Mangrove stores five times more organic carbon (blue carbon) stocks per unit area compared to terrestrial forests, but an understanding of blue carbon sequestration mechanisms is limited. This research assesses carbon stocks, carbon burial rate, and carbon origins of mangrove soils in West Papua, Indonesia — a region represents nearly 15% of the world’s mangrove area. A series of field biophysical data collections were conducted across three study sites representing coastal geomorphology gradient; namely coastal mudflat, fringe mangrove, and interior mangrove. Soil carbon stocks were determined by analysing physicochemical properties. A 210Pb radionuclide dating and stable isotopes (δ13C and δ15N) approaches were used to estimate the burial rates and origins of soil carbon. The largest carbon stocks and burial rates of the top 50 cm soil were obtained at interior mangrove with 179 Mg C ha–1 and 0.86 Mg C ha-1 yr-1. The ages of buried soil carbon ranged between 74 and 87 years. Carbon stored in mangrove soils is originated from multiple sources (autochthonous and allochthonous), with net primary productivity of hinterland forest and mangrove itself induced carbon burial substantially. Our findings indicate that carbon cycling in mangrove and terrestrial forest ecosystems are closely linked, and at least a part of carbon losses from terrestrial forests can be captured and buried in mangrove ecosystems. This research suggests that ecosystems connectivity within a catchment scale play a key role in optimizing the blue carbon sink capacity of mangrove and therefore should be considered in the mangrove conservation management policy.