Enhancing combined composting with cattle slurry solids as a bulking agent: Synergistic improvements in earthworm performance, lignocellulose degradation, and greenhouse gas mitigation.

Coupling membrane-covered composting with vermicomposting is an emerging strategy for livestock manure management. However, crop straw, the conventional bulking agent, poses challenges due to inconsistent supply and slow biodegradation, prolonging the composting cycle. Therefore, identifying readily available and biodegradable alternative bulking agents is crucial. This study evaluated the effects of the solid fraction of cattle slurry (SCM), the solid fraction of pig slurry, and water hyacinth as alternative bulking agents on the combined composting process, using wheat straw as the control. The results demonstrated that SCM outperformed other bulking agents. Compared with the control, SCM not only increased cocoon production and average weight gain by 72.3% and 76.6%, respectively, but also maintained the highest survival rate and substrate reduction efficiency. Moreover, SCM enhanced lignin degradation and the humification index to 12.4% and 2.0, respectively. It also enhanced bacterial diversity and lignocellulose-degrading genera like Actinomadura. Additionally, SCM reduced copper content and bioavailability and improved final product quality, characterized by high total nutrient content (54.5 mg/g), low electrical conductivity (2.7 mS/cm), and low bulk density (0.3 g/cm3). Finally, SCM reduced greenhouse gas emissions (carbon dioxide equivalents) by 13.1% relative to the control group (P < 0.05). These findings provide theoretical support and technical guidance for livestock waste management and vermicomposting.