From Grasslands to Forests, Nitrogen Impacts all Ecosystems

By Ashley Mayrianne Jones

Can there be too much of a good thing?

That’s the case with nitrogen, an essential element for plant growth that, in overabundance, can also be potentially damaging. Nitrogen moves from the air to the land, soil, and water via a process called nitrogen deposition. Atmospheric nitrogen deposition has increased ten-fold or more since pre-industrial levels due to increased emissions from the burning of fossil fuels, fertilizer use, and other human activities.

Blue Ridge Mountains at the Roan Highlands State Park in North CarolinaOnce nitrogen is emitted into the atmosphere, it can travel vast distances and deposit in the environment, making it a national as well as local problem. Elevated nitrogen deposition can increase leaf biomass in the canopy, shading ground-dwelling plants from the sun. Additionally, physical and chemical reactions that occur when nitrogen compounds are deposited can lead to more acidic soils. Both effects restrict plant growth and increase competition for limited resources, resulting in a loss of local biodiversity.

To date, most U.S. biodiversity studies on the effects of nitrogen deposition had been focused on individual sites, where fertilizer was applied and small plots were monitored through time. It was unknown whether the resulting reductions in plant biodiversity at these small scales translated to meaningful changes at the landscape level. A series of recent studies had indicated that across the European continent, many ecosystems were experiencing reductions in plant biodiversity due to nitrogen deposition. However, it remained unclear whether the same held true in the U.S., which historically, has experienced lower atmospheric deposition levels.

That’s why EPA researcher Chris Clark and a team of scientists from EPA, U.S. Geological Survey, the U.S. Forest Service, the University of Colorado, and multiple other universities are exploring the effects of nitrogen deposition on herbaceous plants (those with non-woody stems such as grass) in a first-of-its-kind study focused on multiple ecosystems across the nation. The new research expands the focus to not only grasslands, but into habitats that have not received much attention, including the forest understory.

The study, recently published in Proceedings of the National Academy of Sciences, assesses how nitrogen deposition affects herbaceous plants at over 15,000 forest, woodland, shrubland, and grassland sites throughout the United States. The research addresses how physical, chemical, and climatic factors such as soil acidity, temperature, and precipitation can affect an area’s vulnerability to nitrogen deposition.

Nearly a quarter of the sites were vulnerable to nitrogen deposition-induced species loss, and those with acidic soils tended to be more vulnerable. At extremely low levels of nitrogen deposition, the number of individual plant species tended to increase. However, above a certain threshold level, or “critical load,” diversity began to decline.

The study indicated that on average, forests can tolerate slightly higher levels of nitrogen deposition than other ecosystems before showing a negative impact on biodiversity. The reasons for this are unclear, but scientists hypothesize part of the reason is that forest species living under the canopy are already adapted to low-light conditions and are less susceptible to shading effects caused by increased nitrogen. Both grasslands and forests, however, were quite vulnerable to nitrogen deposition, with critical loads in the range of current deposition levels.

Moving forward, EPA scientists and their partners will attempt to determine which individual plant species are most at risk, and which native and invasive species may increase with elevated nitrogen deposition.

Examining multiple ecosystems across the country gives us more information about how different locations may respond to the effects of nitrogen deposition and will help set monitoring and conservation priorities that protect plant biodiversity.

Learn more about EPA’s Air, Climate, and Energy Research.

About the Author: Ashley Mayrianne Jones is a student contractor and writer working with the science communication team in EPA’s Office of Research and Development.

Citation: Biological Sciences – Ecology: Samuel M. Simkin, Edith B. Allen, William D. Bowman, Christopher M. Clark, Jayne Belnap, Matthew L. Brooks, Brian S. Cade, Scott L. Collins, Linda H. Geiser, Frank S. Gilliam, Sarah E. Jovan, Linda H. Pardo, Bethany K. Schulz, Carly J. Stevens, Katharine N. Suding, Heather L. Throop, and Donald M. Waller. Conditional vulnerability of plant diversity to atmospheric nitrogen deposition across the United States. PNAS 2016 113 (15) 4086-4091; published ahead of print March 28, 2016, doi:10.1073/pnas.1515241113

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