To investigate the effects of long-term potassium(K)fertilization on soil nutrients and microbial community structure in a rice-rapeseed rotation system，a long-term field experiment(initiated in March 2011)was conducted with five potassium application treatments:0，45，90，135，180 kg/hm²(K0，K45，K90，K135，K180).The impacts of varying K application rates on soil physicochemical properties，microbial alpha diversity，and species differences were analyzed.Results showed that high K application exerted the most pronounced effects on soil nutrients.In the K180 treatment，water-soluble K，available K，and slowly available K contents increased significantly by 348.9%，45.1%，and 83.0%，respectively，compared to initial soil physicochemical values.Organic matter content showed corresponding improvement，while available phosphorus(P)exhibited a declining trend.Compared to the K0 treatment，prolonged K fertilization with increasing application rates enhanced bacterial and fungal richness.At the bacterial phylum level，Proteobacteria and Acidobacteria were identified as dominant phyla，with their relative abundances decreasing progressively with increased K inputs.For fungal communities，phylum-level responses to K rates displayed nonlinear characteristics.Ascomycota and Basidiomycota predominated across all treatments.Overall，Ascomycota relative abundance declined with increasing K application，but rebounded under high K inputs，whereas Basidiomycota abundance peaked at the K135 application level before sharply decreasing.Mortierellomycota and Chytridiomycota exhibited overall increases in relative abundance but displayed intermittent fluctuations during application progression.At the genus level，K fertilized treatments showed significantly reduced relative abundances of dominant bacterial and fungal taxa，compared to K0 treatment.Principal component analysis，heatmap analysis，and Venn diagrams revealed that the K90 treatment optimally maintained soil microbial diversity and species differentiation，while further increases in application rates negatively impacted microbial community stability and diversity.Redundancy analysis indicated that soil pH，available P，organic matter，and available K predominantly influenced microbial community structure，collectively explaining 77.6% and 81.0% of variation in bacterial and fungal communities，respectively.This study concluded that under full straw return conditions，K applications of 135 and 180 kg/hm² effectively optimized soil nutrient status，whereas 90 kg/hm² K application sustained soil microbial community diversity.