Organic carbon preservation in wetlands: Iron oxide protection vs. thermodynamic limitation

•OC in surface sediments is primarily protected by Fe (oxyhydr)oxides.•The Fe-bound OC decreased by 2 to 10-fold with increasing sediment depth.•DOC degradation in bottom sediments is stabilized by thermodynamic limitation.•Fe oxides and organics are heterogeneous distributed at the (sub-)microscale. The sequestration of organic carbon (OC) in wetland sediments is influenced by the presence of oxygen or lack thereof. The mechanisms of OC sequestration under redox fluctuations, particularly by the co-mediation of reactive iron (Fe) protection and thermodynamic limitation by the energetics of the OC itself, remain unclear. Over the past 26 years, a combination of field surveys and remote sensing images had revealed a strong decline in both natural and constructed wetland areas in Tianjin. This decline could be attributed to anthropogenic landfill practices and agricultural reclamation efforts, which may have significant impacts on the oxidation–reduction conditions for sedimentary OC. The Fe-bound OC (CBD extraction) decreased by 2 to 10-fold (from 8.3 to 10% to 0.7–4.5%) with increasing sediment depth at three sites with varying water depths (WD). The high-resolution spectro-microscopy analysis demonstrated that Fe (oxyhydr)oxides were colocalized with sedimentary OC. Corresponding to lower redox potential, the nominal oxidation state of C (NOSC), which corresponds to the energy content in OC, became more negative (energy content increased) with increasing sediment depth. Taken together, the preservation of sedimentary OC is contingent on the prevailing redox conditions: In environments where oxygen availability is high, reactive Fe provides protection for OC, while in anoxic environments, thermodynamic constraints (i.e., energetic constraints) limit the oxidation of OC. [Display omitted]