Effects of nitrogen fertilization on antibiotic resistance gene spread from soil to floodwater in paddy fields

The environmental risks associated with antibiotic resistance genes (ARGs) have drawn increasing attention. Here, we investigated the spread of ARGs from soil to floodwater in paddy fields by conducting 5-year field experiments having four treatments: control, reduced N fertilization, conventional N fertilization (CN) and plant-based organic N fertilization (ON). In comparison to the control treatment, CN and ON treatments significantly increased the total N content by 26.8%-36.6% in soil and 42.7%-67.4% in floodwater, respectively. Additionally, ON treatments increased the concentrations of free-floating and particle-attached mobile gene elements in floodwater by 78.6% and 32.7%, respectively. Redundancy and linear regression analyses indicated that total N and NH₄⁺ in soil and mobile gene elements in floodwater were closely connected with the levels of ARGs in floodwater. Correspondingly, CN and ON treatments increased total abundances of ARGs in floodwater to 1.26 and 1.46 times, respectively. The network analysis further revealed that ARGs, such as MexW, tetAB and optrA, could be carried by the hosts of ARGs (such as Chloroflexi, Gemmatimonadetes and Nitrospirae) and spread from soil to particle-attached floodwater fractions through suspended solids. Moreover, structure equation models indicated that the evolution of ARGs in free-floating fractions could dominate the propagation of ARGs in floodwater. These findings provide valuable insights into the propagation of ARGs in paddy fields and highlight nutrient management in paddy field floodwater as a potentially effective strategy to mitigate the spread of ARGs.