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Carbon Sequestration 101

Coastal wetland ecosystems (salt marshes, mangroves, and seagrass beds) can store large quantities carbon for two main reasons:

  1. Their plants usually grow a lot each year, and in the process, capture (or sequester) large amounts of carbon dioxide (CO2).
  2. Their soils are largely anaerobic (without oxygen) so carbon that gets incorporated into the soils decomposes very slowly and can persist for hundreds or even thousands of years (carbon storage).

The diagram below details the mechanisms by which carbon moves into and out of coastal wetlands. (please click on the image to enlarge)

Carbon Cycle

Carbon Pathways

Coastal wetlands tend to be very productive ecosystems—meaning that the plants grow a lot each year. As part of the growth process, plants capture carbon dioxide from the air and convert it to plant parts such as leaves, stems, or roots. This process is called “fixation” or “uptake” of carbon dioxide.

Carbon is also lost back to the atmosphere when plants respire (exhale) carbon dioxide, the same way people exhale carbon dioxide. This carbon dioxide is the byproduct of the plants breaking down sugars (i.e. food) and converting it to energy.

Some of the carbon that plants capture gets added to soils either via internal transport in the plant or when plant parts, such as leaves and roots, die and become incorporated into the soil.

Once carbon is in the soil some of it is respired by microbes and returns to the atmosphere as carbon dioxide. However, some of the carbon stays stored in the soils, often for hundreds or even thousands of years, buried deep underground.

Why are Anaerobic Soils Important?

One reason coastal wetlands are particularly good at storing carbon is because the soils are largely anaerobic, which means they lack oxygen. 
In most coastal wetlands there is usually a thin layer of soil that is oxygenated and above water, but the remainder of the soil is submerged in water. Oxygen diffuses very slowly through water, so saturated (wet) soils in these wetland habitats tend to have little to no oxygen present.

Decomposition of organic plant material is much slower when there is no oxygen present, so the carbon present in this plant material remains intact, rather than being broken down by microbes and respired back to the atmosphere. As a result, wetlands are very good carbon sinks (meaning they store a lot of carbon).

In summary, coastal wetlands are particularly good at storing carbon because the plants annually sequester (capture) a lot of carbon and then these ecosystems store carbon for long periods of time in their soils.