Variation of heavy metal speciation, antibiotic degradation, and potential horizontal gene transfer during pig manure composting under different chlortetracycline concentration

Overuse of heavy metal and antibiotics in livestock husbandry has led to the accumulation of heavy metal resistance genes (HMRGs) and antibiotic resistance genes (ARGs) in environment. This research aims to reveal the variation of heavy metal speciation and potential horizontal gene transfer (HGT) of HMRGs and ARGs in manure composting under different initial chlortetracycline (CTC) concentrations. Treatments spiked with 20 mg/kg CTC (treatment P1), 100 mg/kg CTC (treatment P2), and the control (treatment CK) were operated. Results showed that CTC could be completely removed in the thermophilic phase of all the treatments despite of the initial concentrations. Bioavailable Cu in treatments CK, P1, and P2 declined by 14.5%, 27.1%, and 26.7% and bioavailable Zn declined by 15.3%, 29.5%, and 12.1%, respectively, after the composting, respectively. Relative abundance of HMRGs decreased by 6.49 log, 8.88 log, and 5.77 log, respectively, in treatments CK, P1, and P2. Relative abundance of ARGs decreased by 3.37 log, 4.86 log, and 3.32 log, respectively, in treatments CK, P1, and P2. Composting could effectively reduce genespcoD,pcoA,zntA,tetQ, andtetA, which might locate on the same plasmid. CTC of 100 mg/kg promoted the co-selection of ARGs and HMRGs and increased the potential HGT of genecusA.