AMES, Iowa — Fifteen years of wetlands research by Iowa State University – a study thought to be the largest and longest running project of its kind in the country – clarifies their performance as highly beneficial systems for reducing nitrogen pollution.
“Even under highly variable conditions over many years, the wetland systems we studied worked as expected and removed significant amounts of nitrate,” said William Crumpton, the lead researcher and University Professor of ecology, evolution and organismal biology.
Now, during National Wetlands Month, it’s a good time to consider the benefits wetlands can provide. The Iowa State research outlines the potential of wetland systems to improve water quality in agricultural landscapes and provides extensive data to better assess their costs and benefits.
Nonpoint-source loads of nitrogen and phosphorus are a major concern, both for their impact on local receiving waters and their contributions to hypoxia in the Gulf of Mexico. The Iowa Nutrient Reduction Strategy identifies wetland development as an integral conservation practice to achieve the state’s nitrogen-reduction goals.
However, not all wetlands are alike. There is considerable variability among wetlands and a lack of information on how restored or created wetlands can impact water quality.
“We set out to evaluate nitrogen removal by wetlands across a wide range of conditions and to identify the most important factors that impact their performance,” Crumpton said.
The study’s findings are published in a February article in the peer-reviewed “Journal of Environmental Quality.”
Crumpton’s co-authors, all in the Department of Ecology, Evolution and Organismal Biology, were associate scientists Greg A. Stenback and David I.S. Green, and assistant scientists Stephen W. Fisher and Jana Z. Stenback. The research was funded by the Iowa Department of Agriculture and Land Stewardship and USDA’s National Institute of Food and Agriculture.
From 2004 through 2018, Crumpton and his team monitored multiple wetlands subject to wide-ranging weather conditions that included extended droughts and record floods. The nitrogen concentrations in water entering the wetlands under study also ranged widely, from near zero to over 50 milligrams per liter. Seventy percent of the daily inflows had nitrate nitrogen values exceeding the drinking water standard of 10 milligrams per liter.
All the wetlands had been drained and cropped and were predominantly characterized by poorly drained, wet soils. Many had been restored through Iowa’s CREP, the Conservation Reserve Enhancement Program, designed to filter tile water drainage from agricultural land.
Crumpton’s team found a 35% average reduction in nitrate loads for the 26 study wetlands that had pools ranging from 0.25% to 3.5% of their watershed area. On average, the wetlands removed 1340 pounds of nitrate nitrogen per acre of wetland per year.
“These removal rates are substantially higher than typically reported for Corn Belt wetlands, but comparable to smaller studies of systems receiving very high nitrate loads,” Crumpton said.
The results of the study demonstrate wetlands’ substantial capacity to reduce nitrogen loads over a broad range of weather and loading conditions.
An additional benefit of the research has been to build more accurate models to predict expected wetland performance in tile-drained areas. This is important to help farmers and conservation planners balance costs with likely impact.
The researchers’ prediction model is based on three factors: the concentration of nitrate entering the system, the water’s temperature (microbes process nitrate more quickly in warmer water) and a wetland’s hydraulic loading rate. Nitrate removal efficiency for the study wetlands ranged from 9% to 92%, primarily due to a wide range in hydraulic loading rate.
“We can’t do anything about the first two factors, so the main thing we can adjust is the hydraulic loading rate — basically the size of the wetland compared to the amount of water it intercepts,” Crumpton said. “By adjusting this, we can predict the nitrate reduction expected from a wetland of a specific size receiving drainage from a specific area.”
Crumpton added that a wetland’s ability to reduce nitrate can be enhanced by improving hydraulic efficiency – in effect, providing more contact time and minimizing short circuiting of flows between the inflow and outflow. Newer CREP wetlands designs are incorporating berms and pools to accomplish that, but this can add to the cost.
“Our research found that even during high-rainfall periods, when the time water stays in the wetland can be quite short, a wetland can still significantly reduce the overall amount, or mass, of nitrate headed to the stream. That’s what matters most for water quality,” Crumpton said.
Applying their models to the Iowa CREP, Crumpton estimated that CREP wetlands restored through 2019 remove over 600 tons of nitrogen per year from their combined wetland pool area covering just over 900 acres. “To put that in perspective,” Crumpton said, “the level of nitrate reduction achieved by these wetlands would be comparable to retiring about 44,000 row crop acres.”
“Iowa State’s research provides added confidence in the modeling approaches employed by the Iowa Nutrient Reduction Strategy’s science team in establishing nutrient reduction expectations for conservation practices,” said Jake Hansen, water resources bureau chief for the Iowa Department of Agriculture and Land Stewardship.
Hansen added that IDALS has long supported ISU’s wetland monitoring program to determine optimal methods for siting, designing and building nutrient-reduction wetlands in tile-drained areas.
“In consultation with Dr. Crumpton and other ISU researchers, our CREP wetlands design approaches have evolved to see which modifications are most effective and enhance nutrient reduction performance.”
Crumpton and his graduate students are researching other aspects of wetlands, including quantifying their impact on the greenhouse gas nitrous oxide. They are also in the process of developing a study site for a new “tile zone” type of wetland on an ISU research farm near Ames. Sources of support for these projects include IDALS and its Water Quality Initiative, the U.S. Environmental Protection Agency, USDA NIFA and the Iowa Nutrient Research Center at Iowa State.
— Iowa State University
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