Coupled shifts in microbial and mineralogical Fe cycling destabilize organic carbon in converted estuarine wetlands

Estuarine wetlands are critical organic carbon sinks, where Fe oxides bind with organic carbon to form Fe-bound organic carbon (Fe-OC), which plays an important role in carbon sequestration within these ecosystems. The conversion of natural estuarine wetlands into aquaculture ponds leads to notable changes in both the Fe content and the Fe-OC pool. This study analyzed three typical Chinese estuarine wetlands to investigate changes in Fe fractions and Fe-OC during aquaculture pond conversion, employing 16S rDNA sequencing to examine Fe-related bacterial communities (Fe-oxidizing bacteria and Fe-reducing bacteria) dynamics, thereby revealing interactions between these bacteria, soil Fe, and Fe-OC throughout the transformation process. The results showed that after land-use change, Fe-OC and the molar OC:Fe rations (OC:Fe) in all soil layers decreased significantly by over 54% and 49%, respectively, while the Fe crystalline ratio (the ratio of crystalline Fe oxides to free Fe oxide) increased significantly by more than 100% across all layers. Among the Fe fractions, amorphous Fe oxides (Fe_o), complexed Fe oxides (Fe_p), and Fe-OC were key factors regulating Fe-reducing bacteria (p < 0.01). In turn, those Fe-related bacteria affected the Fe cycle and the transformation of Fe oxides (mainly Fe_o and Fe_p), thereby influencing Fe-OC and OC:Fe. The transformation process leads to Fe reduction, Fe_p depletion, and Fe_o crystallization, resulting in the loss of Fe-OC. To safeguard the carbon storage function of estuarine wetlands, it is essential to minimize wetland exploitation and implement strategies to curb Fe oxide loss and crystallization, thereby enhancing the stability of these critical carbon pools.