Carbon capture from constructed wetlands declines as they age

Researchers examined soil core samples taken from two constructed freshwater wetlands and compared them to data from previous studies of the same wetlands over 29 years to determine how well human-made wetlands sequester — or capture and store — carbon as they age.

Findings showed both wetlands captured similar amounts of carbon over the decades, but neither has shown a net gain or loss since year 15.

But their value in sequestering carbon is remarkable, the researchers said.

“Wetlands are generally thought of as the kidneys of our world because they can clean water naturally and sequester carbon well,” said Jay Martin, a distinguished professor in food, agricultural and biological engineering at The Ohio State University and a co-author of the study. “As we try to combat climate change, they also provide habitat for many species that are important to us.”

The researchers analyzed data from the Schiermeier Olentangy River Wetland Research Park (ORWRP), a site ideal for long-term study due to the overwhelming amount of environmental data it generated over the past three decades.

Previous studies of the park revealed that its soil has shown an increase in carbon levels. But by using detailed measurements taken in the wetland’s 29th year post-construction, Martin’s team found that wetlands’ ability to sequester carbon diminishes as they mature.

The study was recently published in the journal Ecological Engineering.

Under current conditions, the wetlands have become a stable ecological force, and this equilibrium isn’t expected to change anytime soon.

“When you first construct a wetland, the initial plant growth is often what causes carbon to be sequestered so quickly,” said Daniel Ruane, a former master’s student in ecological engineering and the lead author of the study. “But it just isn’t possible to have infinite growth.”

Although there are limits to how much atmospheric carbon artificial wetlands can effectively store, since their carbon sequestration and storage rates are still far greater than other ecosystems, they still represent a potential solution to counter climate change, said Ruane.

As a result, future research into the health of the ORWRP is likely to analyze the various plant communities that grow within the area as well as investigate methane emission levels to determine how long the land can function as a carbon sink.

“The benefits that wetlands provide are increasingly positive,” said Martin. “Our findings emphasize that these ecosystems should be looked at in a better light now than ever before.”

Due to an increase in urban and agricultural land use, more than 50% of Earth’s natural wetlands have disappeared over the last few centuries. This decline has impacted ecosystem services all around the U.S., but most notably in the Midwest, said Martin.

In Ohio, for example, projected wetland loss is closer to 90%, jeopardizing many essential processes that humans rely on, like water quality improvement and flood mitigation.

This provides even more reason why policymakers should be trying to build and maintain wetland ecosystems, Ruane said.

“If we started to create and restore more wetlands now, that could solve a lot of our problems down the road,” he said.

Co-authors of the study include Michael Brooker and William Mitsch of Ohio State, Blanca Bernal of Greencollar US Inc., Chris Anderson of Auburn University, and Robert Nairn of the University of Oklahoma.