Earthworms Enhance γ-Hexachlorocyclohexane Degradation by Mediating Bacterial Community Composition in Fluvo-Aquic Soil

Earthworms, vital ecosystem engineers, significantly influence soil health and are known to enhance the degradation of organic pollutants by mediating microbial communities. However, the mechanisms by which earthworms affect the degradation of persistent pollutants like γ-hexachlorocyclohexane (γ-HCH) remain poorly understood. Here, a 28-day microcosm experiment was conducted to investigate the roles of two ecological types of earthworms, Eisenia fetida and Pheretima guillelmi, in promoting the degradation of γ-HCH in soil. It was found that the addition of earthworms significantly enhanced the degradation efficiency of γ-HCH by 34.96% and 40.02% in treatments with E. fetida and P. guillelmi, respectively, compared to controls, with both species exhibiting similar effectiveness. Mechanistically, earthworms lowered soil pH and increased organic matter consumption, which boosted microbial biomass and activity. They altered the soil bacterial community composition, promoting the relative abundance of γ-HCH-degrading microorganisms such as Nocardioides, Sphingomonas, and norank c Acidobacteria. Structural equation modeling revealed that earthworms influenced soil pH, affecting bacterial community composition and diversity, which led to changes in microbial biomass nitrogen (MBN) and microbial biomass carbon (MBC); the increase in MBC, in turn, promoted γ-HCH degradation. Network analysis demonstrated that earthworms enhanced the stability and complexity of microbial networks, further facilitating the degradation process. These findings highlight that earthworms can significantly accelerate the degradation of γ-HCH by modulating soil physicochemical properties and microbial communities. This study underscores the potential of utilizing earthworms as effective bioremediation agents in soils contaminated with persistent organic pollutants, offering new insights into sustainable strategies for environmental remediation.