Depth-dependent mineralization reveals contrasting disturbance vulnerability of seagrass blue carbon

Seagrass sediments represent major blue carbon reservoirs, yet assessments of their vulnerability to disturbance commonly rely on standing sediment organic carbon (SOC) stocks, with limited consideration of how much carbon may be mineralized following sediment exposure. We conducted a 59-day aerobic incubation of sediments from contrasting muddy and sandy seagrass habitats across three depths (0-10, 10-20, and 20-30 cm) to examine SOC mineralization, organic matter transformation, and associated enzymatic processes. Muddy sediments contained substantially larger SOC stocks and showed greater absolute cumulative mineralization, but only 6.94% of the initial SOC was mineralized, compared with 33.7% in sandy sediments. Across habitats, changes in thermally defined organic matter fractions and enzyme activities were consistent with a continuous mineralization pathway in which enzyme-mediated consumption of refractory organic matter was linked to subsequent turnover of intermediate stable fractions. Sediment depth further modified these transformation processes, particularly the consumption of stable and labile organic matter. These contrasting responses indicate that high SOC stocks do not necessarily correspond to high proportional mineralization vulnerability: carbon-rich muddy sediments may represent a larger absolute emission risk when physical protection is disrupted, whereas deeper sandy sediments may be particularly sensitive to oxygen exposure. Blue carbon vulnerability assessments should therefore integrate carbon stock size with sediment depth, stabilization mechanisms, and post-disturbance mineralization dynamics to support habitat-specific conservation and disturbance management.