How do tidal marshes store carbon?

Tidal marshes are wetlands that are covered with incoming tidal water twice a day. These marshes may be as small as narrow fringe along a tidal creek or miles across in estuaries with adjacent flat landscapes. There are nearly 40 million acres of tidal marshes along the Atlantic, Gulf, and Pacific shores of the US.

Tidal marshes are also common in many estuaries such as the Chesapeake Bay and along rivers that enter these coastal estuaries. Tidal marshes are some of the most productive ecosystems on the planet.

Tidal marshes are important parts of the landscape because they provide:

• flood protection,
• erosion control,
• wildlife food & habitat,
• commercial fisheries support,
• water quality support,
• areas for recreation, and,
• storage for carbon in the marsh soil.

Roots and rhizomes in tidal marshes add to the organic matter of the soil. Recent research found that tidal marsh soils are able to store 3 to 5 times the amount of carbon than forest soils. Credit: Mark Stolt

Regarding carbon storage, let’s compare a forest to a tidal marsh. Forest soils store about 65% of the carbon held in a forest, with the rest of the forest’s carbon stored in plant life. Tidal marshes store 3 to 5 times that amount in the soil.  

Why do tidal marshes store so much carbon? There are two major factors – the plant life, and the soil conditions.

Marsh grasses and shrubs are drenched with full sunlight and the soils are rich in nutrients. This results in prolific plant growth above the ground. There is also growth below the ground in the form of roots and rhizomes. Marsh plants thrive in saturated conditions.

The plants take carbon dioxide out of the atmosphere (photosynthesis) and turn it into plant materials for growth. Each year, as plants die out, their leaves and stems drop to the soil surface creating soil organic matter which forms the carbon rich marsh peat.

Tidal waters that flood marshes bring in organic matter via sediment that is suspended in the flooding waters. That carbon-rich sediment is trapped by the marsh grasses. It settles to the bottom becoming part of the marsh peat.

With twice-daily flooding, the soils are constantly under water, or saturated. Because the soil is under water, soil microbial activity is inhibited by the limited amount of oxygen. Without oxygen, soil microbes can’t decompose organic matter as well as oxygen-rich environments. This is the big reason why tidal marshes can accumulate much more carbon than forest soils. Let’s look at how.

Researchers collected soil samples in a tidal marsh area that was affected by Hurricane Sandy in 2012. They were measuring the amount of organic matter added to the tidal marsh after 8 years. Tidal marshes are productive ecosystems and can help store carbon, mitigating climate change. Credit: Mark Stolt

In oxygen-rich environments, there is a balance to the amount of organic matter deposited, and the amount that microbes decompose. When microbes decompose organic matter, carbon is released back into the environment as carbon dioxide. However, in tidal marshes, the saturated conditions limit the organic matter decomposition. This means that more carbon is added to the marsh as plant detritus and sediment than is released as carbon dioxide, and the carbon accumulates over time.

Understanding the amount of carbon that is stored in the soil is important. However, knowing the rate that carbon accumulates, what we call carbon sequestration, may be even more important.

In our changing climate, we are looking for any mechanism that can remove carbon dioxide from the atmosphere and store it for the long term. When we determine what types of ecosystems that are the most efficient at carbon storage, we can focus on them. We can prioritize time and resources to manage, conserve, and restore these ecosystems as one piece of the puzzle to mitigate climate change.

Accurately measuring carbon sequestration – the rate of carbon storage – is not easy. The main issue is trying to determine the point in time that the carbon first started to accumulate (time zero). In a recent study, we took a different approach to measure all the carbon that is added to the soil during tidal marsh carbon sequestration. We researched what happened with carbon storage rates after Hurricane Sandy in 2012.

Hurricane Sandy pounded the coastal regions of the Mid-Atlantic and southern New England. Storm surges pushed vast quantities of sand from the barrier island beaches over the top of the barrier. It landed in the tidal marshes behind the island.

A soil sample taken by researchers from University of Rhode Island who were studying carbon storage in tidal marshes. In the upper inch, you can see plant roots from marsh plants. At about two inches, you can start to see a sandier soil – sand and sediments brought in from tides rising and falling. Starting at about 8 inches, you can see a denser organic layer full of decayed plant litter. Tidal marsh soils can store about 3-5 times the amount of carbon than forest soils. Credit: Mark Stolt

Eight years later we sampled the marsh soils behind three separate barrier islands that formed in these sandy overwash sediments. We measured carbon sequestration rates using 2012 as time zero.

We asked the question—Does the elevation above sea level in the marsh impact the carbon sequestration rate?  This is important because we wanted to know if a single carbon sequestration rate is representative of a tidal marsh, or does the rate depend on where you sample in the marsh.

Eight years after the overwash events of Hurricane Sandy, 55 to 94% of the sandy deposits were covered with typical tidal marsh vegetation. In that time, thin layers 2 to 3 cm thick had developed in the soil that were rich in carbon. Roots were found throughout the upper 10 cm of the soil.

Sequestration rates ranged from 465 to 5680 pounds of carbon per acre year depending the on the marsh and elevation. At lower elevations, carbon storage rates increased.

Two reasons likely explain the elevation effects. First, lower elevations are under water longer and more often than higher elevations on the marsh. That means soil microbes are deprived of oxygen longer and can’t decompose organic matter as quickly. Secondly, marsh vegetation at the lower elevations likely traps much of the sediment that is brought in by the flooding tidal waters. This adds more carbon to the soils that may not be received at higher elevations.

Our study found that on average, back-barrier salt marsh soils sequester 3 to 4 times the amount of carbon sequestered in adjacent upland forest soils. In addition, the lower elevation back barrier salt marshes sequester even more. This could lead scientists and policy makers who are making climate change decisions to focus on using tidal marshes to store more carbon.

Answered by Mark Stolt, University of Rhode Island

Dr. Stolt’s team published their research in Soil Science Society of America Journal.

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