The effects of oxytetracycline (OTC) and sulfamethoxazole (SMX) antibiotics on potential nitrification and alkaline phosphatase and urease activities in a calcareous soil

Pharmaceutical antibiotics such as oxytetracycline (OTC) and sulfamethoxazole (SMX) are highly consumed for the treatment of infectious diseases and to growth improvement in livestock and poultry industry every year. More than 90 percent of consumed antibiotics may be excreted from the animal's body as main compounds and bioactive metabolites, and introduced into the soil environment which may impose adverse effects on non-target microorganisms. The effects of antibiotics on soil microbial functions have not been well determined yet. In this study, in order to assess the impact of oxytetracycline and sulfamethoxazole on soil microbial functions, a factorial experiment was conducted based on completely randomized design with factors of concentrations (0, 1, 10, 25, 50 and 100 mg.kg-1 soil), time (1, 4 and 21 days) and kind of antibiotics in the laboratory conditions. Soil microbial community functions were evaluated by measuring the activities of alkaline phosphatase and urease and potential nitrification. The results showed that oxytetracycline severely affected alkaline phosphatase and urease activities in the first day of incubation. But, the activities of these enzymes were recovered with increasing the incubation time. While, sulfamethoxazole significantly inhibited activities of alkaline phosphatase and urease enzymes compared to control treatment during the incubation. The effect of oxytetracycline and sulfamethoxazole on potential nitrification followed the same pattern so that nitrification rate increased with increasing concentrations of antibiotics in the early days of incubation, but these antibiotics had adverse effects on potential nitrification over time. Overall, OTC antibiotic exerted adverse effects on soil microbial functions in the early days of incubation, while SMX antibiotic exerted long term effects on soil microbial parameters.