Changes to tetracyclines and tetracycline resistance genes in arable soils after single and multiple applications of manure containing tetracyclines

The influence of manure containing tetracyclines (TCs) on the prevalence of antibiotic resistance genes in soils remains poorly understood. Here, three different TCs (oxytetracycline (OTC), tetracycline (TC), and chlortetracycline (CTC)) were mixed respectively with unpolluted manure to fertilize arable soil. The soil received either a single application of 0 μg kg−1, 300 μg kg−1 (TC and CTC), or 700 μg kg−1 (OTC) or multiple applications every 14 days for 140 days. Four tetracycline resistance genes (TRGs), including tet(A), tet(L), tet(M), and tet(Q), were monitored. Although the abundances of the four TRGs in the single application treatment initially increased rapidly, they decreased over time and were significantly lower than those of the repeated treatments after day 112. All additions of TCs stopped on day 140, but we continued to assess the long-term accumulation of TRGs. Most of the TRGs were detected even after the TC-containing manures had not been applied for more than 15 months. The abundance of the TRGs after ceasing fertilization with the TC-containing manures was higher in the repeated application treatments than in the single application treatments. Therefore, more attention should be paid to repeated applications of antibiotic-containing manure to arable soils.