Iron-Amended Pleurotus citrinopileatus Residue Composting: Lignocellulose Degradation and Microbial Functional Potential

Spent mushroom substrate (SMS) from Pleurotus citrinopileatus cultivation is a lignocellulose-rich by-product whose efficient composting is hindered by lignocellulose recalcitrance and slow humification. This study compared the effects of Fe3O4-based magnetic powder (MP), Fe0 beads (IB), and Fe2O3-rich crude iron ore (RI) with those of an unamended control during 30-day aerobic co-composting with chicken manure. Physicochemical characterization and metagenomic sequencing were combined to assess compost maturity, lignocellulose degradation, humic substance transformation, microbial succession, and functional potential. MP achieved the most balanced overall performance, exhibiting the highest total lignocellulose degradation (59.48%), the lowest final organic matter content (357.70 g/kg), a final C/N ratio of 13.04, and the highest germination index (151.50%). RI achieved the highest hemicellulose degradation and final contents of humic acid and total humic substances, whereas IB promoted rapid initial heating and relatively high cellulose degradation but caused greater physicochemical fluctuations. Metagenomic analyses revealed amendment-specific microbial succession and distinct CAZy-annotated gene and MetaCyc pathway profiles associated with lignocellulose transformation and carbon metabolism. Overall, Fe3O4-based magnetic powder showed the greatest potential for simultaneously promoting lignocellulose degradation, organic matter transformation, and compost maturity, providing a practical basis for the sustainable valorization of edible mushroom industry by-products.