The influence of ciprofloxacin on nitrogen transformation in unsaturated zone under alternate drying and wetting

Groundwater nitrate contamination is a critical global environmental and public health issue. Antibiotics often enter agricultural soils with manure and coexist with nitrogen under wet–dry cycle conditions. This study used ciprofloxacin (CIP) as a representative quinolone antibiotic to investigate the characteristics of nitrate–antibiotic co-contamination and the rules of nitrogen transport and transformation in agricultural soils under wet–dry cycles through soil column incubation experiments. The results showed that CIP mainly accumulated in the outer soil layer with low mobility and a cumulative desorption rate of only 4%. We further identified that CIP exerts a selective inhibitory effect on denitrifiers (e.g., Bacillus and Nocardioides) while having a much weaker impact on nitrifiers, thereby driving the continuous accumulation of nitrate in the soil. During the second wetting phase, the nitrate leaching concentration in the CIP treatment group reached 40.84 mg L⁻¹, significantly higher than that in the control group, which substantially increased the risk of groundwater pollution. This study reveals the long-term interference mechanism of CIP on soil nitrogen cycling and provides important scientific basis for the control of antibiotic contamination in agricultural soils and the protection of groundwater quality.