Contrasting roles of pyrochar and hydrochar in mitigating antibiotic resistance genes during swine manure composting

The propagation of antibiotic resistance genes (ARGs) in livestock manure presents a significant risk to environmental safety and human health. Biochar amendment is a promising approach to reduce ARGs during composting, yet the performance and mechanisms of different types of biochar are poorly understood. This study compared the effects of pyrochar and hydrochar on the dynamics of ARGs, mobile genetic elements (MGEs) and bacterial communities during swine manure composting. Results showed that hydrochar amendment resulted in a significantly higher removal of total detected ARGs (94.54%) compared to pyrochar (83.91%) and conventional composting (70.15%). Hydrochar was also associated with a greater reduction in ISCR1, a genetic element linked to ARG mobilization, in contrast to the greater reduction in the integron gene intI1 by pyrochar. Network analysis showed that while both biochar types reduced the abundance of putative host genera such as Anaerococcus, Lactobacillus and Streptococcus, hydrochar amendment was associated with a lower pH environment in the compost, which may have contributed to the suppression of putative bacterial hosts. Redundancy analysis and structural equation modeling indicated that MGEs and putative bacterial hosts were the primary direct factors affecting ARG dynamics during biochar-amended composting. These findings underscore the different roles of pyrochar and hydrochar in mitigating antibiotic resistance during composting, and provide new insights into biochar-mediated strategies for safer organic waste management.