Vertical migration and leaching behavior of antibiotic resistance genes in soil during rainfall: Impact by long-term fertilization

The vertical migration and leaching behavior of antibiotic resistance genes (ARGs) during rainfall in soils subjected to long-term fertilization remain largely unclear. In this study, ARGs in vertical profiles (0-60 cm) and leachates from three soils (acidic, neutral, and calcareous) in a long-term (13 years) field fertilization experiment were monitored by high-throughput quantitative PCR after each rainfall event throughout an entire year. The results showed that, compared with unfertilized soils, long-term manure fertilization mainly promoted the vertical migration and leaching of aminoglycoside, beta-lactam, and multidrug resistance genes in the soil profiles. As a result, the annual cumulative loads of ARGs in leachates from the three soils with long-term manure fertilization were significantly increased compared to the controls and were in the order of acidic soil > neutral soil > calcareous soil. SourceTracker analyses revealed that manured soil was the predominant source of the ARGs in the soil leachate samples. Pseudomonas, Anaeromyxobacter, IMCC26256, and MND1 were identified as the dominant potential hosts responsible for the vertical migration and leaching of ARGs in the three soils. PiecewiseSEM analysis further showed that long-term manure fertilization affected the vertical migration of ARGs during rainfall mainly by altering soil properties (i.e., pH, soil organic carbon, and sand). Our results suggest that the ARGs in soils with long-term manure fertilization are a significant potential source of ARG pollution in groundwater, and the measures should be taken to mitigate the vertical migration and leaching of ARGs during rainfall.