Fe-S interactions and geological background shape phosphorus bioavailability in mangrove sediments

Mangrove sediments act as critical phosphorus (P) sinks and potential sources in coastal ecosystems, with P release risk dominated by bioavailable phosphorus (BAP), including exchangeable P, iron-bound P (Fe-P), and organic P (OP). While Fe-S cycles tightly regulate P speciation, how the geological background mediates Fe-S-P coupling across heterogeneous mangroves remains unclear. We integrated Fe-S-P fractionation, water-soluble organic matter (WSOM) fluorescence spectroscopy, and metagenomics to compare volcanic weathering (V) and Quaternary coastal (Q) sediments in Dongzhai Harbor. Key findings are as follows: Total P in shallow V sediments (8.89-30.90 μmol/g) is higher than in Q (6.95-17.09 μmol/g). OP dominates V's BAP (48.87%) linked to OP mineralization genes (e.g., appA), whereas Fe-P dominates Q's BAP (57.31%) and is stabilized by amorphous Fe oxides. In deep V sediments, Fe-P is positively correlated with acid-volatile sulfide (AVS; R² = 0.57) and Fe/S reduction genes (fsr, omcF), indicating Fe-S-coupled P mobilization and release. In deep Q sediments, Fe-P remains stabilized by Fe_ox1, and P mobilization is driven by microbial iron reduction, with iron-reduction genes (e.g., mtrA) being significantly enriched. Two distinct P release pathways are identified: in volcanic weathering zone sediments, P release shifts from OP dominance to Fe-S-coupled regulation, while in Quaternary coastal zone sediments, Fe-P mobilization relies on microbial iron reduction. These findings clarify the geological controls on Fe-S-P interactions in mangrove sediments, providing a scientific basis for site-specific P risk assessment and the formulation of targeted strategies for mangrove ecosystem restoration and coastal eutrophication control.