Note to editors: Curtis Richardson can be reached for additional comment at (919) 613-8006 or curtr@duke.edu. Scott Winton can be reached at scott.winton@duke.edu.

DURHAM, N.C. – To add or not to add, that is the question.

When scientists restore a freshwater wetland, they sometimes amend its depleted soil with salvaged topsoil or composted yard waste to promote healthy new plant growth. The practice is especially common in the eastern United States.

But there’s a trade-off. As the organic matter decomposes, it causes the wetland to emit more methane than it stores, turning the wetland, at least initially, into a greenhouse gas source.

“A hotly debated question in science and policy circles is at what point in time – if ever – do these wetlands switch into greenhouse gas sinks,” says Curtis J. Richardson, director of the 91 Wetland Center and professor of resource ecology at Duke’s Nicholas School of the Environment. “Some scientists have suggested evaluating the issue on a 100-year timeframe, but that’s not especially useful from a real-world policy perspective.

“Our new study provides a more relevant point of reference,” he says.

The study, published by Richardson and Duke PhD student Scott Winton this month in the peer-reviewed journal Wetlands, measured methane and carbon dioxide emissions coming from a 10-year-old restored upland wetland in coastal Virginia.

“We found little evidence that adding in moderate amounts of composted yard waste significantly increased methane emissions coming from the site ten years after restoration,” says Winton, who led the study.

“Methane emissions by that point were significant only under flooded conditions during warmer months – and even then, they represented a relatively modest contribution to the global warming potential, compared to the naturally occurring soil carbon dioxide flux that would take place if the wetland had remained an abandoned farm field,” Winton says.
 
Richardson and Winton’s analysis shows that adding larger amounts of composted yard waste into the wetland, however, was detrimental to the site’s greenhouse gas budget because of increased carbon dioxide emissions linked to soil respiration.

“There’s a tipping point you reach – a point of diminishing returns,” Richardson says. “As you add larger amounts of composted organic material – greater than 11.2 kilograms, or about 24.7 pounds, per square meter – we found that the environmental and economic costs spiral up and wetland productivity does not keep pace. It may even reverse.”     

Large areas of freshwater wetlands have been restored in the eastern United States in recent decades to comply with environmental mitigation measures mandated under section 404 of the Clean Water Act. Richardson and Winton’s study site was a former farm field in Charles City County, Virginia, just east of Richmond.  The field is owned and managed by the Virginia Department of Transportation as part of its compensatory mitigation program. In 2001, scientists from Virginia Tech began using the site to assess the impacts of incorporating different amounts of composted organic matter into four test plots. One plot was amended with 5.6 kilograms of composted material per square meter. Another plot was amended with 11.2 kilograms; the third plot received 22.4 kilograms; and the final plot was amended with 33.6 kilograms.  A control plot received no organic amendments.

“Differences in local geology, weather, hydrology and land-use can all affect a restored wetland’s rate of emissions, so this is not a one-size-fits-all finding,” Winton cautions. “But it does provide a useful new insight to help guide restoration policies and practices.”

“Few studies have measured greenhouse gas emissions from created or restored wetlands, and fewer still have done so at sites amended with organic material,” notes Richardson. “So this breaks new ground.”

Support for the new study came from the Peterson Family Foundation, the 91 Wetland Center and Wetland Studies and Solutions Inc.  
 
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CITATION: “The Effects of Organic Matter Amendments on Greenhouse Gas Emissions from a Mitigation Wetland in Virginia’s Coastal Plain,” R. Scott Winton, Curtis J. Richardson; published July 3, 2015, in .