Ecologists are embracing the potential to increase the storage of carbon in terrestrial ecosystems. This could reduce the build-up of carbon dioxide in Earth’s atmosphere. At least a portion of their enthusiasm derives from the observation that wetlands are very efficient in soil carbon storage, because the rate of decomposition is slow in anoxic (without oxygen) conditions. Most wetlands store carbon at rates about 10X greater than the rate of carbon storage in upland soils.
Now, at least several investigations suggest some tempering of enthusiasm for carbon storage in restored or constructed wetlands, inasmuch as wetlands yield a greater flux of methane to the atmosphere, and methane has a much greater effect on global warming than carbon dioxide—the normal pathway returning carbon to the atmosphere from upland soils. The enhanced carbon storage in wetland soils must be discounted by their greater flux of methane to the atmosphere. Flooding can convert soils from a methane sink to a methane source to the atmosphere—even in uplands as shown in a new paper by Ni and Groffman.
One of the earliest investigations of the comparative flux of methane and carbon dioxide from peatland soils found that the ratio of carbon dioxide to methane was >2500 in relatively dry soils, but decreased to values of 4 to 173 when the soils were flooded. Any ratio less than about 80 means that the impact of flooding soils with the goal of enhanced carbon storage is completely negated by the higher flux of methane from flooded soils to the atmosphere.
Many restored wetlands in California are net sources of global warming potential, despite storing more carbon in their soils. Reservoirs created by hydroelectric dams in the Amazon basin are also large sources of greenhouse gases to the atmosphere.
The “discount” to the role of wetlands as a carbon sink is less in mangrove forests, salt marshes, and seagrass meadows inasmuch as the methane flux from saline habitats is less than that from flooded freshwater soils. Mangroves are seldom constructed to provide a new sink for atmospheric carbon dioxide. Nevertheless about 20% of the benefit of carbon storage in mangrove soils is lost due to methane flux to the atmosphere.
Wetlands are great for wildlife, improving water quality, and for tempering the flow of water to the sea. But, as tools to mitigate global warming, we need to look more carefully at their net gas flux.
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