Down the Drain: Wetlands as Sinks for Absorbing Reactive Nitrogen
EPA scientists conduct the first continental-scale analysis to estimate how much nitrogen is removed by wetland ecosystems across the U.S.
Duck hunters and those who fish, both recreationally and commercially, are already well acquainted with the value of wetlands. The economic benefits alone include providing essential habitat to some 75 percent of the fish and shellfish that are commercially harvested in the U.S. On top of that, wetlands absorb storm runoff, help prevent flooding, and naturally filter water.
EPA researchers are working to quantify another important value of wetlands: the ability to act as natural sinks that absorb “reactive” nitrogen (Nr). While perhaps not as obvious as providing sought-after recreational destinations or habitat for valuable fisheries, wetlands’ natural ability to absorb nitrogen released into the environment can be just as important.
As one of life’s “essential elements,” nitrogen is required for the normal growth and maintenance of all biological tissue. The development and widespread use of nitrogen-based fertilizers helped revolutionize agriculture, dramatically increasing crop yields and food production.
But too much nitrogen can mean trouble.
Released into the environment through agricultural practices and as a byproduct from sewage treatment and the combustion of fossil fuels, reactive nitrogen can lead to complex and far-reaching environmental and human health problems. Nitrogen carried off croplands by storm water runoff can harm aquatic systems even far downstream, sparking the growth of oxygen-depleting algae and leading to harmful algal blooms and hypoxia (low oxygen). Excess nitrogen in the form of nitrate is a drinking water contaminant and an increasing health concern.
As hotspots of biological productivity, especially for plants, algae, and microorganisms that absorb nitrogen as they grow, wetlands serve as natural “sinks” for removing and storing reactive nitrogen as it cycles through the environment in one form or another.
EPA researchers are exploring innovative ways to better quantify the benefits—what they refer to as “ecosystem services”—that wetlands and other natural systems provide to society. As part of that effort, Agency scientist Stephen J. Jordan, Ph.D. and his partners recently compiled a comprehensive database on nitrogen removal from the scientific literature on wetland studies. He and his colleagues conducted an extensive search across a host of scientific publications, and then added supplemental data through strategic internet-based queries reaching as far back as 1970. “Looking at nitrogen removal across such a large geographic scale had never been done before,” says Jordan.
After identifying between 400 to 500 journal articles, Jordan and his team analyzed the data through statistical and quality assurance methods, building a robust dataset based built from 190 separate studies. Using latitudes and other environmental data recorded in the studies, along with the extent of major wetland classifications from the National Wetlands Inventory, the researchers were able to base their overall analysis across the entire contiguous U.S.
Finally, the team was able to use statistical analyses to calculate the estimated total amount of reactive nitrogen removed by wetlands across the continent. “Having such a large set of data made a huge difference, allowing us to tease out differences that emerged from looking at nitrogen removal across a broad scale,” explains Jordan.
The researchers estimated that the major classes of wetlands found across the contiguous U.S. removed approximately 20-21% of the total amount of reactive nitrogen added to the environment by human activities. That adds up to some 5,803,140 metric tons of nitrogen removed before it could taint drinking water or contribute “dead zones” off the coast by sparking algal blooms.
Extrapolating further, the researchers found that wetlands remove roughly 17% of worldwide human-caused nitrogen releases, at least 26 metric tons.
The team’s findings were recently presented in the scientific journal Ecosystems (Jordan, S., Stoffer, J., & Nestlerode, J., 2010. Wetlands as Sinks for Reactive Nitrogen at Continental and Global Scales: A Meta-Analysis Ecosystems, 
14 (1), 144-155 DOI).
The results of the research add considerable data that can be added to the “balance sheet” of ecosystem services provided by wetlands. And as some 50% of the worlds’s historic wetlands have been lost to human activities, there is a growing need to quantify and understand their true value so that decision makers, land use planners, and water quality managers can better protect human health and the environment.