Seedling Density as a Key Determinant of Topsoil Organic Carbon and CNP Stoichiometry in Intertidal Mangrove Ecosystems: Insights from Avicennia marina Stands

Background and Aims Mangrove ecosystems are vital "blue carbon" sinks, yet their degradation threatens global carbon sequestration efforts. While forest structure influences carbon storage, the role of specific life-history stages, particularly that of seedlings, remains poorly quantified. This study aims to identify the key drivers of soil organic carbon (SOC) retention in mangroves, with a particular focus on the understudied role of seedling density. Methods We conducted a field study in a monodominant Avicennia marina forest on Techeng Isle, China, sampling along seaward-landward transects across the intertidal gradient. We measured stand structure and topsoil properties (0–20 cm), analyzing for SOC, total nitrogen (TN), total phosphorus (TP), and stoichiometric ratios (C:N:P). The relative importance of drivers was assessed using random forest regression. To uncover the underlying mechanisms, we derived proxy variables for belowground carbon input and sediment oxygen demand, which were integrated into structural equation modeling (SEM) to test causal pathways. Results From the seaward to landward fringe, the mangrove population structure shifted from declining to growing, with a significant increase in seedling density ( P  < 0.001). Concurrently, the levels of SOC, TN, TP and their stoichiometric ratios increased significantly from the outer to the inner fringe ( P  < 0.05). Among all predictors, seedling density emerged as the most vital for SOC, a finding substantiated by random forest analysis. Structural equation modeling demonstrated that seedling density enhances SOC primarily by promoting sediment anoxia and, to a lesser extent, by increasing root-driven carbon inputs, with the model explaining a substantial proportion of the variance (R² = 0.97). Conclusion Our findings demonstrate that mangrove population characteristics, especially seedling density, exert a stronger influence on topsoil carbon and nutrients than sediment physical properties alone. We propose high-density seedling planting as a novel and effective restoration strategy to enhance blue carbon sequestration, improve nutrient retention, and bolster ecosystem resilience against sea-level rise.