Context Soil active organic carbon, the labile fraction of soil organic carbon (SOC), serves as a sensitive indicator of soil health. However, its dynamics under long-term restoration in SOC-depleted sandy soils remain poorly understood.Aims To unravel how multi-decadal Salix cupularis shrub restoration (0-34 years) in alpine desertified grasslands of northwestern Sichuan modulates soil active organic carbon fractions - dissolved organic carbon (DOC), microbial biomass carbon (MBC), and readily oxidizable organic carbon (ROC) - and SOC sequestration mechanisms.Methods A chronosequence approach was applied to compare unrestored and restored sites (6, 10, 18, and 34 years), analyzing depth-specific (0-100 cm) DOC, MBC, ROC, and SOC dynamics alongside carbon pool management index calculations.Key results (1) Soil carbon sequestration capacity increased with restoration duration. Although no significant increase was observed during the 6-18 year period, a critical threshold emerged at 18 years, after which SOC content rose significantly during 18-34 years of restoration. (2) DOC, MBC, and ROC in the 0-20 cm layer responded acutely to restoration, showing progressive enrichment of active carbon components. (3) Restoration duration significantly influenced both the proportion of active carbon fractions in SOC (P < 0.05) and carbon pool lability.Conclusions Salix cupularis restoration drives SOC stabilization through progressive soil active organic carbon enrichment.Implications These findings provide a mechanistic basis for optimizing shrub-based restoration strategies in sandy ecosystems, prioritizing longer-term interventions to maximize carbon sink potential and accelerate degraded soil recovery.