rates of deposition follow a strong west to east gradient with the heaviest depositions occurring east of the Mississippi.

UntitledIn order to assess the impact of nitrogen deposition, the authors define a critical load as “the quantitative estimate of exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge” and identify a range of critical values for every national park. These values depend mostly on the primary receptor of nitrogen in the park (forests, lichen, waterways, etc.) and, thus, the prevailing ecosystem present in the park. Unfortunately, due to the types of ecosystems present across the United States, the critical loads an ecosystem can handle of nitrogen deposition are much lower precisely in the locations with higher levels of deposition. Additionally, the critical loads follow a strong east to west gradient, with the lowest values in the east, just the opposite of the level of deposition. Alarmingly, the highest levels of deposition are occurring in some of the most vulnerable areas.

What does all of this mean for the average visitor to the national parks? In the short term, probably not much. The nitrogen build up takes time, and its impacts are generally felt over a longer time frame. However, in the long-term this could have significant landscape level impacts for the national parks including changes in the “function and structure of the community as a whole.”

The importance of this paper is not limited to describing the impacts of increased nitrogen deposition on the 58 national parks. It also serves as an excellent empirical example of how human activity is upsetting natural, planetary processes — in this case the nitrogen cycle — described in this landmark 2009 paper in Nature. Even then the nitrogen cycle was identified as one of three — along with climate change and biodiversity loss — of ten critical planetary boundaries that had already been exceeded. With this recent evidence from national parks it is obvious that disruptions to the natural nitrogen cycle are getting worse rather than better. While the impacts of disruption in the nitrogen cycle on a global scale are not well understood, the impacts on a local level suggest that it could lead to major changes in the distribution plant life around the world.

Fortunately, there is still a chance for things to turn around for the national parks; they, and much of the east coast of the United States, have been here before. For decades sulfur dioxide emissions from coal fired power plants in the Midwest fell as acid rain on national parks on the east coast, leading to concerns about the continued survival of places like Shenandoah and Great Smoky Mountains National Parks. Like the problems with nitrogen described here, acid rain was generated by pollutants released hundreds or thousands of miles from the park that threatened large scale, land-scape level changes due to tree die-offs in the eastern parks. However, while the problem still exists, amendments to the Clean Air Act in the 1990s have led to reductions in the incidence of acid rain along the east coast.

Solutions to these large scale, dispersed pollution problems do exist but they call for the collective action of multiple government agencies and private entities. The EPA has taken some steps to regulate nitrogen emissions and is considering more stringent regulation based on the effect that deposition can have on ecosystems. This new evidence makes a strong case that the continue preservation of our national parks depends on the passage of these new regulations. And as the experience with sulfur dioxide demonstrates, this is a solvable problem.

Imade Credit: By on Flickr (USDA Natural Resources Conservation Service) (Flickr) [CC-BY-2.0 (], via Wikimedia Commons


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