Dynamics of Bacterial Communities in Salt Marshes Across Reclamation Ages in the Yangtze River Estuary

Salt marsh soils store substantial organic carbon, yet the persistence of this carbon depends on microbial community composition that governs key transformation processes. Coastal reclamation buries former marshes, but how burial alters microbiomes and carbon-relevant functions through time remains unclear. We investigated successional shifts in soil microbiomes across a reclamation chronosequence in the Yangtze River Estuary and compared buried (historically reclaimed) soils with modern marsh soils using 16S rRNA high-throughput sequencing. Bacterial diversity declined significantly in buried soils, with Shannon index values decreasing from 7.07–7.24 in modern soils to 6.13–6.55, accompanied by a depth-related shift from aerobic to anaerobic dominance. Burial duration was the strongest predictor of community composition (r² = 0.6759, p = 0.001), indicating that prolonged burial after reclamation is associated with physicochemical changes that restructure microbiomes. Total organic carbon, total nitrogen, and pH were the main environmental correlates; total organic carbon decreased with depth, consistent with the expansion of anaerobes and intensified carbon and sulfur cycling. Redundancy analysis showed that variation in these soil properties explained 56.61% of community differences. By linking burial-driven microbial reassembly to measurable soil properties, these findings elucidate the spatiotemporal dynamics of microbial communities in buried salt marshes induced by reclamation and their environmental constraints, providing a theoretical basis for assessing the stability of soil carbon sinks of salt marshes.