Long-term continuous agricultural activities can significantly increase the soil carbon storage in the reclaimed wetlands of estuarine coastal islands

Coastal reclamation alters wetland carbon cycles by modifying hydrological conditions, vegetation composition, and soil physicochemical properties. However, the relative contributions and underlying pathways by which reclamation period and land-use type influence soil organic carbon (SOC) content remain unclear. This study analyzed soil physicochemical properties, SOC content, and δ¹³C values across reclamation chronosequences on Chongming Island, Shanghai, and employed random forest modeling and partial least squares path modeling (PLS-PM) to elucidate the key factors and causal mechanisms governing SOC variation. The results showed that SOC content was positively correlated with reclamation period ( p < 0.05) and was higher in paddy fields and other croplands than in artificial forests and natural wetlands. Among the measured environmental variables, SWC had higher variable importance for predicting the spatiotemporal distribution of SOC than did pH, D₅₀, and land-use type. PLS-PM revealed that SWC and soil chemical properties were the primary factors directly influencing SOC ( p < 0.001), whereas D₅₀ had a significant negative direct effect only on surface SOC ( p < 0.01). Both land-use type and reclamation period indirectly influenced SOC primarily by modulating soil physicochemical properties, with land-use type exhibiting a larger total effect on SOC. Furthermore, earlier reclaimed areas exhibited a higher proportion of new carbon derived from agricultural or forest inputs compared to areas reclaimed during the 1990s, consistent with the observed SOC enrichment patterns. These findings highlight the critical role of soil physicochemical properties and carbon input in SOC dynamics and the potential of long-term croplands for achieving carbon neutrality goals.