Anecic earthworms and residue management synergistically enhance soil carbon sequestration via macroaggregate formation

• Earthworms alone increase large macroaggregates but not intra-aggregate SOC. • Earthworms and residue incorporation increase large macroaggregates by 67.4%. • Earthworms and residue incorporation synergistically increase SOC by 21.7%. • Sufficient residue inputs resolve the “earthworm dilemma” for SOC sequestration. Soil organic carbon (SOC) sequestration is critical for climate change mitigation. Anecic earthworms, as key ecosystem engineers, are hypothesized to promote SOC storage by forming stable soil aggregates, but their interaction with plant residue inputs remains unclear. To address this knowledge gap, we conducted a one-year field experiment in a poplar plantation to investigate the individual and combined effects of earthworm inoculation ( Metaphire guillelmi ) and poplar leaf residue addition (surface mulching vs. soil incorporation) on SOC dynamics. Compared to the control, residue incorporation and earthworm inoculation alone increased the proportion of large macroaggregates (>2000 μm) by 23.3% and 37.9%, respectively. However, their combination yielded a significant synergistic effect, increasing large macroaggregates by 67.4%. Notably, earthworms alone did not increase SOC concentration within aggregates and marginally reduced whole-soil SOC (≈ 0.46%), whereas their combination with residue addition, either mixed into soil or placed on the soil surface, significantly boosted SOC in large macroaggregates and raised whole-soil SOC by up to 21.7%. Structural equation modeling confirmed that residue addition directly enhanced SOC pools, whereas earthworms acted primarily by promoting soil large macroaggregates formation and increasing aggregate stability (quantified by mean weight diameter). This synergistic interaction resolves the “earthworm dilemma” by offsetting short-term carbon mineralization through long-term SOC protection in stable aggregates. We conclude that anecic earthworms (e.g., Metaphire guillelmi) act as critical regulators of soil physical structure, and their contribution to soil organic carbon sequestration efficiency depends critically on their interaction with fresh plant residue inputs. Our findings provide actionable insights for climate-smart forestry, highlighting that integrating earthworm conservation with optimized residue management is a key strategy to strengthen terrestrial carbon sinks in forest plantations.