Jia Lin, Xiaoguang Ouyang, Siqi Nie, Wangli Xing, Yunlin Jiang, Fen Guo, Yuan Zhang, Zhifeng Yang
Blue carbon ecosystems contain disproportionately large carbon stocks and have therefore become priority targets for ecological restoration. Yet long-term effectiveness of restoration remains uncertain because species choice and hydrogeomorphic settings are rarely evaluated together in relation to carbon sequestration. Here, we analyzed sediment C and N accumulation in 90 cores from natural and restored mangroves, seagrass beds, and tidal flats across estuarine and open-coast settings in South China, integrating multi-decadal dating, stable-isotopic analysis, and an organic matter diagenetic model to improve comparisons of historical C and N inputs. Our results suggest that natural mangroves were the principal sedimentary C and N reservoirs, exhibiting the highest burials and stocks. However, restoration effects on C and N burial were strongly context dependent. Specifically, native-species restoration outperformed exotic-species in large-river estuaries, whereas exotic-species restoration outpaced native-species restoration in open-coast and small-river estuarine mangroves. Nevertheless, exotic-restored stands in coastal mangroves showed lower apparent stability of organic matter despite high burial rates. Restored seagrass meadows had C and N burial rates comparable to natural meadows. Decadal dating and δ13CSOC profiles indicated that hydrogeomorphic setting and restoration strategy shaped temporal trajectories of C and N inputs. Structural equation modelling further identified vegetation type and factors associated with hydrogeomorphic setting, including position, sedimentation rate, and porewater chemistry, as key controls on C and N stocks. Together, these results highlight that restoration planning should begin with an assessment of hydrogeomorphic conditions to guide restoration priorities, thereby strengthening the climate credibility of blue carbon restoration.