Mechanisms of the effects of humic acid on antibiotic resistance genes and microbial communities in Cd-contaminated soils

Humic acid (HA) is an organic macromolecular compound that widely exists in nature, but its effect on antibiotic resistance genes (ARGs) in Cd-contaminated soil is unknown. This study investigated the effects of Cadmium (Cd) and HA in soil on ARGs, mobile genetic elements (MGEs), and bacterial communities. The relative abundance of ARGs and MGEs increased 0.89-fold and 1.12-fold after the addition of 2 mg kg(-1) Cd to the soil, and 1.06-fold and 1.68-fold after the addition of 8 mg kg(-1) Cd. Redundancy analysis further revealed that among environmental factors, available Cd was the dominant factor influencing ARGs. In Cd-contaminated conditions, the addition of high concentrations of HA suppressed the expression of ARGs and MGEs. The abundance of ARGs and MGEs decreased by 0.12-fold and 0.44-fold after the addition of HA (50 mg kg(-1)) to the soil at a Cd concentration of 2 mg kg(-1), and by 0.18-fold and 0.41-fold at a Cd concentration of 8 mg kg(-1). IntI1 was significantly associated with all ARGs (sul1, sul2, tetG, tetW), and intI2 was significantly correlated with most ARGs (sul2, tetG, tetW). Network analysis illustrated that Proteobacteria were the main host bacteria causing changes in the abundance of ARGs and MGEs. HA has an important role in agricultural production. To reduce the enrichment of ARGs in Cd-contaminated soil, the addition of higher concentrations of humic acid can be considered to mitigate the environmental and biological hazards. (c) 2022 Published by Elsevier Ltd on behalf of Institution of Chemical Engineers.

Automated summary

This study investigated the effects of Cd and HA on ARGs, MGEs, and bacterial communities in soil. The results showed that the addition of Cd increased the abundance of ARGs and MGEs, while high concentrations of HA suppressed their expression. Available Cd was found to be the dominant factor influencing ARGs, and Proteobacteria were identified as the main host bacteria. Therefore, adding high concentrations of humic acid to Cd-contaminated soil can reduce the enrichment of ARGs.