Vermicomposting of biogas residue and poplar leaf mixture employing Eisenia fetida

This study aimed to evaluate the vermicomposting of a 7:3 (biogas residue: poplar leaves, dry weight) mixture employing Eisenia fetida over 90 days, focusing on physicochemical evolution, humification, heavy metal dynamics, microbial community composition, and final product maturity. The results indicated that vermicomposting significantly reduced pH, total organic carbon (TOC), and organic matter (OM). The carbon-to-nitrogen (C/N) ratio decreased to 12.0, meeting maturity standards. The germination index (GI) reached 88.41%, confirming the absence of phytotoxicity. Earthworm population size and total biomass increased by 60.7-fold and 25.28-fold, respectively, demonstrating excellent substrate suitability. Notably, Fe and Cu concentrations decreased significantly by 47.72% and 39.95%, respectively, owing to combined mechanisms of earthworm bioaccumulation, leaching of mobilized fractions, and organic matter mineralization. The bacterial community was dominated by Pseudomonadota, Actinomycetota, and Bacillota, while the fungal community was overwhelmingly dominated by Ascomycota (89.32%), providing a microbial explanation for efficient lignocellulose degradation and nitrogen enrichment. Scanning electron microscopy and Fourier transform infrared spectroscopy confirmed the porous, granular structure and progressive stabilization of organic matter. These findings demonstrate that vermicomposting effectively converts biogas residue and poplar leaves into a stable, non-phytotoxic, and agriculturally safe organic fertilizer.