Among the various providers of ecosystem services, forested wetlands might be the champions. With their sponge-like abilities, they supply and purify water, protect communities from flooding, offer habitat for diverse species, produce timber and other goods, and present many opportunities for recreation and general enjoyment. Hidden in wetland soils is another critically important benefit: storage of carbon that would otherwise enter the atmosphere as carbon dioxide (CO2), a climate-warming greenhouse gas.

But what happens to this carbon when wetlands dry out because of seasonal water level fluctuations, climate variability, or land use changes related to human development? A team of researchers from North Carolina State University (NCSU) and the U.S. Forest Service Eastern Forest Environmental Threat Assessment Center may have the answer.

In a recently published study, researchers sought to characterize the factors that drive soil carbon loss as the water table rises and falls in a forested wetland. The study area was a seasonally flooded, forested wetland along the North Carolina coast where, for 17 months, they continuously measured CO2 released from soils. The researchers set up monitoring sites along three microtopographic gradients—areas where variations in the soil surface create different conditions that influence the movement of water and associated biological processes. These gradients determine the distribution of plants and soil organisms that release CO2 from the soil surface, known as soil respiration.

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The study area is pictured during flooded conditions.

“We observed seasonal variations in CO2 release at each monitoring site caused by the transitions between flooded and non-flooded conditions,” says Guofang Miao, an NCSU researcher collaborating with the Eastern Threat Center and the study’s lead author. When researchers simulated these observations with a computer model, they concluded that soil temperature and water table depth were responsible for seasonal variations in soil CO2 loss. “Across the three monitoring sites, 93 percent of CO2 loss occurred during non-flooded periods. This indicates that soil respiration is highly linked to microtopography and the period of time during which forested wetland soils are exposed and drying.”

While wetland soils cover only 2 to 3 percent of the total land area across the world, they may store up to 30 percent of global soil carbon. As wetlands experience pressures from land use and climatic changes, including drought, vast amounts of CO2 could potentially enter the atmosphere. “Compared to upland soils, models suggest that CO2 loss from wetland soils may be more sensitive to rising temperatures and altered precipitation patterns,” says Eastern Threat Center research ecologist Steve McNulty. “There is a need for long-term monitoring and models that account for wetland hydrology dynamics to improve our understanding of carbon cycling in wetland soils. The vulnerability of wetland soils’ carbon storage capacity must be considered as we develop management solutions to adapt to and mitigate the impacts of a changing climate.”
For more information, email Steve McNulty at smcnulty@fs.fed.us.