Tree species diversity influences soil carbon persistence by reconfiguring stabilization pathways

1. Increasing tree species diversity is known to enhance soil organic carbon (SOC) stocks, but its effect on SOC stability remains a critical uncertainty for climate mitigation.2. Through examination of a subtropical karst forest diversity gradient in southwest China, we reveal a fundamental shift in SOC stabilization mechanisms using physical fractionation, 13C nuclear magnetic resonance spectroscopy, and metagenomic sequencing.3. Higher tree species diversity increased total SOC content but paradoxically decreased the ratio of mineral-associated to particulate organic carbon (MAOC:POC), a key metric traditionally linked to lower stability. This decrease, however, was accompanied by a critical reduction in SOC mineralization rate. Further analysis revealed that this enhanced persistence under high tree species diversity was associated with a trade-off between stabilization pathways. The role of traditional iron/aluminum oxide-mediated protection diminished, while two alternative mechanisms strengthened, i.e., (1) enhanced physical protection of POC through calcium carbonate aggregation, and (2) a profound shift in microbial ecology towards more efficient anabolism.4. Synthesis. This research demonstrates that tree species diversity actively reconfigures SOC stabilization pathways, emphasizing that ecosystem carbon persistence emerges from a dynamic interplay of physical, microbial, and context-specific mineral controls. These findings suggest that managing for high species richness can enhance both the quantity and the resilience of forest carbon sinks, providing a robust nature-based solution for climate change mitigation.