Objective Aerobic composting, as a major approach for the resource utilization of bacterial residue, can effectively remove antibiotics from bacterial residue, yet poses risks of antibiotic degradation products and enhanced propagation of antibiotic resistance genes (ARGs). In this study, metagenomic methods were employed to evaluate the effectiveness and safety of fertilizer utilization of avermectin bacterial residue.

Method Pot experiments were conducted with soybean and peanut, including four treatments: no-fertilizer control (CK), commercial organic fertilizer (CO), avermectin bacterial residue (ABM), and avermectin bacterial residue organic fertilizer (ABM_T). The technical indicators of the tested avermectin bacterial residue and its prepared organic fertilizer all complied with the Chinese agricultural industry standard Organic Fertilizer (NY/T 525-2021). After peanut and soybean harvest, plant, grain and rhizosphere soil samples were collected to determine soil physicochemical properties, soil bacterial community composition and ARGs abundance, and the contents of avermectin in soil and crops were analyzed.

Result Compared with CK, the three treatments of CO, ABM and ABM_T significantly increased soil contents of total nitrogen, available phosphorus, available potassium and organic matter, and significantly decreased soil pH (P<0.05). No avermectin residue was detected in soil or crop grains under ABM and ABM_T treatments. Compared with CK, ABM_T had no significant effect on soil bacterial community diversity (P>0.05), but altered bacterial community composition: the abundances of Proteobacteria, Myxococcota and Acidobacteriota were significantly increased, while the abundance of Actinobacteriota was significantly decreased (P<0.05). In terms of ARGs, ABM_T significantly increased the types and relative abundances of multidrug resistance genes and macrolide-lincosamide-streptogramin (MLS) resistance genes compared with ABM and CK. Moreover, the genera Microvirga, Steroidobacter and Archangium were positively correlated with macrolide resistance genes (macB, vgaE, macA, oleB, vgaA).

Conclusion The organic fertilizer prepared from avermectin bacterial residue via aerobic composting meets the organic fertilizer product standards and can improve soil fertility. No avermectin residue is detected in soil or crop grains after application, but it enriches macrolide ARGs in soil. The potential risk of residual ARGs transmission through host bacteria still needs further evaluation.