Effects of heavy metals and antibiotics on antibiotic resistance genes and microbial communities in soil

With rapid developments in livestock and poultry farming, antibiotics and heavy metals are entering the soil environment as feed additives. However, their relationships with antibiotic resistance genes (ARGs), microorganisms, and environmental factors are unclear. In this study, cadmium (Cd) and sulfadiazine (SD) were selected as representative heavy metal and antibiotic substances, respectively, to investigate the relationships among environmental factors, microorganisms, mobile genetic elements (MGEs), and ARGs. Addition of Cd and SD, both individually and in combination, increased the abundances of total ARGs and MGEs. The abundance of total ARGs increased by 0.89-fold and 1.06-fold in C2 and C8 (only-Cd-added groups), 1.61-fold and 6.53-fold in S10 and S100 (only-SD-added groups), and 7.29-fold, 7.62-fold, 2.32-fold, and 6.80-fold in C2S10, C2S100, C8S10, and C8S100 (groups with Cd and SD in combinations), respectively. Proteobacteria and Bacteroidetes were the most abundant bacterial phyla. MGEs (especially intI1) had the greatest impact on ARGs and played a crucial role in their propagation and expression. Exploring the effects of heavy metals and antibiotics in soil on ARGs is important for comprehensively assessing their ecological and environmental risks to ARGs.