Compost-bulking agents reduce the reservoir of antibiotics and antibiotic resistance genes in manures by modifying bacterial microbiota

Sawdust, rice husk, and mushroom residues are commonly used as bulking agents during manure composting; this work evaluated their potentiality for enhancing the removal of antibiotics and antibiotic resistance genes (ARGs) under a temperature-consistent condition. Results indicated that the addition of all the compost-bulking agents increased antibiotic removal in swine manure with increasing rates of 14.9%–33.4%; however they showed less effect on the reduction of residual antibiotics in chicken manure where fluoroquinolone (FQs) antibiotics are the dominant antibiotics, partly owing to the weak promoting effects of bulking agents on FQs degradation. The addition of bulking agents somehow hindered the reduction of ARGs within swine manure, whereas there were obvious reductions in the total relative abundance of ARGs in chicken manure with bulking agents added. Among the three bulking agents, sawdust was the most efficient enhancer for antibiotic removal in both manures, and rice husk exhibited the best performance on ARGs reduction in chicken manure. The relationship between antibiotics, ARGs and bacteria communities was subsequently delineated. Proteobacteria was proposed to play key roles on the effect of bulking agent addition on antibiotics and ARGs in swine manure. Particularly, the increased Xanthomonadaceae contributed much to the promoted antibiotic degradation as well as the high level of ARGs in swine manure with sawdust added. By contrast, the changes in dominant bacterial families by the addition of bulking agents into chicken manure were not strong enough to effectively enhance antibiotic removal, but largely influenced the ARGs abundance. The large reductions of Paenibacillaceae and Staphylococcaceae by rice husk addition were closely related to the reduced ARGs in chicken manure with rice husk added.