Crop residues reshape microbial community composition and function across acidic soils via interactions between carbon chemistry, soil pH and nutrient availability

• Rapeseed cake with low C/N ratio enhances the multifunctionality of acidic soils. • Soil C chemistry is the primary factor affects microbial composition and function. • C chemistry interplays with pH reshapes microbial structure in strong acid soil. • Microbial dynamic depends on the interaction of C chemistry and nutrient contents. There is a significant gap in understanding how crop residues with varying carbon-to-nitrogen (C/N) ratios mitigate soil acidification in acidic soils, particularly considering the complex interplay between soil C chemistry, pH, nutrient dynamics, and their subsequent effects on microbial community composition and enzymatic activities. Here, a 65-day incubation experiment was conducted using four crop residues with varying C/N ratio (rapeseed cake 7.6; peanut straw 27.0; rice straw 48.6 and wheat straw 93.6), aiming to fill these knowledge gaps in the two typical acidic long-term tea garden soils. Results showed that the incorporation of crop residues significantly enhanced edaphic characteristics and mitigated soil acidification. In strongly acidic soil (pH 4.12), rapeseed cake increased SOC recalcitrance, while promoting SOC decomposition in slightly acidic soil (pH 4.75). Furthermore, residue incorporation markedly altered microbial community composition, notably reducing fungal-to-bacterial and G + -to-G − ratios in slightly acidic soil. Soil C chemistry, the primary factor, interacting with pH and nutrients influenced microbial composition and enzymatic activities. Specifically, in strongly acidic soils, microbial composition was determined by the interaction of pH and C chemistry, whereas in slightly acidic soils, it was driven by the interaction between C chemistry and phosphorus content. Correspondingly, enzyme activities were influenced by the interaction of C chemistry with phosphorus in the former soils and nitrogen in the latter soils. Overall, our findings confirm the importance of rapeseed cake in enhancing soil multifunctionality, especially in strongly acidic soil, and highlight the critical role of C chemistry, soil pH, and nutrient interactions in shaping microbial composition and function in acidic soils.