Responses of Soil Organic/Inorganic Carbon Concentrations in the Lower Yangtze River to Soil Development and Land Use

Understanding the evolution and regulation of soil carbon (C) across different stages of geological development is essential for elucidating soil’s role in C storage and release processes. In this study, 1029 soil samples were collected from the alluvial layers of the lower Yangtze River. The chemical index of alteration (CIA) was employed to establish a gradient sequence of soil development, facilitating an investigation into the evolution of organic carbon (OC) and inorganic carbon (IC) in both surface and deep layers across various development stages, as well as their influencing factors. The results demonstrated that as soil develops, surface OC content increases significantly, while the deep layer exhibits no substantial changes. Notably, IC loss was particularly pronounced in surface soils, decreasing from 4.90 g/kg to 0.07 g/kg. Furthermore, the impacts of land use were more evident during the early stages of soil development. Paddy–dryland rotation (paddies) was found to enhance OC sequestration while maintaining IC levels comparable to those of drylands. Soil development directly influenced IC content, whereas its effect on OC content at different depths was primarily mediated by changes in the zirconium-to-rubidium (Zr/Rb) ratios and metal cation concentrations. This study highlights that OC accumulation during soil development predominantly occurs in surface layers, while IC leaching can also be detected at greater depths. At highly developed sites, paddies are recommended as beneficial for preserving C reserves.