Wastewater-borne pollutants influenced antibiotic resistance genes and mobile genetic elements in the soil without affecting the bacterial community composition in a changing wastewater irrigation system

Over decades, untreated wastewater from Mexico City was used to irrigate crops in the Mezquital Valley. With the construction of a wastewater treatment plant (WWTP), irrigation was replaced by WWTP effluent. To investigate how this shift affects the abundance, diversity, and dispersion of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in the short-term, we conducted a soil microcosm experiment over 4 weeks. Soils from the Mezquital Valley with a long history of irrigation with untreated wastewater were irrigated with WWTP influent or effluent, both spiked with or without antibiotics and disinfectants. The spiking of irrigation water increased water-extractable soil concentrations of sulfamethoxazole. ARG and MGE abundances were affected by the spiking, whereas soil type and water quality had no effect. Contrarily, the soil microbiome was only determined by the soil type and the duration of incubation. Among the most abundant soil amplicon sequence variants (ASVs), one affiliated to Pseudomonas was affected by the spiking. Two ASVs affiliated to Methylotenera detected among the most abundant in wastewater and soil increased in abundance in soil after wastewater irrigation, indicating that they were likely introduced with the irrigation water or benefited from it. Most isolates from WWTP influent or effluent belonged to proteobacterial genera. These findings highlight the importance of water pollution on the dissemination and spread of ARGs.