Positive microbial associations underpin soil organic carbon stabilization under manure legacy in a poplar plantation

Forest soils are major terrestrial carbon reservoirs, yet the long-term legacy effects of forest management on soil organic carbon (SOC) stabilization remain unclear. In particular, whether historical manure inputs can continue to influence microbial- and mineral-mediated carbon sequestration after management cessation is poorly understood. We investigated soils from a poplar (Populus × euramericana ‘Neva’) plantation managed as two treatments: a forest-poultry compound system that received duck manure for three years and a control without poultry management. Soils were sampled seven years after duck rearing had ceased to assess manure legacy effects on SOC stability. Manure legacy increased SOC by 11%–27%, microbial necromass carbon (MNC) by 26%–67%, and mineral-associated organic carbon (MAOC) by 59%–73%, while enhancing SOC stability, as indicated by a higher MAOC:SOC ratio. Improved nutrient availability promoted positive microbial associations, greater metabolic efficiency, and a lower microbial respiratory quotient. Soil organic matter became richer in polysaccharides and less aromatic, and iron shifted toward more reactive and organically complexed forms. Overall, manure legacy effects persisted long after poultry farming had ceased and enhanced forest soil carbon stabilization through the combined action of microbial community organization, iron oxide reactivity, and organic matter composition. These findings provide new insights into the enduring effects of agroforestry management on forest soil carbon sinks and a theoretical basis for designing management practices to enhance carbon sequestration under global environmental change.