Depth-dependent drivers of soil organic carbon thermal stability across Tibetan alpine grasslands

Abstract

The response of soil organic carbon (SOC) to climate warming is contingent on its stability. However, the mechanisms sustaining SOC stability across the soil profile in Tibetan alpine grassland ecosystems remain inadequately understood. This study investigated how mineral protection and molecular composition influence SOC thermal stability across soil depths under changing climatic conditions. We assessed SOC stability using thermogravimetric (TG-T50) and differential scanning calorimetry (DSC-T50) analyses, characterized molecular composition via nuclear magnetic resonance spectroscopy, and evaluated mineral protection through targeted chemical extractions. The results showed that the TG-T50 and DSC- T50 values in the topsoil (363.24°C and 360.62 °C) were lower than those in the subsoil (373.09 °C and 363.16 °C), suggesting enhanced thermal stability at greater depths. Molecular characteristics and mineral protection explained 21 % and 43 % of SOC stability in the topsoil, and 29 % and 39 % in the subsoil, respectively. Molecular characteristics exerted a stronger influence on subsoil SOC stability, whereas mineral protection played a more dominant role in both the topsoil and subsoil. The complex interactions between minerals and the molecular characteristics that govern SOC stability were emphasized. Moreover, mean annual temperature significantly and directly influenced the molecular resistance in the subsoil. Despite the inherent thermal stability of subsoil SOC, climate change may undermine its biochemical stability and accelerate SOC losses in the subsoils of the Tibetan Plateau. These findings highlight the critical role of mineral-molecule interactions in controlling carbon persistence, and offer process-based insights for forecasting SOC dynamics in vulnerable alpine ecosystems.