Bio-geomorphologic effects on blue carbon accumulation in mangrove sediments

Blue carbon habitats, such as mangroves, salt marshes and seagrasses, are vital carbon sinks, storing large amounts of organic carbon in sediments, which often exhibit pronounced spatial variability even within a single site. Here, we examined the bio-geomorphologic controls underlying this spatial heterogeneity through year-round field investigation in a mangrove ecosystem along China's southern coast. Field observations across an elevation gradient revealed that sediment organic carbon stocks generally decreased with lowering elevation, with vegetated zones storing significantly more organic carbon than adjacent bare mudflats. Further analysis demonstrated that wave-induced bed level variability—rather than sediment input alone—was the primary factor governing carbon storage in the sediment. Vegetated zones had much higher sediment organic carbon stocks than bare mudflats due to the reduced hydrodynamic disturbance provided by the vegetation. Despite receiving six times more sediment input, the sediment carbon stock in the top 15 cm of bare mudflats was less than 25 % of that in the vegetated zones, due to experiencing at least three times more erosion and carbon loss. These findings establish a bio-geomorphologic framework for understanding heterogeneous carbon sequestration within mangroves and extend to other vegetated blue carbon habitats. By providing empirical evidence for the regulating role of bio-geomorphologic feedbacks, this study enhances predictive understanding of organic carbon burial processes and informs long-term assessments of coastal carbon sink dynamics under changing environmental conditions.