Fe2O3 nanoparticles drive enhanced composting humification by modulating bacteriophage-bacteria interactions.

Although bacteria are responsible for decomposing organic matter and forming humic substances (HS) during composting, the role of bacteriophages in carbon metabolism cannot be ignored. In this study, integrated metagenomics and untargeted metabolomic analysis were used to explore the interaction mechanism between bacteriophages and bacteria on the humification process in composting with added Fe2O3 nanoparticles (NPs). The results showed that Fe2O3 NPs optimized the function of the bacterial community by maintaining a high relative abundance of Bacillota, promoting organic matter degradation, and significantly increasing the HS concentration by 27 % on day 30 of composting. Specifically, the higher Bacillota relative abundance directly enhanced the relative abundance of cellulose phosphorylase (GH94), activated the glycolysis pathway, and led to a significant enrichment of metabolites such as phenols, organic acids, and amino acids on day 14 of composting, thereby strengthening energy metabolism. Furthermore, lysis of the host (Bacillota) by bacteriophages released cellular contents, providing key precursors for HS condensation. Concurrently, lysis by bacteriophages interrupted metabolism of the host, preventing complete mineralization of some carbon, thereby converting easily mineralizable carbon into sequestered HS carbon. Fe2O3 NPs also accelerated lignin depolymerization, enriching aromatic precursors and providing core structural units for humification. This study reveals that Fe2O3 NPs stimulate the synergistic action of the functional bacterium Bacillota and remodeling of the microbial-virus interaction network to drive efficient lignocellulose degradation and humification, thus providing a new strategy for optimizing composting processes based on viral regulation.