Whole-Profile Soil Carbon Responses to Climate Change Modulated by Vertical Carbon Transport and Priming Effect Gradients

The vertical transport (VT) of soil organic carbon (SOC) mixes carbon pools of varying depth-origin and decomposability, regulating whole-profile SOC dynamics through altered carbon pool interactions, such as the priming effect (PE). However, quantifying this process in situ is challenging. Using global data sets on SOC stocks and carbon inputs, we trained a depth-resolved SOC model incorporating VT and PE to assess the vertical gradient of VT and PE, and explore their roles in regulating whole-profile SOC dynamics in response to climate change. The results indicate that VT-induced redistribution of SOC is essential for capturing observed profile distribution of SOC stocks. Transported carbon from neighboring layers accounted for 8%–27% of total layer-specific carbon inputs, varying by depth and ecosystem type, and regulated SOC turnover behavior via the PE, especially in deeper layers. Precipitation emerged as the most important factor influencing layer-specific VT. While the PE was higher in upper layers, it was far from its maximum potential in deeper layers, making SOC dynamics in these layers more sensitive to carbon input changes. If VT and PE gradients are not considered, the sensitivity of whole-profile SOC to warming will be underestimated, and the impact of carbon input changes will be overestimated, particularly in deeper layers. Our findings highlight the critical role of VT and PE in controlling whole-profile SOC dynamics, underscoring the need to explicitly include these processes in Earth system models for reliable whole-profile SOC predictions under climate change.