Abstract: The climate-change-induced northward shift of mangroves in China has widely been recognized. However, its impacts on the dynamics of soil organic carbon (SOC) stock and underlying mechanisms remain poorly understood. Using a space-for-time substitution approach, we investigated the temporal dynamics of SOC content and density (SOCD), as well as their vertical distribution patterns, during the development of northward mangroves in southern Zhejiang. We further compared our results with that in 30-year-old mangrove stands in Fuding, Fujian Province. Results showed that the SOC content and SOCD of the northward-afforested mangroves generally increased with stand age. In 15 a mangroves, SOC exhibited a pronounced decreasing trend with increasing soil depth, and both SOC content and SOCD in the surface layer(0-20 cm) were significantly higher than those of other stand ages. Average SOC content (0.91%±0.18%) and SOCD (103.58±7.06 t C·hm^-2) of the 15-year-old northward mangroves were significantly higher than the 2- and 5-year-old mangroves, but were not different from the 30-year-old mangroves in Fuding (P>0.05). The accumulation of SOC and SOCD was mainly driven by pH and nitrogen content, and also driven by multiple environmental factors such as clay content and salinity during mangrove restoration in southern Zhejiang. These results suggest that the conservation and restoration of northward-afforested mangroves in southern Zhejiang can enhance coastal wetland organic carbon storage within a relatively short term, contributing to carbon sequestration and climate change mitigation.

Key words: northward mangrove, blue carbon, soil organic carbon, organic carbon density, ecological restoration