Threshold effect of seagrass transplantation density on blue carbon sequestration

Seagrass restoration is increasingly promoted as a key strategy for climate change mitigation. However, a key design parameter - transplantation shoot density - remains poorly understood in terms of its influence on blue carbon capture and storage. This study investigated the regulation of suspended particulate matter deposition by shoot density in transplanted tropical Enhalus acoroides, aiming to identify optimal density thresholds for enhancing particulate carbon deposition. The deposited particles were differentiated into organic and inorganic fractions, and the thermal stability spectrum of particulate organic carbon was characterized using thermogravimetric analysis (TGA) to quantify labile, recalcitrant, and refractory pools. Seagrass survival exceeded 95 % across all transplanted plots. Particulate matter deposition rate was significantly higher in summer, due to increased algae and seagrass contributions. A nonlinear relationship between seagrass density and particulate deposition was observed, where maximum sequestration of particulate organic carbon (POC) and total particulate organic matter (POM) occurred at 113-118 shoots m^-2. Particulate inorganic carbon (PIC) and particulate inorganic matter (PIM) deposition peaked at higher densities of 127 shoots m^-2 and 140 shoots m^-2, respectively. Our findings translate into actionable operational guidance: leveraging the identified density thresholds (113-118 shoots m^-2) can enhance blue carbon burial. This density-driven carbon optimization framework provides critical science-based tools for: (1) Enhancing the efficacy of global seagrass restoration programs aimed at climate mitigation; (2) Supporting nationally determined contributions under the Paris Agreement through quantifiable blue carbon gains; (3) Strengthening coastal ecosystem resilience via engineered sediment carbon stabilization.