【Objective】This study aimed to clarify the respective roles of plant- and microbial-derived residues in soil organic carbon (SOC) accumulation in Robinia pseudoacacia plantations by examining their dynamic changes across a chronosequence of stand ages.【Method】Farmland was used as the control. Robinia pseudoacacia plantations aged 10, 15, 20, 35, and 50 years on the Loess Plateau were selected as the study sites. Lignin phenols and amino sugars were quantified in surface (0–20 cm) and subsurface (20–40 cm) soils to assess the contributions of plant- and microbial-derived residues to SOC. In addition, forest characteristics, soil chemical properties, and microbial biomass were analyzed to explore the potential mechanisms underlying carbon sequestration in these plantations.【Result】The results showed that: (1) the establishment of R. pseudoacacia plantations significantly increased SOC content in surface soils, whereas no significant change was observed in subsurface soils. After 50 years of afforestation, SOC content in the surface soil increased by 93.5% relative to farmland. (2) In surface soils, the contents of amino sugars and their monomers were significantly higher in R. pseudoacacia plantations than in farmland and peaked at a stand age of 35 years. By contrast, amino sugar contents in subsurface soils showed no significant variation, ranging from 0.39 to 0.57 g·kg-1 (3) In surface soils, lignin phenol contents under R. pseudoacacia plantations were significantly higher than those in farmland soils. However, lignin phenol contents in subsurface soils remained relatively low and showed little variation with increasing stand age, ranging from 0.1 to 0.3 g·kg-1. (4) In surface soils, the relative contribution of plant residue carbon to SOC decreased with stand age, ranging from 12.5% to 2.8%. In contrast, microbial residue carbon made a substantially greater contribution, accounting for 44.0%–53.0% of SOC. In subsurface soils, the contributions of plant residue carbon and microbial residue carbon ranged from 8.9%–31.8% and 30.6%–65.0%, respectively.【Conclusion】Overall, the restoration of R. pseudoacacia plantations importantly stimulated SOC accumulation, but this effect was largely confined to surface soils. Microbial residue carbon was the dominant source of SOC in these plantations and contributed more substantially than plant residue carbon.