Marshes, intertidal ecosystems dominated by herbaceous vegetation, can be found globally on coastlines from the arctic to the subtropics. In the tropics, marshes are replaced by mangroves as the dominant coastal vegetation.
Marshes have high productivity, with a large portion of primary production in belowground biomass. This belowground biomass can form deposits up to 8m deep. Marshes provide valuable habitat for plants, birds, and juvenile fish, protect coastal habitat from storm surge and flooding, and can reduce nutrient loading to coastal waters. Similarly to mangrove and seagrass habitats, marshes also serve as important carbon sinks. Marshes sequester C in underground biomass due to high rates of organic sedimentation and anaerobic-dominated decomposition. Salt marshes cover approximately 22,000 to 400,000 km globally, with an estimated carbon burial rate of 210 g C m yr.
Salt marshes may not be expansive worldwide in relation to forests, but they have a C burial rate that is over 50 times faster than tropical rainforests. Rates of burial have been estimated at up to 87.2 ± 9.6 Tg C yr which is greater than that of tropical rainforests, 53 ± 9.6 Tg C yr. Since the 1800s salt marshes have been disturbed due to development and a lack of understanding of their importance. The 25% decline since that time has led to a decrease in potential C sink area coupled with the release of once buried C. Consequences of increasingly degraded marsh habitat are a decrease in C stock in sediments, a decrease in plant biomass and thus a decrease in photosynthesis reducing the amount of CO taken up by the plants, failure of C in plant blades to be transferred into the sediment, possible acceleration of erosive processes due to the lack of plant biomass, and acceleration of buried C release to the atmosphere.
Tidal marshes have been impacted by humans for centuries, including modification for grazing, haymaking, reclamation for agriculture, development and ports, evaporation ponds for salt production, modification for aquaculture, insect control, tidal power and flood protection. Marshes are also susceptible to pollution from oil, industrial chemicals, and most commonly, eutrophication. Introduced species, sea-level rise, river damming and decreased sedimentation are additional longterm changes that affect marsh habitat, and in turn, may affect carbon sequestration potential.