# Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis
## 4 Soil Carbon Biogeochemistry  
### Soil-Air/Water Fluxes  
Salt marsh and mangrove soils are waterlogged and saline and inundated for a part of every day. Usually but not always composed of silt and clay particles, soil of both ecosystems is penetrated by roots, rhizomes, and pieces of dead plant material. Having similar C and nutrient contents (Section 2), it is not surprising that they have rates of soil respiration that are not significantly different (one-way ANOVA on ranks; \(p>0.05\)), except for significantly lower rates of oxygen uptake in mangrove soils (Table 3); these oxygen fluxes are significantly lower than those of dissolved inorganic carbon (DIC) and CO\({}_{2}\)[94]. In contrast, rates of CH\({}_{4}\) release are, on average, nine times greater from salt marsh soils than from mangrove deposits (Table 3). Methanogenesis is not a large decomposition pathway in mangrove \begin{table}
\begin{tabular}{c c c} \hline
**Component** & **Mangrove Forests** & **Salt Marshes** \\ \hline GPP & 35.3 \(\pm\) 6.0 (26.6) & 17.7 \(\pm\) 1.9 (16.6) \\ Aboveground NPP & 13.2 \(\pm\) 1.7 (9.0) & 5.0 \(\pm\) 0.3 (3.7) \\ Belowground NPP & 5.2 \(\pm\) 4.4 (5.1) & 12.6 \(\pm\) 1.1 (9.4) \\ R\({}_{\rm{C}}\) & 22.3 \(\pm\) 4.9 (15.7) & 8.4 \(\pm\) 1.2 (6.4) \\ Microalgal GPP & 4.4 \(\pm\) 2.2 (3.3) & 1.8 \(\pm\) 0.8 (1.3) \\ Microalgal NPP & 2.1 \(\pm\) 0.7 (1.8) & 1.5 \(\pm\) 0.2 (1.1) \\ P\({}_{\rm{GFP}}\)/R\({}_{\rm{C}}\) & 1.6 \(\pm\) 0.1 (1.5) & 1.0 \(\pm\) 1.3 (1.0) \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of mean (\(\pm\)1 SE) GPP and above- and belowground net (NPP) primary production, canopy respiration (R\({}_{\rm{C}}\)) and benthic microalgal GPP and NPP between salt marsh and mangrove ecosystems. Units = Mg C\({}_{\rm{ORG}}\) ha\({}^{-1}\) a\({}^{-1}\). Median values are in parentheses. Mangrove data from [6, 52, 53]. Salt marsh g dry weight (DW) production converted to g C\({}_{\rm{ORG}}\) using percent C\({}_{\rm{ORG}}\) content in roots + rhizomes (41.5 \(\pm\) 1.2%) and shoots (38.5 \(\pm\) 0.7%) of various species (references in Table 1). Salt marsh above- and belowground production data from ([54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 80, 82, 84, 85, 87, 89, 91, 92, 93]. \begin{table}
\begin{tabular}{c c c} \hline
**Component** & **Mangrove Forests** & **Salt Marshes** \\ \hline Soil respiration & 6.13 \(\pm\) 0.62 (DIC, CO\({}_{2}\)); 3.88 \(\pm\) 0.29 (DO, O\({}_{2}\)) & 5.64 \(\pm\) 0.63 (3.66) \\ Soil DIC production & 18.27 \(\pm\) 2.30 (15.5) & 6.92 \(\pm\) 1.61 (3.8) \\ Soil CH\({}_{4}\) release & 0.015 \(\pm\) 0.006 (0.004) & 0.142 \(\pm\) 0.02 (0.07) \\ C\({}_{\rm{org}}\) burial & 1.62 \(\pm\) 0.67 (1.33) & 3.82 \(\pm\) 0.58 (1.84) \\ \hline \end{tabular}
\end{table} 
Table 3: Comparison of mean (\(\pm\)1 SE) rates of soil respiration, dissolved inorganic carbon (DIC) production (to a depth of 1 m), CH\({}_{4}\) release and C\({}_{\rm{org}}\) burial between mangrove and salt marsh ecosystems. Units = Mg C ha\({}^{-1}\) a\({}^{-1}\). Soil respiration data averages measurements taken at the soil surface during air exposure and tidal inundation and includes both gas (O\({}_{2}\) and CO\({}_{2}\)) and solute (dissolved oxygen (DO) and DIC) measurements together for salt marshes and separately for mangroves. Mangrove oxygen and carbon fluxes are kept separate because of significant differences [94]. Median values are in parentheses. Mangrove data from [94]. Burial rates for both ecosystems from [4]. Salt marsh data from [56, 58, 76, 95, 96, 97, 98, 99, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 12, 13, 14, 15, 16, 17, 18, 19, 130, 18]. soils [50], probably due to the vertically depth-elongated zones of sulfate, iron and manganese reduction, in which rates vary irregularly and even peak at various depths to 1 m [94].