Silicon fractionations in coastal wetland sediments: Implications for biogeochemical silicon cycling

Coastal wetland sediment is important reservoir for silicon (Si), and plays an essential role in controlling its biogeochemical cycling. However, little is known about Si fractionations and the associated factors driving their transformations in coastal wetland sediments. In this study, we applied an optimized sequential Si extraction method to separate six sub-fractions of non-crystalline Si (Si_noncry) in sediments from two coastal wetlands, including Si in dissolved silicate (Si_dis), Si in the adsorbed silicate (Si_ad), Si bound to organic matter (Si_org), Si occluded in pedogenic oxides and hydroxides (Si_occ), Si in biogenic amorphous silica (Si_ba), and Si in pedogenic amorphous silica (Si_pa). The results showed that the highest proportion of Si in the Si_noncry fraction was Si_ba (up to 6.6 % of total Si (Si_tot)), followed by the Si_pa (up to 1.8 % of Si_tot). The smallest proportion of Si was found in the Si_dis and Si_ad fractions with the sum of both being <0.1 % of the Si_tot. We found a lower Si_occ content (188 ± 96.1 mg kg^-1) when compared to terrestrial soils. The Si_dis was at the center of the inter-transformation among Si fractions, regulating the biogeochemical Si cycling of coastal wetland sediments. Redundancy analysis (RDA) combined with Pearson's correlations further showed that the basic biogenic elements (total organic carbon and total nitrogen), pH, and sediment salinity collectively controlled the Si fractionations in coastal wetland sediments. Our research optimizes sediment Si fractionation procedure and provides insights into the role of sedimentary Si fractions in controlling Si dynamics and knowledge for unraveling the biogeochemical Si cycling in coastal ecosystems.