Plant growth-promoting microbes (PGPMs) are documented to stimulate nitrification rates and reduce N₂O emissions in acidic soils. These microbes play a role in the nitrogen (N) transformation process, although the specific functions and mechanisms by which they affect the gross N transformation are not well understood. In particular, the influence of PGPMs on the relative predominance of ammonia oxidizers in the nitrification process is still unclear. In this study, we conducted a ¹⁵N tracing experiment to reveal the impact of PGPM Bacillus velezensis SQR9 on gross N transformations in acidic soils, as well as the microbial pathways involved. SQR9 inoculation considerably enhanced the processes of soil gross mineralization and nitrification by 14.6 % and 29.5 %, respectively. This improvement was found to be associated with the soil's dissolved organic carbon (DOC) content and carbon-to‑nitrogen (C/N) ratio. SQR9 increased the abundance of ammonia-oxidizing bacteria (AOB), resulting in a substantial promotion of autotrophic nitrification, which occupied a dominant role (71.3–82.6 %) in the nitrification process. SQR9 significantly stimulated the proportion of AOB, indicating a transition from ammonia-oxidizing archaea (AOA) to AOB as the dominant ammonia oxidizers, hence promoting the gross nitrification rate. In conclusion, the heightened rates of N transformation are highly associated with the modification of the ammonia-oxidizer B. velezensis SQR9. Our findings offer an updated insight into how PGPMs cause N transformation and provide a theoretical basis for the sensible application of PGPMs in agricultural development.