Temporal and depth-dependent variations in soil aggregate-associated organic carbon in reclaimed coastal poplar plantations

Coastal reclamation alters the ecological environment of wetlands and influences global carbon cycles. However, variations in soil organic carbon fractions and stability in the soil profile during reclamation, particularly in the subsoil, remain unclear. In this study, soil aggregate-associated organic carbon and its stability were investigated in soil profiles (0-100 cm) at different reclamation times (0, 24, 44, and 64 years) in coastal poplar plantations in East China. Total soil organic carbon concentrations in the topsoil (0-40 cm) increased with increasing reclamation time but varied little in the subsoil (40-100 cm). In the topsoil, the soil organic carbon concentrations in different aggregate fractions tended to increase with increasing reclamation time, which was enhanced mainly by particulate organic carbon within macro-aggregates. In the subsoil, soil organic carbon concentrations increased in the micro-aggregate fraction and decreased in the silt and clay fraction with increasing reclamation time, whereas the opposite changes led to smaller variations in total soil organic carbon. This relative balance was regulated by micro-aggregate formation via new organic matter input (e.g., root litter) and old mineral-associated organic carbon depletion. Regardless of the soil horizon, soil aggregate stability increased with reclamation time, whereas the total particulate organic carbon/mineral-associated organic carbon ratio increased with increasing reclamation time, implying that the soil organic carbon stability decreased, which may be attributed to an increase in the vulnerability of soil organic matter to mineralization as the reclamation progresses. These findings indicate that long-term plantation development influences aggregate-associated organic carbon accrual throughout the soil profile in reclaimed coastal land. Thus, effective management is required to improve soil organic carbon accrual in coastal woodland subsoil, which is crucial for increasing soil carbon sequestration.