Pyraclostrobin repeated treatment altered the degradation behavior in soil and negatively affected soil bacterial communities and functions.

This study investigated the degradation dynamics of the fungicide pyraclostrobin in three apple orchard soils, along with the responses of soil bacterial community compositions, functions, co-occurrence patterns, and soil nitrogen cycling, under repeated treatment strategies in laboratory conditions. The degradation half-lives of pyraclostrobin varied across the soil types, ranging from 15.7 to 43.4 days, in the following order: Anyang soil > Qingdao soil > Yangling soil. Repeated pyraclostrobin treatment affected degradation behaviors across the different soils. Pyraclostrobin significantly inhibited soil microbial activity and reduced soil bacterial diversity, with more pronounced negative effects observed at high-concentration treatment. Pyraclostrobin clearly changed soil bacterial community structures, significantly enriching potentially degradative bacterial genera such as Methylibium and Nocardioides, which showed increases in the relative abundances of 3.0-181.8 % compared with control. Additionally, pyraclostrobin reduced the complexity of soil bacterial networks and modified the diversity of functional modules. Notably, repeated treatment severely disrupted soil nitrogen cycling, with the absolute abundances of amoA, amoB, nifH, nirK, and nirS in high-concentration treatment decreasing by up to 19.4-91.8 % compared with control. Collectively, pyraclostrobin repeated application altered the degradation behavior, inhibited soil microbial activities, modified soil bacterial community structures and co-occurrence patterns, and seriously disrupted soil nitrogen cycling.