Abstract: Mangroves play a crucial role in carbon cycle and budget of coastal ecosystems, and it is important to identify the source and variation characteristics of mangrove soil organic carbon. In March (spring) and November (autumn) of 2016, mangrove wetland in the Jiulong River Estuary Mangrove Provincial Nature Reserve in Longhai, Fujian Province was selected as study object, soils of landward mangrove, seaward mangrove, and tidal flat in half-mature and mature forests of Kandelia obovate were collected to analyze the changes of soil organic carbon contents and storages at 0-100 cm depths, and the contributions of different potential sources of soil organic carbon were identified using the Stable Isotope Mixing Model (SIMMR). The results showed that the soil organic carbon contents in half-mature and mature forests were ranged from 9.80 to 30.10 g/kg and 9.40 to 30.97 g/kg, respectively, and their storages in autumn were 25.6% and 19.1% higher than those in spring, respectively. The δ¹³C and δ¹⁵N values in all soils ranged from -28.20‰ to -23.68‰ and 2.39‰ to 10.98‰, respectively. The δ¹³C values in seaward mangrove and tidal flat soils were higher than in landward mangrove. Maricultural particulate organic matter was the main source of soil organic carbon, with an average relative contribution of (32.6±3.7)%, followed by riverine particulate organic matter (23.3±10.2%) and C3 plants (20.1±0.9%). The contribution proportion of mariculture and C3 plants was highest in the landward mangrove, while the contribution of river was highest in the tidal flat soil. Moreover, the contribution of autochthonous sources to soil organic carbon showed seasonal differences. Soil organic carbon storage in mangrove wetland decreased from landward to seaward, and soil organic carbon mainly originates from allochthonous sources. Therefore, mangrove ecosystems have strong capacity for carbon capture and sequestration.