Successive utilization of carbon from different biogenic sources leads to continuous enhancement of soil respiration

Climate change affects soil organic carbon (SOC) by altering plant carbon inputs and microbial metabolic processes. On the Tibetan Plateau, which is rich in SOC and sensitive to climate change, investigations on the absolute SOC content affecting soil respiration (Rs) were extensive. However, the relationship between Rs and the SOC composition remains largely unclear. Employing a combination of data collection, large-scale field surveys, and analysis, with lignin phenols and amino sugars indicating plant and microbial SOC components, we observed an increase in Rs (3.747 kg C ha⁻¹ yr⁻¹) correlating with rising SOC stocks (0–30 cm; 8.6 kg C ha⁻¹ yr⁻¹). Our results showed that Rs is predominantly driven by plant-derived carbon, especially cinnamyl phenol carbon, which is significantly influenced by vegetation characteristics and soil properties. Although microbial-derived carbon has a minimal overall impact on Rs, fungal necromass carbon critically regulates Rs, underscoring the complex interactions between microbial- and plant-derived components under diverse environmental conditions. The rapid, short-term accumulation of plant-derived carbon significantly enhanced Rs and led to substantial microbial carbon accumulation. As the levels of microbial-derived carbon increase, the Rs process tends to utilize this carbon, potentially altering and reducing the SOC composition and stability, respectively, thereby leading to a continuous increase in soil respiration. These findings offer new insights into Rs and SOC dynamics within the grassland ecosystem of the Tibetan Plateau and provide a scientific basis for predicting the response of soil carbon to climate change.