There are several reasons why mangrove forest ecosystems have high ecosystem C stocks. Coastal ecosystems sequester CO₂ from the atmosphere through plant primary production and store it in plant biomass (mostly for woody plants) and soil [30]. Although C accumulation rates vary among coastal wetlands, plant primary production in coastal wetlands in general is comparable to that of terrestrial forests [29]. However, the low decomposition rate of soil C gives coastal wetlands a higher potential to sequester C in sediments [29]. Thus, coastal ecosystems are generally believed to accumulate C up to 100 times faster than terrestrial forest ecosystems [19,31,32,33]. Compared to other coastal ecosystems, mangrove forests are believed to have higher organic C stocks because of their high growth rates [34]. Furthermore, unlike the herbaceous salt marshes, where most organic C stocks are stored in soil, C stocks in mangrove forests are distributed more in plant biomass than soil [35]. Previous research found that most mangrove plant-fixed C is stored in biomass and only 3%–11.7% of it is transferred to and stored in sediment [36].

 

The soil C stored in mangrove forests can vary widely, but it is generally higher in the tropical regions than the sub-tropical ones [35,37,38,39,40] (Table 1). Different environmental and soil physicochemical factors may explain this difference. Different tidal ranges may create different soil anaerobic conditions among mangrove forests, and thus affect C decomposition rates [40,41]. Moreover, fine soil texture in some mangrove forests may also reduce groundwater drainage and facilitate soil C accumulation [42].

 

Aboveground and belowground biomass production in mangrove plants is another major contributor to the ecosystem C stocks in mangrove forests. Unlike herbaceous plants, which have a fast C turnover rate, mangrove plants may be able to fix atmospheric CO₂ and store it as biomass for a long period of time (i.e., up to centuries); this would lead to a considerable amount of C stock [49]. Mangrove plants have different degrees of root volumes and aboveground structures that may create a wide range of C storage rates [22,50]. Indeed, field surveys from previous studies in Atlantic coastal mangrove forests showed that aboveground plant biomass comprised 50–250 Mg C ha⁻¹ and the belowground biomass comprised 10–50 Mg C ha⁻¹ [35,39].

 

The abundant C that mangrove forests provide facilitates the development of soil microbial communities. Studies have shown that the microbial genus Bacteroidetes is abundant in the mangrove rhizosphere, which may be due to the high particulate organic matter in the environment [51,52]. Furthermore, the abundant root systems of mangrove plants may create environmental niches for Proteobacteria, one of the important microbial genera for N and S cycling in mangrove ecosystems [52,53].

Although mangrove forests provide high ecosystem C stocks, their wide ranges of anoxic soil conditions also make them a considerable source of greenhouse gases and decrease their net contribution to CO₂ reduction (Figure 1). In addition, the presence of sulfate (SO₄^2-) in the saline water can serve as an alternative electron acceptor and help soil microbes yield more energy than methanogens, resulting in CO₂ efflux in coastal ecosystems [54,55,56]. As a result, the ecosystem respiration rates in tide-influenced coastal forest wetlands are typically higher than those observed in inland freshwater wetlands [57]. The average CO₂ emission from mangrove forests was calculated to be 0.7–3 g C m⁻² d⁻¹ [58,59,60,61], which is comparable to CO₂ emissions from coastal marshes (0.3–2 g C m⁻² d⁻¹) [56,62], but slightly higher than those from inland wetlands (0.8–1.6 g C m⁻² d⁻¹) [57] (Table 2).

CH₄ efflux in coastal wetlands is considerably lower than in freshwater wetlands, mostly because of the presence of SO₄^2- [56,82]. The CH₄ fluxes reported from previous literature show a decreasing trend with increasing salinity (Table 2). Compared to other coastal ecosystems, mangrove forests generally emit 0–23.68 mg C m⁻² h⁻¹ of CH₄ [58,60,65,74,76,77,83], which is generally higher than in brackish marshes (−0.17–0.23 mg C m⁻² h⁻¹) [56], but lower than in tidal freshwater marshes (0.01–10.8 mg C m⁻² h⁻¹) [62,84] and freshwater ecosystems such as rice paddies (0–630 mg C m⁻² h⁻¹) [79,80,85] or ponds (0.75–40.5 mg C m⁻² h⁻¹) [81] (Table 2). In addition, species in mangroves with pneumatophores had significantly lower CH₄ emission rates than in mangroves without pneumatophores because pneumatophores increase soil aeration [86]. Moreover, anthropogenic nutrient loading from upland drainage also contributes to the high CH₄ emission rates [72,78].

 

CO₂ in mangrove soils is generated by chemoheterotrophs during respiration, but the CH₄ fluxes are mainly attributed to methane-producing archaea in soils. However, until now, few studies have focused specifically on identifying the quantity, composition, and environmental niches of methanogenic communities in mangrove soils. The soil total organic C concentrations may stimulate CH₄ production and increase the mcrA gene expression (i.e., methanogenic population) in soil [87]. Furthermore, studies on other coastal ecosystems also found that methanogens may be sensitive to soil pH and showed optimum growth at soil pH 6.5–7.5 [88,89].

 

Along with high SO₄^2- concentrations, CH₄ efflux can be reduced by methanotrophs in surface mangrove soils that use CH₄ as an energy source and oxidize it into CO₂ (Figure 1) [90]. This mechanism can reduce CH₄ before it reaches the atmosphere [91,92,93]. However, most previous studies on methanotrophs have been performed in freshwater, not coastal, ecosystems. In fact, mangrove soils may have high CH₄ oxidation potentials that are comparable to those of freshwater ecosystems, such as rice paddies and lakes [94,95,96,97,98].

 

Compared to freshwater ecosystems, mangrove forest soils typically contain more Type I methanotrophic communities [97], which are believed to have higher CH₄ oxidation potentials, than Type II methanotrophs, which are typically found in freshwater ecosystems [99,100,101]. Moreover, the Type I methanotrophs Methylosarcina, Methylomonas, and Methylobacter in mangrove forest soils contained the most active CH₄-oxidizing genes, despite the fact that the dominant methanotrophs in mangrove soils were uncultured and their genes belong to the deep-sea 5 cluster, which is one of the five major sequence clusters retrieved from marine environments [102]. The presence of NaCl in mangrove soils was proven to be one of the reasons why this environmental niche contains more Type I methanotrophs than Type II ones [103]. As shown in a previous study, Methylobacter is better adapted to various salinity conditions and can be found in water with NaCl concentrations up to 3% [104]. In addition, alkaline environmental conditions may also be an important factor influencing the growth of Type I and Type II methanotrophs [98]. Previous studies revealed that the Type I methanotrophs Methylomonas and Methylobacter are mostly adapted to pH 6.5–7.55, which is generally the pH of saline ecosystems [97,104,105]. This ecological niche provided by the coastal mangrove forests may be one of the key factors resulting in the large Type I methanotrophic populations and low CH₄ emissions in this ecosystem.

 

 

 

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