Microbial-mediated mechanisms of iron-bound organic carbon formation in mangrove rhizosphere soils: An integrated biogeochemical and metagenomic analysis along a salinity gradient

Background and Aims Mangrove ecosystems serve as critical blue carbon sinks, yet the microbial mechanisms governing iron-bound organic carbon (Fe-OC) formation under varying salinity conditions remain poorly understood. This study aims to elucidate the microbial-mediated pathways controlling Fe-OC formation in Aegiceras corniculatum rhizosphere soils across a natural salinity gradient (1.5–2.4‰). Methods We employed an integrated biogeochemical and shotgun metagenomic approach to analyze rhizosphere and bulk soils from the Nanliu River Estuary, China. Structural equation modeling (SEM) was utilized to identify the dominant mechanistic pathways linking environmental factors, microbial metabolism, and Fe-OC formation. Results Under increasing salt stress, absolute Fe-OC content declined from 15.2 to 4.0 mg·g⁻¹, yet its proportion within total organic carbon increased from 34.2% to 61.8%. Metagenomic analysis revealed stable functional gene diversity despite significant taxonomic turnover, demonstrating inherent functional redundancy. Iron oxidation genes were enriched in saline flats, while carbon fixation genes concentrated in fresher sites. SEM identified salinity as the master environmental control (R² = 0.67), operating primarily through a dominant pathway: salinity → iron oxidation genes → iron oxides → Fe-OC. Conclusion Mangrove root metabolism and microbial functional genes synergistically mediate soil iron-carbon binding. Salinity acts as the primary environmental control through both direct geochemical effects and indirect pathways via microbial community restructuring, highlighting the importance of functional redundancy in maintaining ecosystem resilience under environmental stress.