Amendment soil with biochar to control antibiotic resistance genes under unconventional water resources irrigation : Proceed with caution

The spread of antibiotic resistance genes (ARGs) has become a cause for serious concern because of its potential risk to public health. The use of unconventional water resources (e.g., reclaimed water or piggery wastewater) in agriculture to relieve groundwater shortages may result in an accumulation of ARGs in soil. To counter this, biochar addition to soil has proven to be a beneficial method to alleviate the pollution of antibiotics and ARGs in manure-amended soil. However, the role of biochar on ARGs in soil-plant systems irrigated with unconventional water resources is unknown. Under reclaimed water or piggery wastewater irrigation, rhizobox experiments using maize plants in soil amended with biochar were conducted to investigate the variation of typical ARGs (tet and sul genes) in soil-plant systems during a 60-day cultivation. Only piggery wastewater irrigation significantly increased the abundance of ARGs in rhizosphere and bulk soils and root endospheres. Following 30-day cultivation, the abundance of ARGs in soil was significantly lower due to biochar addition. However, by day 60, the abundance of ARGs in soil supplemented with biochar was significantly higher than in the control soils. Antibiotics, bio-available heavy metal, nutrients, bacterial community, and mobile gene elements (MGEs) were detected and analyzed to find factors shaping ARGs dynamics. The behavior of ARGs were associated with antibiotic concentrations but not with bio-available heavy metals. The correlation between ARGs and available phosphorus was stronger than that of ARGs with total phosphorus. MGEs had good relationship with ARGs, and MGEs shift contributed most to ARGs variation in soil and root samples. This study provides insights into potential options for biochar use in agricultural activities.