Tracking the transformations and by-products of antimicrobials in soil by means of high-resolution mass spectrometry

Beyond soil composition, several critical factors-such as organic matter content, soil pH, and the concentration of antimicrobial substances (AMs)-significantly influence both the adsorption and persistence of AMs, as well as the potential development of AM transformation products (TPs) in soil. Although AMs and their TPs contribute to soil environmental pollution, there is still a lack of research on the degradation of AMs and the identification of TPs. In this context, this study investigated the degradation of four AMs from different families-sulfamethoxazole (SMX), oxytetracycline (OTC), enrofloxacin (ENRO), and trimethoprim (TMP)-in a soil:compost mixture (97.2:2.8 w/w). The analysis was carried out at two concentration levels (1 mg kg^-1 and 150 mg kg^-1) under controlled light and humidity conditions. The results showed a higher degradation rate for SMX, regardless of the initial concentration, and a higher persistence in soil for the fluoroquinolone ENRO. In addition, nineteen potential TPs were identified using a suspect screening analysis approach, many of which, to our knowledge, have not been previously identified in soil:compost samples. The higher degradation rate observed for SMX coincided with the highest recorded abundances of TPs of SMX. Conversely, the highest diversity of identified TPs was observed for TMP. This work also extends the information on the transformation mechanism leading to the detected TPs. However, there is still a large gap regarding the possible activity of the TPs and their influence on resistance propagation. Further research is therefore needed in this area, combining chemical and biological assays.