Vertical distribution of antimicrobial resistance genes in soil adjacent to contaminated sites with animal manure.

With the rapid expansion of livestock farming, antibiotic resistance genes (ARGs) in the environment have become an increasing concern. However, studies on the vertical distribution of ARGs in soils surrounding livestock farms remain limited. This study investigates the distribution of antibiotics, potentially toxic elements (PTEs), and physicochemical properties in soils from three livestock farm types (pig, cattle, and layer) in Henan Province, China, and evaluates their effects on ARG prevalence. Antibiotic concentrations were highest in soils from layer farms, followed by those from pig and cattle farms, and decreased significantly with soil depth. Heavy metal concentrations varied across different soil layers. Soil organic matter decreased with depth, while water content increased. The soil pH remained relatively stable. Bacterial diversity analysis revealed Proteobacteria, Actinobacteria, Nitrospirae, and Acidobacteria as the dominant phyla across all farm types, with notable differences in bacterial genera abundance among farm types. qPCR detected six types of ARGs and two mobile genetic elements (MGEs) in soils, with sulfonamide resistance genes (SRGs) being the most abundant and quinolone resistance genes the least. Redundancy analysis indicated that environmental factors explained 82.20, 74.30, and 78.44% of the variation in soil ARGs and MGEs in pig, cattle, and layer farms, respectively, this highlights the critical role of environmental conditions in ARG transfer. Spearman correlation and network analysis identified potential hosts of ARGs across different farm types. These insights provide a scientific basis for developing more effective strategies to control the spread of antibiotic resistance in soils surrounding diverse livestock operations.