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Coastal wetlands host large and dynamic reservoirs of organic carbon (C) and are also biogeochemical hotspots for a wide range of Fe (hydr-)oxides with different chemical reactivities, properties, and functions. The cycling of these iron (hydr-)oxides is closely coupled to that of organic C, which in turn strongly influences the magnitude and dynamics of organic C turnover in these ecosystems. This review synthesizes and summarizes current knowledge of distribution, turnover, and controls of Fe (hydr-)oxides, as well as their ecological roles and impacts on organic C turnover in coastal wetland ecosystems globally. Regional hydro-geochemical processes and anthropogenic activities in the uplands as well as soil texture exert a first-order control on the abundance and distribution of Fe (hydr-)oxides in coastal wetland soils, while the activities of plant roots and macro-organisms act as important biological drivers for the formation, transformation, and turnover of Fe (hydr-)oxides as well as associated organic C in both rhizosphere/burrows and bulk soils. The reported rates of dissimilatory Fe reduction (DFeR) are correlated with incubation temperature and the sizes of reactive Fe(III) phases. However, the contributions of DFeR to total anaerobic carbon oxidation were found to be correlated only with the size of reactive Fe(III) pools, meaning that all the identified processes contributing to the accumulation and formation of Fe hydroxides could increase the importance of the DFeR-dominated respiratory pathway and suppress sulfate reduction and methanogenesis. Additionally, Fe plaques dominated by amorphous Fe hydroxides are formed and cycled in close interaction with the activities of wetland plant roots, and likely provide several important ecological functions and contribute to maintaining high levels of plant productivity in coastal wetlands under different environmental stresses. The features and findings presented in this review not only contribute to an improved understanding of the biogeochemical cycle and ecological roles of Fe (hydr-)oxides in coastal wetlands, but also provide a basis for future studies on some highlighted key research areas. Such future studies will further increase our ability to understand and predict how the size, stability, and turnover of Fe (hydr-)oxides and organic C in coastal wetlands will respond to and affect global climate change. [ABSTRACT FROM AUTHOR]

• A sequential chemical extraction method was used to assay soil Si fractionations along different slope positions at the margin of a coastal wetland. • The Na 2 CO 3 -Si fraction accounted for 82%–90% of the labile Si in soil and sediment, mainly resulting from phytoliths and diatoms. • The storage of labile Si in sediment was significantly lower than that in soil. • With the influence of margin erosion under sea level rise, coastal wetland ecosystems will be an important source of Si to the estuary. Silicon (Si) and its biogeochemical cycling play an important role in maintaining the functions of terrestrial and marine ecosystems. However, the effects of sea level rise on the biogeochemical cycling of Si in coastal wetlands remain poorly understood. To explore this impact on biogeochemical Si cycling, we sampled a gradient from sediment to soil without the impact of tidal inundation in the Beidagang Wetland Nature Reserve, and then assayed non-crystalline Si (labile Si), including mobile Si (CaCl 2 -Si), adsorbed Si (Acetic-Si), Si bound to soil organic matter (H 2 O 2 -Si), Si occluded in pedogenic oxides/hydroxide (Oxalate-Si), and amorphous Si (Na 2 CO 3 -Si) fractions. Analytical results showed that the content of CaCl 2 -Si ranged from 13.0 to 53.3 mg kg−1 and the content of Acetic-Si ranged from 32.3 to 80.9 mg kg−1, both of which were lower in sediments compared to soils. The content of H 2 O 2 -Si (84.1–160.1 mg kg−1) and Oxalate-Si (306.6–655.1 mg kg−1) in the soil profiles showed non-significant variations along the sampling slope. The Na 2 CO 3 -Si fraction accounted for 82%–90% of labile Si in soil and sediment, mainly being contributed from phytoliths or diatoms. Diatoms were only detected in sediment profiles. The storage of labile Si in sediment was significantly (p = 0.0009) lower than the storage in soil, suggesting that the coastal wetland ecosystems are an important source of Si to the estuary. With future sea level rise and increased margin erosion, the inter-transformation processes among different Si fractions would likely be weakened to increase dissolved Si for marine diatoms. [ABSTRACT FROM AUTHOR]