Blue carbon and microplastic dynamics in natural and planted mangroves, Thailand

Mangrove forests (MFs) play a crucial role in climate change mitigation due to their capacity to store significant amounts of blue carbon. This study investigates carbon sequestration dynamics in MF sediments, encompassing natural stands (sites BD1 and BD3) and planted stands (sites PN2, PN3, and PN4), focusing on vertical soil profiles up to 1 m depth. Results indicate that natural mangroves function as blue carbon hotspots, benefiting from mangrove, terrestrial, and marine organic matter (OM) inputs. In comparison, a 10-year-old plantation (PN2) stored approximately twice the carbon of a 2-year-old plantation (PN4), suggesting that effective carbon sink development requires more than a decade. Sediment cores revealed that BD1 dates back to 1952, whereas PN2 is older, dating to 1919. Fine sand layers at BD3 and PN4 are likely remnants of tropical storm deposits, illustrating how mangroves archive century-scale hydroclimatic events. Microplastic (MP) pollution is emerging as a critical concern, with both natural and planted mangroves accumulating MPs in their sediments. A negative correlation between MPs and total organic carbon (TOC) in natural forests suggests that organic-rich sediments may reduce MP retention through competitive adsorption or enhanced biodegradation—an unprecedented finding requiring further investigation. Overall, this study underscores the importance of mangrove restoration for climate resilience and highlights the necessity of integrating MP pollution into blue carbon management strategies. It advances understanding of plantation effectiveness, sediment carbon stability, and anthropogenic stressors influencing mangrove ecosystems. • Natural mangrove act as blue carbon hotspots from diverse organic matter. • Ten-year plantations store double carbon compared to two- year ones. • Fine sand (>30 %) indicates storm-affected sediment layers. • Negative microplastic-TOC correlation found in natural forests. • Microplastics may disrupt sediment carbon sequestration processes.