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Enlighten Me: UD research challenges established thinking on salt marshes and carbon

Salt marshes, which line much of Delaware’s coast, are often thought of as “sinks” for global warming-causing carbon. But researchers at the University of Delaware are challenging this understanding.

A study in next month’s issue of the journal Agricultural and Forest Meteorology took a different approach to measuring the movement of greenhouse gases in and out of a Delaware salt marsh, and found something surprising: the ecosystem was a net source of carbon to the atmosphere over the study period. 

Salt marshes, which are tidal wetlands, are considered important ecosystems for global carbon sequestration. 

“Healthy coastal habitat is not only important for seafood and recreation, it also plays an important role in reducing climate change,” the National Oceanic and Atmospheric Administration (NOAA) says on its website. “Salt marshes, mangroves, and seagrass beds absorb large quantities of the greenhouse gas carbon dioxide from the atmosphere and store it, thus decreasing the effects of global warming.”

The local research, led by UD doctoral student Alma Vázquez-Lule, tracked gas fluxes at the St. Jones Reserve near Kitts Hummock over three years—throughout all the growth phases of the marsh grasses. 

The researchers found that low but constant carbon dioxide (CO2) emissions from the marsh during the plants’ dormancy period outweighed the carbon the plants absorbed during their growing season. The researchers also found the marsh emitted methane, which is an even more potent greenhouse gas than carbon dioxide. 

“When we do the sum up of all the COemissions and all the emissions of methane, we end up with a different story,” said Rodrigo Vargas, professor in UD’s College of Agriculture and Natural Resources and co-author of the study. “The different story is that these ecosystems are net carbon sources—or at least could be net carbon sources for the years that we measured them.”

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Delaware Public Media's Sophia Schmidt interviews Vargas late last year about other UD research that found unexpected methane emissions from salt marshes

The UD researchers’ discovery is thanks to when they conducted their research. Vargas says many researchers tend to do field work only during the warmer months, when the marsh plants are more active, and miss what happens during the winter.  

“If we walk through these ecosystems in the winter, we just see dead grasses,” Vargas said. “Nothing is green. Nothing is happening—or apparently nothing is happening.”

Vargas says the team expected microbes, which digest organic matter and release greenhouse gases, to be fairly inactive during the colder winter months.

“But we were able to measure that,” Vargas said. “And what we found was that in the wintertime, what we call this dormancy period of the plants, we have high emissions of CO2 and also relatively high emissions of methane.”

Vargas says even if salt marshes are net sources of carbon to the atmosphere, it is still important to preserve them. That’s because these ecosystems provide other services, such as habitat for animals, storm buffering and runoff filtration. And disturbing salt marshes could result in even bigger releases of carbon to the atmosphere. 

“If we just remove all that carbon that is stored there... and suddenly we expose it to oxygen, basically what you will be doing is creating a party for all these microbes,” Vargas said. “It’s [like] going to a buffet. Food that was not accessible to these microbes now is going to be accessible, and all that carbon that was stored there is just going to ... go back quickly to the atmosphere. So that is a big mistake.”

Vargas says it is unclear whether the conclusions of the study would hold under different weather conditions, so his team will continue collecting data.

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