Antibiotics released in agricultural soils alter soil bacterial communities, inducing antimicrobial resistance and, in turn, canceling the efficiency of antibiotic drugs used for human and animal health. In soils, antibiotic impact on nitrogen cycling is poorly known, notably when antibiotic mixtures are applied. We hypothesized that the impact of antibiotic mixtures would have higher effects on denitrification. We exposed soil denitrifying bacteria enrichments to tetracycline, ofloxacin, sulfamethoxazole and tylosin, either applied single or as mixture of three antibiotics, during 7 days under denitrifying conditions. We measured the minimum inhibitory concentration of the N₂O-reducing capacity of the bacterial enrichment, we deduced the half maximal effective concentration (EC₅₀) from the experimental data and from the concentration addition hypothesis, and we quantified nosZ gene abundances. Results show that single antibiotic exposure inhibited N₂O-reduction only for tetracycline at 64 mg/L. Inhibition by antibiotic mixtures always exceeded the modeled inhibition calculated by concentration addition. At high-antibiotic exposure, nosZ gene clade I denitrifiers remained abundant, of 10⁷–10⁸ copies/ng DNA. NosZ gene clade II denitrifiers increased with antibiotic concentrations. Our findings reveal for the first time the synergistic effects of antibiotic mixtures on soil nitrogen cycling.

在农业土壤中释放的抗生素改变了土壤细菌群落，引起了抗药性，从而降低了用于人类和动物健康的抗生素药物的效率。在土壤中，抗生素对氮循环的影响知之甚少，特别是在使用抗生素混合物时。我们假设抗生素混合物的影响将对反硝化产生更高的影响。在反硝化条件下的7天中，我们将土壤反硝化细菌富集暴露于四环素，氧氟沙星，磺胺甲恶唑和泰乐菌素（单独施用或三种抗生素混合施用）。我们测量了细菌富集的N ₂ O还原能力的最小抑制浓度，得出了最大有效浓度的一半（EC ₅₀）从实验数据和浓度加法假设，我们量化了nosZ基因的丰度。结果表明，单次抗生素暴露仅在64 mg / L时抑制四环素的N ₂ O还原。抗生素混合物的抑制作用始终超过通过添加浓度计算得出的模型抑制作用。在高抗生素暴露下，nosZ基因进化枝I反硝化剂仍然丰富，为10 ⁷ –10 ⁸拷贝/ ng DNA。NosZ基因进化枝II反硝化剂随抗生素浓度的增加而增加。我们的发现首次揭示了抗生素混合物对土壤氮循环的协同作用。