Opposite Effects of Planting on Antibiotic Resistomes in Rhizosphere Soil with Different Sulfamethoxazole Levels

Achieving consensus about the rhizosphere effect on soil antibiotic resistomes is challenging due to the variability in antibiotic concentrations, sources, and the elusory underlying mechanisms. Here, we characterized the antibiotic resistomes in both the rhizosphere and bulk soils of soybean plants grown in environments with varying levels of antibiotic contamination, using sulfamethoxazole (SMX) as a model compound. We also investigated the factors influencing resistome profiles. Soybean cultivation altered the structure of antibiotic-resistant genes (ARGs) and increased their absolute abundance. However, the rhizosphere effect on the relative abundance of ARGs was dependent on SMX concentrations. At low SMX levels, the rhizosphere effect was characterized by the inhibition of antibiotic-resistant bacteria (ARBs) and the promotion of sensitive bacteria. In contrast, at high SMX levels, the rhizosphere promoted the growth of ARBs and facilitated horizontal gene transfer of ARGs. This novel mechanism provides new insights into accurately assessing the rhizosphere effect on soil antibiotic resistomes.