Vertical migration of bacteria bearing antibiotic resistance genes and heavy metal resistance genes through a soil profile as affected by manure

Untreated chicken manure causes a large amount of antibiotics and heavy metals to enter the soil environment. Currently, there is limited research on antibiotic resistance genes (ARGs) and heavy metal resistance genes (HMRGs) in soil profile. In this study, we conducted a preliminary investigation on the soil profile of vegetable field contaminated by chicken manure. The results showed that the absolute abundance of some resistance genes was higher at the 20-60 cm. Subsequently, we further analyzed the vertical migration of bacteria bearing ARGs and HMRGs through a soil profile as affected by manure using metagenomic sequencing. The findings revealed that long-term application of chicken manure significantly increased the alpha (alpha) diversity of the 0-20 cm soil layer ARGs and HMRGs, the plasmids relative abundance of soil profile substantially increased. Furthermore, long-term application of chicken manure changed the community composition of the 0-20 cm soil layer resistance genes, and also affected the community composition of the 20-40 cm soil layer with the increase of manure rates. Additionally, long-term application of chicken manure significantly increased the alpha diversity of the 0-20 cm soil layer bacteria. Structural equation modeling (SEM) further analysis revealed that bacterial relative abundance was the primary driving factor for the distribution of ARGs in vertical space, while mobile genetic elements (MGEs) were the main driving factor for HMRGs. This study strengthens our understanding of the vertical spatial distribution of soil resistance genes following long-term application of chicken manure, and also provides the basis for the management of subterranean environment.