The effect of soil amendments on the occurrence forms and bioavailability of antibiotics in saline-alkali soil.

Antibiotic contamination in soil and soil salinization are both hot issues in environmental research. Amendments can modify the physicochemical properties of saline-alkali soil, thus affecting the occurrence forms and bioavailability of antibiotics. This study investigated the effects of pig manure and biochar on oxytetracycline (OTC) and sulfamethoxazole (SMX) in saline-alkali soil. The results showed that appropriate amendment dosages effectively reduced antibiotic bioavailability. At 7 days post-application, OTC bioavailability was minimized with 2% biochar, achieving a 50.34% reduction. However, higher biochar dosages increased OTC bioavailability. In contrast, SMX bioavailability consistently decreased with increasing dosages of both amendments. Soil salinization was found to increase antibiotic bioavailability, but under increasing soil salinity (from a non-saline control to 7 g/kg), the addition of 2% biochar inhibited OTC bioavailability by 39.40% and constrained the increase in SMX bioavailability to a mere 3.83%. In comparison, 2% pig manure initially increased and then decreased OTC bioavailability, while directly reduced SMX bioavailability by 32.06%. The amendments promoted the transformation of soil organic pollutants into more bound or residual forms, with biochar being more effective for OTC and pig manure for SMX. After 60 days of applying the two amendments, the bioavailability of OTC and SMX in the soil were significantly reduced. Specifically, 2% biochar decreased their bioavailability by 83.03% and 65.21% respectively, whereas 2% pig manure led to reductions of 50.47% and 60.42%, respectively. These findings provide a theoretical basis for the remediation of antibiotic-contaminated saline-alkali soils and the mitigation of associated ecological risks.