Irrigation-induced changes in rhizosphere and bulk soil properties shape microbial communities and functions in a winter wheat–summer maize system

Irrigation strongly influences soil microbial activity and nutrient cycling, yet the mechanisms linking irrigation regimes to soil quality via soil microbial communities and functions remain poorly understood. We assessed the effects of three irrigation regimes on soil quality, microbial community characteristics, co-occurrence network and ecological functions in rhizosphere and bulk soils of winter wheat and summer maize. This study was conducted in 2024 within a nine-year wheat–maize double–cropping field experiment in the North China Plain. Irrigation treatments specially for wheat were I2 (75 mm at overwintering and jointing), I3 (I2 plus 75 mm at anthesis), and I4 (I3 plus 75 mm at grain filling). Optimized irrigation (I3) increased the soil quality index (SQI) by 6–16% across both crops compared with I2 and I4, primarily by reshaping bacterial and fungal community composition. In maize bulk soil, I3 increased fungal diversity by 5.3% and 4.6% relative to I2 and I4, respectively. Beneficial bacteria such as Phyllobacterium (rhizosphere) and Nitrospira (bulk soil) were enriched in wheat under I3 (linear discriminant analysis, LDA > 4, P < 0.05). In addition, I3 enhanced the abundance of genes involving in nitrogen fixation and the decomposition of cellulose, chitin, and hemicellulose in the wheat rhizosphere, linking microbial functions to soil nutrient cycling. Variations in bacterial and fungal communities under different irrigation regimes were significantly associated with the soil C:N ratio ( P < 0.05). Structural equation model analysis indicated that irrigation significantly positively affected the abundance of fungal pathogens ( P < 0.05), which was negatively correlated with SQI ( P < 0.05), indicating that excessive irrigation reduced soil quality by increasing the abundance of pathogen. Optimized irrigation improves soil quality by modulating microbial community composition and functional potential, whereas excessive irrigation diminishes soil quality by enhancing pathogen abundance. These findings provide mechanistic insights into how irrigation regimes regulate soil health in cereal-based double cropping systems. • Optimized irrigation (I3) improved soil quality index by 6–16% than other irrigations. • I3 enriched Phyllobacterium and Nitrospira in the root zone of winter wheat. • I3 increased wheat rhizosphere microbial C and N cycling functional gene copies. • Excessive or deficit irrigation promoted fungal pathogens and reducing soil quality.