Industrial composting of sewage sludge mitigates antimicrobial resistance risks and preserves bacterial dynamics in...

Industrial-scale thermophilic composting of sewage sludge (SS) offers a promising strategy to reduce antimicrobial resistance risks in agricultural soils, although the impacts of its application on soils remain unclear. This study evaluated the impact of SS compost, produced thermophilically with and without lime, on antibiotic resistance genes (ARGs) related to fluoroquinolones, sulfonamides, and tetracyclines, mobile genetic elements (MGEs), and bacterial communities in tropical clay and sandy loam soils over 100 days, using high-throughput qPCR and 16S rRNA gene sequencing. Actinobacteriota followed by Pseudomonadota, Chloroflexota, Acidobacteriota, and Bacteroidota dominated both soils, representing 80-96 % of total community. Fresh SS reduced microbial complexity and transiently enhanced ARGs (mainly sulfonamides) as well as MGEs (transposon and integrons), and enriched potential ARG hosts. Non-limed compost (NLC) initially disrupted bacterial community richness, diversity, and structure, reducing Pseudomonadota and Acidobacteriota abundances by 15 % and 5 %, respectively, while increasing Actinobacteriota by 19 % in average at both soils. Industrial composting effectively reduced ARGs, especially sulfonamide-related genes, with transposons and integrons playing central roles in early dissemination. Lime addition did not enhance ARG reduction but improved compost stability and briefly increased microbial diversity. Genera such as Atopobium, Candidatus Competibacter, Clostridium sensu stricto, Coxiella, Kocuria, Lysinibacillus, Micrococcus, Nocardiopsis, Paeniclostridium, and Terrisporobacter were identified as potential ARG carriers. These findings support industrial composting as a viable strategy to mitigate AMR risks while preserving microbial integrity in tropical agroecosystems. However, long-term studies are still needed to assess ARG persistence, horizontal gene transfer, and environmental transmission routes, mainly under tropical field conditions.