Wetlands store carbon by having a slow decomposition rate.

Wetlands, especially peatlands in the northern arctic and boreal areas, have the ability to store large amounts of carbon. The storage of carbon is related to how wetlands function. Wetlands plants are able to take carbon dioxide out of the air and incorporate some of it into their tissues during photosynthesis. Wetlands also accumulate and store carbon within the soil in the form of leaf matter and plant debris. Forested wetlands and peatlands in the north accumulate large amounts of litter and organic matter and can store a high percentage of carbon in comparison to other areas. These organic materials release carbon back into the atmosphere when they’re decomposed by bacteria, but the combination of cool and wet soil conditions in the northern wetlands slows the decomposition of organic materials.

In many wetlands, high water levels prevent large amounts of oxygen from reaching wetland soil, where it would enable aerobic bacteria to decompose organic matter. During the winter, cool temperatures in wetlands and frozen soils in many northern wetlands slow decomposition by slowing the metabolic activity of the decomposing bacteria. This enables wetlands in arctic and boreal regions to be effective carbon sinks for hundreds to thousands of years.

Researchers caution, however, that climate change and increased temperatures could lower wetland water levels and increase decomposition rates. This may affect the ability of wetlands to continue to sequester carbon and may even turn wetlands into a source of carbon dioxide.

References

“Wetlands annually store carbon at rates that can be 10-20 times faster than in terrestrial systems. These high rates are due to slower decomposition rates under anaerobic conditions and cooler temperatures in boreal and arctic regions where most wetlands are found (Fig. 1.8). As a consequence, today wetlands may contain as much as 40% of global soil carbon, most of it in peatlands and forested wetlands. Although they cover only 3% of the world’s land area, peatlands alone may contain as much as 25% of the soil carbon pool. So much carbon is stored in northern peatlands that changes in the amount of carbon stored in peatlands have been hypothesized to be linked to global ice-age cycles (Frazen 1994). Although wetlands are an important sink for carbon, they are not expected to sequester much additional carbon as global CO2 levels increase. In other words, wetlands are not expected to ameliorate global climate change. In fact, the opposite is predicted.” (van der Valk 2006:142-143)

Book
The Biology of Freshwater Wetlands (Biology of Habitats)March 24, 2012
Arnold G. van der Valk

“The release of this carbon may exceed sequestering if temperatures rise due to climate change. This is particularly true if water levels fall…Upon drainage bacteria which live in aerated conditions will oxidize much of the carbon and return it to the atmosphere…Release and sequestration will depend upon atmospheric carbon dioxide levels, temperature, water levels, fires, tree harvesting, and land management practices. Fires may accelerate this release.” (Kusler 2006:13)

Report
Common Questions: Wetland, Climate Change, and Carbon SequestrationAssociation of State Wetland Managers, Inc. [accessed April 25, 2017]January 1, 2006
Kusler J

Journal article
Wetlands, carbon, and climate changeLandscape EcologyJanuary 1, 2013
Mitsch WJ; Bernal B; Nahlik AM; Mander U; Zhang L; Anderson CJ; Jørgensen SE; Brix H

Journal article
Carbon storage in US wetlandsNature CommunicationsJanuary 1, 2016
Nahlik AM; Fennessy MS