Implications of Vermicompost on Antibiotic Resistance in Tropical Agricultural Soils - A Study in Hainan Island, China

The contamination of antibiotic resistance genes (ARGs) associated with animal manure fertilization have attracted a global concern. Vermicompost has been widely popularized as an eco-friendly alternative to recycle animal manure on Hainan Island, China. However, the effects of vermicompost application on ARG spread and environmental fate in tropical agricultural soils remains undefined. Herein, the spatial prevalence and vertical behavior of ARGs in the soil profiles of vermicompost-applied agricultural regions were explored by a large-scale survey across Hainan Island. The results showed that although vermicompost application marginally enhanced the load of ARG pollution in the soil in Hainan, the ARGs derived from vermicompost did not eventually accumulate in the soil profile. The increase rate of ARGs in 40-60 cm soil layer was only 0.0015 % compared with that of unfertilized soil. Interestingly, vermicompost application reduced the abundance of high-risk ARGs, such as bla and bla, by approximately one order of magnitude. Vermicompost was also observed to increase the abundance of beneficial bacteria, like Clostridium, and decrease those of Acidobacteriae, Planctomycetes and Verrucomicrobiae, which caused changes in the potential host bacteria of soil ARGs. Mobile genetic elements were further proven to be an essential factor that regulated the vertical dynamics of ARGs in vermicomposted soil, with a direct influence coefficient of 0.9975. This study demonstrated that the controllable risk associated with vermicompost application provided useful information to effectively reduce the threat of ARGs and promote the development of sustainable agriculture on Hainan Island.