The enrichment of antibiotic resistance genes in swine manure compost was related to the bulking agent types

Composting is a major method to produce organic fertilizers, and the variation in antibiotic resistance genes (ARGs) during composting is crucial for the safe utilization of mature compost. The effects of different bulking agents (cellulose-rich cornstalk and lignin-rich garden waste) on ARGs variations during swine manure composting were investigated. The results showed that composting thermophilic could effectively reduce ARGs (58–61 %), whereas ARGs rebounded and were enriched with decreasing temperature during the maturation stage. Compared to their initial abundance, ARGs were enriched 6.97 times (cornstalk) and 22.27 times (garden waste) during the maturation period. The ARGs enrichment mechanism in swine manure composting differed for cornstalk and garden waste amendments. The cornstalk was used as the bulking agent, the selective pressure of continuous high temperature resulted in the proliferation of spore-forming bacteria (Bacillus, Sporosarcina and Psychrobacillus), which are potential host bacteria of ARGs and cause ARGs enrichment through vertical gene proliferation. In the garden waste treatment, the enriched ARGs in the final compost were related to horizontal gene transfer mediated by mobile genetic elements (intl1), with the primary potential host bacteria being Bacillus, Saccharomonospora, and Caldicoprobacter. The types of enriched ARGs were consistent across different bulking agents composting process, and the risk genes enriched in the final mature compost included ermB, ermF, sul1, sul2, tetO, and tetX.