Irrigation with liquid manure promotes the vertical migration of antibiotic resistance genes in soil-maize systems...

The wide persistence of antibiotic resistance genes (ARGs) caused by manure application has received considerable attention. However, whether liquid manure irrigation could promote the vertical migration of ARGs in the soil-plant systems compared to the surface application of compost has not yet been assessed. This study investigated the vertical migration of ARGs in the soil-maize system via field experiments among different fertilizer types. A total of 115 ARGs and 22 mobile genetic elements (MGEs) were detected in the system. Roots had the highest abundance of ARGs, followed by leaves and stems. Compared to compost, liquid manure resulted in 2.20, 1.10, and 1.45-fold increases in the absolute abundance of ARGs in soil, roots, and stems, respectively. ARG abundance exhibited a 4.30-fold increase, even at depths of 45-60 cm. It indicated that liquid manure promoted the vertical migration of ARGs in soil-maize system, which might cause higher resistance risks. Source Tracker showed that liquid manure contributed 8.04 % to ARGs at 45-60 cm, higher than the 3.87 % of manure compost. Core resistomes dominated the distribution pattern of ARGs and MGEs. The soil bacterial communities and their correlation with ARGs, MGEs, and soil factors were strongly driven by fertilization types. Liquid manure increased the carrying capacity of microorganisms for ARGs by enhancing the gene horizontal transfer. MGEs acted as a direct dominant influence on ARG abundance in the soil-maize system. These findings emphasized that liquid manure irrigation elevated the vertical migration of ARGs to deep soil, even plants, potentially compromising crop and groundwater safety.