# Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis
## 6 Whole-Ecosystem Carbon Mass Balance  
Sufficient data exist to construct carbon mass balance models of mangrove (Figure 3) and salt marsh (Figure 4) ecosystems. The salt marsh model is derived from the data in this review, while the mangrove mass balance is derived from [94], revised and updated from an earlier iteration [188]. This revised mangrove model shows that about 63% of GPP is inspired by the forest canopy. The salt marsh budget indicates that about 47% of GPP is respired by salt marsh plants. Mangrove NPP is vested nearly equally in litter, wood and belowground roots, with about 40% of litter exported to adjacent tidal waters; salt marsh NPP is vested mostly in belowground roots (72%). Unlike in the mangrove mass balance, salt marsh below- and aboveground NPP are inexplicably greater than GPP, implying that either below- and/or aboveground production are overestimates or that GPP is underestimated by about 30%. The standard deviation of the salt marsh GPP data is 73% of the mean with a range of 45 Mg C ha\({}^{-1}\) a\({}^{-1}\) (GPP mean = 17.7 Mg C ha\({}^{-1}\) a\({}^{-1}\)), suggesting that these data have a wide margin of error. Pre-eddy covariance data [143] estimate GPP, plant R and NPP for salt marshes of 112, 10.7 and 11.5 Tg C a\({}^{-1}\), respectively, which nearly balance assuming that the estimate for root production (69 Tg C a\({}^{-1}\)) is correct. However, plant R would be too low, equating to only 9.5% of GPP, resulting in an extraordinarily high P\({}_{\text{GPP}}\)/R\({}_{\text{C}}\) ratio of 10.5; both values are highly unlikely compared to those estimated for other vegetated ecosystems [1]. In both ecosystems, most roots produced (90% for mangroves, 78% for salt marshes) are shunted into the soil CORG pool for eventual decomposition; the remainder is buried (estimates from [189]). The balance of buried carbon is derived equally from litter and the soil pool. About 12% of mangrove NPP and 22% of marsh NPP is \begin{table}
\begin{tabular}{c c c} \hline
**Component** & **Mangrove Forests** & **Salt Marshes** \\ \hline Water-air CO\({}_{2}\) release & 3.35 \(\pm\) 0.35 (2.19) & 2.981 \(\pm\) 0.33 (1.55) \\ Water-air CH\({}_{4}\) release & 0.0116 \(\pm\) 0.003 (0.01) & 0.104 \(\pm\) 0.047 (0.006) \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of mean (\(\pm\)1 SE) rates of water–air CO\({}_{2}\) and CH\({}_{4}\) emissions between mangrove and salt marsh ecosystems, including open water estuaries containing mangrove and salt marsh habitats. Units = Mg C ha\({}^{-1}\) a\({}^{-1}\). Median values are in parentheses. Mangrove data from [94,139]. Salt marsh data from [139, 140, 150, 151, 152] and references within. \begin{table}
\begin{tabular}{c c c} \hline
**Export Component** & **Mangrove Forests** & **Salt Marshes** \\ \hline POC & 1.73 \(\pm\) 0.23 (1.76) & 0.60 \(\pm\) 0.11 (0.31) \\ DOC & 5.90 \(\pm\) 1.95 (1.43) & 2.55 \(\pm\) 0.55 (1.33) \\ DIC & 14.34 \(\pm\) 2.26 (10.82) & 5.28 \(\pm\) 1.212 (3.995) \\ CH\({}_{4}\) & 0.0277 \(\pm\) 0.0135 (0.026) & 0.109 \(\pm\) 0.0786 (0.0081) \\ \hline \end{tabular}
\end{table} 
Table 5: Comparison of mean (\(\pm\)1 SE) rates of particulate organic carbon (POC), dissolved organic carbon (DOC), DIC and CH\({}_{4}\) export from mangrove and salt marsh ecosystems to adjacent tidal waters. Units = Mg C ha\({}^{-1}\) a\({}^{-1}\). Median values are in parentheses. Mangrove data from [52,94,153,154,155,156,157,158]. Salt marsh data from [144,159,150,159,157,159,158,159,160,157,159,161,170,162,163,164,165,166,167,168,169,171,172,173,174,175,176,177]. eventually sequestered in soils, supporting the estimate that salt marshes sequester proportionally more \(\text{C}_{\text{ORG}}\) in soils than mangroves. C\({}_{\text{ORG}}\) decomposition in mangrove soils to a depth of 1 m (158 Tg C yr\({}^{-1}\)) is greater than NPP (114 Tg C a\({}^{-1}\)), implying that (1) the measurements of below-ground microbial decomposition are overestimates (due to methodological shortcomings; variable incubation times, etc.