<p>Coastal wetlands are significant global carbon (C) sinks, but the characteristics and sources of organic carbon (OC) are far from understood due to the difficulties in assessing the different sources of the OC. In this study, sediments from eight reed wetlands around the Bohai Sea were collected. Sediment organic carbon (SOC), total nitrogen (TN) and the stable carbon/nitrogen isotope compositions (δ¹³C, δ¹⁵N) were analyzed to quantify the contributions of different sources to SOC. The results indicated that the SOC content ranged from 0.15% to 4.90%, with an average of 1.16%. The δ¹³C ranged from - 27.10‰ to - 21.70‰ (mean: - 24.76‰) and the δ¹⁵N varied from 1.19‰ to 6.58‰ (mean: 3.16‰), suggesting that the SOC were predominantly derived from reed detritus. Three major sources of the SOC were identified: reed detritus, terrestrial and marine OC. Using the IsoSource model, the reed detritus contribution were calculated as 67.5%, 43.5%, and 51.8% to the SOC of the Liaodong Bay wetlands (LDBW), Bohai Bay wetland (BHBW) and Laizhou Bay wetland (LZBW), respectively. Terrestrial OC accounted for an average of 16.5%, 33.9%, and 40.2% of the SOC in LDBW, BHBW, and LZBW, respectively. In contrast, marine OC contributed 16.0%, 22.7%, and 8.1% to the SOC in the same regions. For LZBW and LDBW, the reed detritus was the main source of SOC of the restored wetland whereas the terrestrial or marine derived OC became increasingly significant in tidal wetlands. Our results suggested that vegetation played a crucial role in carbon sequestration of coastal wetlands, therefore the wetland restoration project for vegetation improvement management need to be strengthened in the future. These findings provide a scientific basis for future monitoring of the organic carbon of the coastal wetlands.</p>