Evaluating the impact of the humic acid amendment on antibiotic resistance genes reduction and product quality during swine manure composting

The overuse of antibiotics in livestock farms and their subsequent accumulation in manures lead to increase antibiotic resistance genes (ARGs) in the compost products. Yet, the function of humic acid (HA) in shaping antibiotics and ARGs abundances, microbial structure, and product quality during composting remains unclear. Here, the efficiency of added HA in reducing antibiotics and ARGs abundances during swine manure (SM) composting was studied using five rates of HA: 0%, 1%, 2%, 4%, and 6%. The findings indicated that the abundance of ARGs types such as LSM, bacitracin, macrolide, sulfonamide, tetracycline, and vancomycin and ARGs subtypes including MacB, MexD, OleB, SrmB, Sul1, VanHB, VanSB, pbp2b, tetPB, and tetX decreased significantly under 6% HA amount compared with CK compost (P < 0.05). Besides, the added 6% HA shaped the abundance of the bacterial community (notably Acinetobacter, Pseudoxanthomonas, and Luteimonas), as well as the level of carbohydrate metabolism and amino acid metabolism (6.9% and 5.4%, respectively), which can be taken as an indicator for microbial activity, thereby accelerating antibiotics decrease. RDA analysis revealed that TOC, moisture content, and pH considerably influenced microbial activity and ARGs reduction (P < 0.05). The estimation of compost quality indicated that 6% HA significantly improved the total content of humic substance and HA to 120.7 & PLUSMN; 1.02 g/kg and 62.3 & PLUSMN; 0.92 g/kg, and reduced the content of fulvic acid to 12.8 & PLUSMN; 1.22 g/kg, thus promoting antibiotics and ARGs reduction. Overall, this study offers a promising option for improving compost quality and safety and mitigating ARGs circulation in the natural environment.