The role of tidal range and seawater pollution in shaping mangrove biomass and carbon stocks.

Understanding the mechanisms that regulate carbon accumulation in mangrove ecosystems remains critical for supporting coastal climate-mitigation strategies. In this study, we integrate high-resolution airborne LiDAR data, sediment geochemistry, and hydrodynamic modeling to quantify the multi-scale controls on biomass and sediment carbon stabilization across 14 representative mangrove stands in subtropical and tropical China. Unlike previous regional assessments that emphasize latitudinal patterns, our approach focuses on how forest structural complexity, root-zone redox conditions, and hydrological connectivity shape long-term carbon storage. We found that stands characterized by dense prop-root networks and high canopy heterogeneity stored up to 47 % more sediment carbon than structurally simple forests. Sediment profiles revealed that prolonged anoxic conditions enhanced organic matter preservation, whereas periodic oxygenation events caused by storm-driven tidal surges accelerated carbon mineralization. Hydrodynamic simulations indicated that limited water exchange-not tidal amplitude alone-was the strongest predictor of belowground carbon retention. Metal pollutants and suspended solids played only secondary roles and mainly affected carbon stabilization in areas with restricted flushing. Nitrogen availability influenced productivity non-linearly: moderate ammonium enrichment enhanced growth, but high concentrations suppressed both net primary production and root respiration. Our findings highlight the importance of forest structure, micro-scale sediment chemistry, and hydrological isolation in governing China's mangrove carbon balance, providing a new framework for restoration programs targeting long-term carbon sequestration rather than biomass recovery alone.