Dynamics and driving factors of the organic carbon fractions in agricultural land reclaimed from coastal wetlands in eastern China

Soils are an important pool for storing organic carbon. Differences in soil managements, the effect of land use, and other factors are often evaluated to explain the dynamics and storage potential of the total soil organic carbon (SOC). As the SOC pool is composed of several subpools with different degrees of stability and turnover rate, insights into the dynamics and driving factors of the functional pools are essential for evaluating carbon sequestration or emission. In China, coastal wetlands are undergoing intensive reclamation due to the ever-increasing population. However, the dynamics and identification of key drivers of the functional SOC pools of reclaimed soils remain largely unclear. In this study, the concentrations of SOC and permanganate-oxidizable carbon (POxC) were determined for soils with different reclamation durations and land uses in an intensified reclamation region in eastern China. A random forest (RF) model was used to identify the importance of the potential drivers of the SOC and POxC at 241 sites in this study area. The result indicated that both SOC and POxC increased significantly with reclamation duration until 65 years of land reclamation. The human-dominated land uses had significantly (p < 0.05) higher levels of SOC and POxC than the tidal flats. For lands with identical reclamation durations, the paddy soils were superior to the upland soils in terms of sequestering the SOC and POxC. Soil total nitrogen (STN), pH and soil total phosphorus (STP) were identified as the most important factors impacting the spatial pattern of SOC, while the three most important drivers of POxC were STN, STP and Cl in coastal reclaimed lands. The relatively higher POxC contents and POxC/SOC ratios of the reclaimed soils in this study compared to previous studies suggest that reclaimed soils are potentially under the great threat of global warming due to the high sensitivity of POxC to environmental changes. However, paddy management practices are recommended as an efficient approach for minimizing the potential negative impacts of global warming because of the larger POxC sequestration ability of paddy soils than that of other land uses.