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Tidal salt marshes are dynamic ecosystems that can be found in a range of sedimentary coastal settings from the tropical to the arctic climate zone (Mcowen et al., 2017). Marsh plants consist of halophytic herbs, grasses, and low shrubs that are adapted to frequent or occasional inundation by tides (saltwater) and lay approximately between mean high‐water neap and mean high‐water spring tides (Mcowen et al., 2017). Located in the intertidal zone, they can share a similar ecological niche and have overlapping distributions with mangroves in temperate regions (Saintilan et al., 2014; Kelleway et al., 2017).

Table 1.1 Summary of local rates and global estimates of carbon burial and stock in salt marshes

(Source: Based on Chmura et al., 2003, and Duarte et al., 2013.)

Local carbon burial rate (g C/m2/yr)	Carbon stock in soil (Mg C/ha)	Global area (km2)	Reference
151		380,000	Woodwell et al. ( 1973 )
218 (18–1,713)		22,000	Chmura et al. ( 2003 )
218 (18–1,713)		400,000	Duarte et al. (2005)
218 ± 24	162	22,000–400,000	Mcleod et al. ( 2011 )
244.7		41,657	Ouyang and Lee ( 2014 )
218 (18–1,713)		55,000	Mcowen et al. ( 2017 )
Global carbon burial (TgC/yr)	Global carbon stock in soil (PgC)	Global carbon burial (reference)
60.4	none available	Duarte et al. (2005)
60.4–70 (max. 190)			Nellemann et al. ( 2009 )
4.8			Mcleod et al. ( 2011 )
87.3			Mcleod et al. ( 2011 )
4.8–87.3	0.4–6.5		Duarte et al. ( 2013 )
10.2 ± 1.1 (0.9–31.4)	none available	Ouyang and Lee ( 2014 )
12 ± 1.3			Al‐Haj and Fulweiler ( 2020 )

As highly productive ecosystems (primary production exceeds respiration), tidal salt marshes sequester large amounts of carbon within their underlying sediments, within their living aboveground biomass (leaves, stems) and belowground biomass, including live and dead roots (Chmura et al., 2003; Ouyang & Lee, 2014). Marsh plants further trap and accumulate litter and allochthonous material/carbon from upstream rivers and tidal exchange (Saintilan et al., 2013). Consequently, carbon burial rates and carbon stocks in salt marshes are exceptionally high, exceeding those of terrestrial forests (Duarte et al., 2005a, 2013; Mcleod et al., 2011).

The global net primary production of tidal salt marsh is estimated at 0.01–0.18 PgC/yr (Duarte et al., 2013). Local burial rates vary from 18 to 1,713 g C/m²/yr with an average of ~230 g C/m²/yr (Table 1.1). Global estimates of carbon burial depend on accurate estimates of the global extent of coastal marsh. Historically, there have been high uncertainties associated with the global distribution of salt marsh (Chmura et al., 2003; Duarte et al., 2013), and combined with high variability in local burial rates, this has resulted in a relatively large range of global burial estimates in salt marsh (4.8 to 190 TgC/yr, Table 1.1). With the most recent estimate of mean carbon sequestration rates (245 g C/m²/yr, Ouyang & Lee, 2014) and the recently updated global salt marsh area of 55,000 km² by Mcowen et al. (2017) we estimate mean global carbon burial in salt marsh to be 13.5 TgC/yr. Uncertainties of global carbon burial and stocks are due to the small number of measurements from few study sites that are extrapolated to large regions. There are significant data gaps, in particular in South America and South Asia (Ouyang & Lee, 2014).

Important controls on local carbon sequestration rates, decomposition, and stocks include climate and latitude, tidal range, and halophyte genera (Kirwan & Mudd, 2012; Ouyang & Lee, 2014; Holmquist et al., 2018), as well as marsh elevation, i.e., the available accommodation space created by sea‐level rise (Kirwan & Megonigal, 2013; Rogers et al., 2019). Loss of salt marsh habitats and therefore the ability of marsh to capture carbon is predominantly caused by anthropogenic disturbance and land‐use change such as dredging, filling, draining, and construction (annual loss rate of 1–2% between 1980–2000, Duarte et al., 2008). Hurricanes can destroy large areas of marsh and trigger instantaneous losses of sequestered soil carbon (DeLaune & White, 2012). For total salt marsh carbon stock, we only include soil carbon, with an estimate of 0.4 to 6.5 PgC (Duarte et al., 2013).