Restoration‐Driven Changes in Soil Organic Matter Stability: Subsoil Contributions and Environmental Controls Across Stand Ages

ABSTRACT Forest soils hold a major reservoir of soil organic matter (SOM), yet how SOM stability and chemical composition respond to long‐term vegetation restoration following degradation remains insufficiently understood. In this study, we examined larch plantation soils established on previously degraded land, sampling across a 0–50 cm profile in stands restored for 10, 30, and 50 years. Thermogravimetric analysis and Fourier transform infrared spectroscopy were used to evaluate changes in SOM thermal stability and chemical composition during the restoration trajectory. With increasing restoration age, SOM thermal stability (TG‐T50), polysaccharide content (rA1034 cm −1 ), and the recalcitrance index (rA1620/2930) all increased, showing clear depth‐specific patterns. Although subsoil layers contained less carbon, they contributed substantially to SOM stabilization during the recovery process. Soil pH, silt, and clay contents were key environmental drivers regulating SOM quality along both depth and restoration gradients. Overall, our findings highlight that vegetation restoration after degradation modifies SOM stability and composition through combined effects of stand age and soil depth, offering improved understanding of SOC recovery mechanisms in forest ecosystems.