Abstract: This study explores the impacts of understory earthworm cultivation on the content and composition of soil organic carbon (SOC) in poplar plantations, providing insights for multifunctional forest management that balances timber production and carbon sequestration. The experiment was conducted in a 7-year-old poplar plantation at Malanghu Forest Farm in Sihong County, Jiangsu Province, with four treatments: control (CK), earthworm inoculation (T1), surface application of cattle manure (T2), and combined cattle manure application and earthworm inoculation (T3). We analyzed soil physicochemical properties, aggregate size distribution and stability, soil organic carbon content (SOC) and its fractions-including particulate organic carbon (POC), mineral-associated organic carbon (MAOC). Structural equation modeling (SEM) was employed to evaluate the relative contributions of earthworms and cattle manure to size distribution of soil aggregates, SOC content and its fractions, and soil properties. The results show that: (1) Compared with CK, both T2 and T3 significantly increased the proportions of large macroaggregates and small macroaggregates, while decreasing the proportions of microaggregates and silt-clay fractions, with the most pronounced changes observed in T3. Additionally, all treatments significantly increased the mean weight diameter (MWD) of aggregates, with T3 exhibiting the highest MWD value. (2) In contrast to CK, only T3 treatment simultaneously increased the contents of POC, MAOC, and SOC. (3) Across all treatments, the MAOC/SOC ratio consistently exceeded the POC/SOC ratio, though neither ratio differed significantly among treatments. SEM results indicate that earthworms and cattle manure directly affected soil large macroaggregates, small macroaggregates, POC, MAOC, SOC, total nitrogen (TN), and total phosphorus (TP). The synergistic effect of T3 induced the most significant SOC accrual and stabilization, suggesting that understory earthworm cultivation can enhance SOC accumulation and stabilization in poplar plantations. These findings provide a theoretical basis for sustainable plantation management aimed at optimizing carbon sequestration efficiency.