Although planting density is known to affect plant‒soil interactions, its specific influence on the sources of soil organic carbon (SOC) in forest ecosystems remains unclear. To address this issue, we examined plant- and microbe-derived carbon (C) pools, using soil lignin phenols and amino sugars, across a planting density gradient (570‒3,000 trees hm^–2) comprising 28 levels in temperate Robinia pseudoacacia forests. The results demonstrated that both the lignin phenols and microbial necromass C contents decreased significantly with increasing stand density (P < 0.05). Between them, lignin phenols (755.6‒2,539.3 mg kg^–1 SOC) contributed more substantially to SOC accumulation than microbial necromass C (767.5‒1742.5 mg kg^–1 SOC). In particular, fungal-derived C ranged from 338.0 to 1260.1 mg kg^–1 SOC, while bacteria-derived C ranged from 329.8 to 570.3 mg kg^–1 SOC. These findings imply that increasing stand density limits the accumulation of SOC by decreasing the lignin phenol and microbial necromass C content. Notably, lignin phenols were primarily influenced by plant composition, while microbial necromass C was mainly determined by microbial properties and soil C content. These results highlight the significance of plant–soil trait interactions in shaping SOC composition and emphasise their relevance for predicting C-climate feedback in forest ecosystems. Overall, our findings provide new insights into the effects of planting density on SOC dynamics, highlighting important implications for sustainable forest management and climate feedback.