Divergent impacts of additives on biological risks associated with antibiotic resistance genes during black soldier fly manure composting: A comparative study of biochar, humic acid, and tea residue amendments.

Black soldier fly (BSF) manure produced through biological conversion technology still raises safety concerns related to antibiotic resistance genes (ARGs) and heavy metal residues. Composting has emerged as a viable approach for mitigating the proliferation of ARGs within livestock manure matrices. This study evaluated the effects of biochar, humic acid, and tea residue on the fate of ARGs, mobile genetic elements (MGEs), heavy metal resistance genes (HMRGs), and virulence factors (VFs) during BSF manure composting. Results indicate that humic acid exhibits the strongest inhibitory effect on ARGs and MGEs, achieving a 95.6% removal rate for intI1 and reducing the abundance of MLS(B) genes by 80.5%. Biochar demonstrates the strongest suppression of HMRGs, substantially decreasing the abundance of genes such as arsC and copA. However, the abundance of certain genes, such as intl2, catB8, arsM and cusA, increased across all treatments. Although humic acid increased the overall abundance of VFs, it achieved a final suppression rate of 86.3% by the end of the composting process. Interactions between bacterial phyla during composting were predominantly positively correlated. Network analysis revealed that humic acid most significantly suppressed co-occurrence associations among ARGs. Environmental factors such as pH and organic matter were associatively linked to ARGs reduction through a pathway that sequentially involved bacterial abundance and VFs (indirect effect = -0.87), suggesting a potential mechanistic cascade that warrants further experimental validation. This study provides a theoretical framework for optimizing the BSF manure composting process while acknowledging both the potential and limitations of additive amendments.