Changes in surface soil organic/inorganic carbon concentrations and their driving forces in reclaimed coastal tidal flats

Intensive coastal wetland reclamation in China followed by agricultural use had significant impacts on soil carbon (C) dynamics due to the alterations in soil hydrology and physicochemical properties. The changes in C and its driving factors in reclaimed soils generally concentrated on soil organic C (SOC) and soil inorganic C (SIC) as major subpools of soil C, but this topic has received little attention. In this study, we have assessed the changes in SOC and SIC concentrations (0-20 cm) with reclamation duration and land use change based on 746 surface soil samples collected from three typical reclamation areas (with differences in sediment sources) in eastern China. Key factors driving SOC and SIC dynamics were identified using a random forest model (RF). The results revealed similar SOC concentration (7.24-7.69 g kg(-1)) and different SIC concentration (7.25-10.9 g kg(-1)) for the three study areas. Overall, SOC increased with reclamation duration, suggesting a positive impact of tidal flat reclamation on SOC accumulation. For SIC, slight increases in the younger land and dramatic decreases in the older land were observed, indicating a fast IC turnover time. The land impacted strongly by human activities had higher SOC than natural tidal flats. Compared with upland soil, paddy soil resulted in greater OC (except for 10 years reclaimed land in Cixi) and comparable levels of IC. Soil nutrient levels (the concentrations of nitrogen and phosphorus) were determined to be important factors for SOC in reclaimed lands. Soil CaO was determined to be the factor with the highest importance in controlling SIC dynamics in reclaimed lands. More attention should be given to the quality of the OC sequestrated by reclaimed soils.