Antibiotic transport and release effects in paddy soil-water system under water level regulation modes

To effectively reveal the kinetic behavior of antibiotics at the soil-water interface in paddy fields under different water level regulation modes, this study established four water level treatments, namely, conventional (W0), low (W1), medium (W2), and high (W3). The spatiotemporal distribution characteristics, migration and release mechanisms, and plant bioaccumulation effects of four typical antibiotics (sulfadiazine (SDZ), norfloxacin (NOR), florfenicol (FLO), and tetracycline (TET)) were systematically investigated within the paddy system. The results showed that with increasing irrigation and drainage control water levels, the concentrations of antibiotics in the overlying water exhibited a gradual upward trend. Specifically, the median concentration of SDZ in the overlying water under the W1 treatment was 3.26 ng·L⁻¹, while those under W0, W2, and W3 increased by 1.59, 2.57, and 2.93 ng·L⁻¹ compared with W1, respectively. In addition, the concentrations of the four antibiotics in the overlying water followed the descending order: TET > SDZ > NOR > FLO. We improved the theoretical model for the deposition-release rate by incorporating the adsorption-desorption process of antibiotics and optimized the calculation theory of deposition-release flux by accounting for the influence of surface flatness. The results demonstrated that at an irrigation-drainage interval of 56 h, the release flux of SDZ under the W1 treatment was 0.16 ng·m⁻²·h⁻¹, and the corresponding values under W0, W2 and W3 treatments increased by 27.50 %, 51.02 % and 87.23 %, respectively, compared to W1. XGBoost-SHAP analysis revealed that water level was the dominant driver of antibiotic transport, followed by hydrodynamic parameters and temperature. Finally, an innovative release model incorporating the thermal degradation of antibiotics was established, which demonstrated that the antibiotic release amount was the highest under the W3 regulation mode and lowest under the W1 treatment. In addition, rice yield was the highest under W2, and the antibiotic bioaccumulation in rice grains was relatively low under both W2 and W0 treatments. This study provides theoretical support for the coordinated development of green irrigation and drainage practices, as well as quality improvement and efficiency enhancement in paddy fields.