AFTER being drained to make way for agriculture, wetlands are staging a small comeback on farms these days.
Some farmers restore or construct wetlands alongside fields to trap nitrogen and phosphorus runoff, and research shows that these systems can also retain pesticides, antibiotics and other agricultural pollutants.
As important as these storage functions of wetlands are, however, Bill Mitsch, director of the Everglades Wetland Research Park at Florida Gulf Coast University and an emeritus professor at Ohio State University, said another critical function is being overlooked: Wetlands also excel at pulling carbon dioxide out of the air and holding it long term in the soil.
Writing in the July/August issue of the Journal of Environmental Quality, Mitsch and co-author Blanca Bernal reported that two 15-year-old constructed marshes in Ohio accumulated soil carbon at an average annual rate of 2,150 lb. per acre — or just more than one ton of carbon per acre per year.
The rate was 70% faster than a natural "control" wetland in the area and 26% faster than either marsh was adding soil carbon five years ago. By year 15, each wetland had a soil carbon pool of more than 30,000 lb. per acre, an amount equaling or exceeding the carbon stored by forests and farmlands.
What this suggests is that researchers and land managers shouldn't ignore restored and manmade wetlands as they look for places to store, or "sequester," carbon long term, Mitsch said. For more than a decade, for example, scientists have been studying the potential of no-tillage, planting pastures and other farming practices to store carbon in agricultural lands, which cover roughly one-third of the Earth's land area.
Yet, when created wetlands are discussed in agricultural circles, it's almost always in the context of water quality. "So, what I'm saying is: Let's add carbon to the list," Mitsch said. "If you happen to build a wetland to remove nitrogen, for example, then once you have it, it's probably accumulating carbon, too."
In fact, wetlands in agricultural landscapes may sequester carbon very quickly, because high-nutrient conditions promote the growth of cattails, reeds and other wetland plants that produce a lot of biomass and carbon, Mitsch said. Once carbon ends up in wetland soil, it can also remain there for hundreds to thousands of years because of water-logged conditions that inhibit microbial decomposition.
"Carbon is a big deal; any carbon sinks that we find we should be protecting," Mitsch said. "Then, we're going even further by saying: We've lost half of our wetlands in the U.S., so let's not only protect the wetlands we have remaining but also build some more."
At the same time, he acknowledged that wetlands emit the powerful greenhouse gas methane, leading some to argue that wetlands shouldn't be created as a means to sequester carbon and mitigate climate change.
However, in a new analysis that modeled carbon fluxes over 100 years from the two constructed Ohio marshes and 19 other wetlands worldwide, Mitsch, Bernal and others demonstrated that most wetlands are net carbon sinks, even when methane emissions are factored in.
The concerns about methane emissions and even his own promising findings point to something else, Mitsch cautioned: "It's easy to undervalue wetlands if we become too focused on just one of their aspects — such as whether they're net sinks or sources of greenhouse gases. Instead, people should remember everything wetlands do.
"We know they're great for critters and for habitat; that's always been true. Then, we found out they cleaned up water and could protect against floods and storms," he said. "Now, we're seeing that they're very important for retaining carbon. So, they're multidimensional systems — even though we, as people, tend to look at things one at a time."