Dissipation of the antibiotic sulfamethoxazole in a soil amended with anaerobically digested cattle manure.

The application of anaerobically digested cattle manure on agricultural land for both improving its quality and recycling a farm waste is an increasingly frequent practice in line with the circular economy. However, knowledge on the potential risk of spreading antibiotic resistance through this specific practice is quite scarce. The antibiotic sulfamethoxazole (SMX) is one of the most heavily prescribed in veterinary medicine. In this study, SMX dissipation and the possible effects on natural microorganisms were investigated in a soil amended with an anaerobically digested cattle manure produced from a biogas plant inside a livestock farm. Microcosm experiments were performed using amended soil treated with SMX (20 mg/kg soil). During the experimental time (61 days), soil samples were analysed for SMX and N4-acetylsulfamethoxazole, microbial abundance, activity and structure. Furthermore, the prevalence of the intI1 gene was also determined. The overall results showed that, although there was an initial negative effect on microbial abundance, SMX halved in about 7 days in the digestate-amended soil. The intI1 gene found in both the digestate and amended soil suggested that the use of anaerobically digested cattle manure as fertilizer can be a source of antibiotic resistant bacteria (ARBs) and genes (ARGs) in agroecosystems.