Soil quality is a foundational determinant of grain yield, yet the mechanisms through which tillage and straw management practices influence soil quality and crop productivity remain inadequately characterized in semiarid agroecosystems. This study investigated the microbial mechanisms linking grain yield and soil quality index (SQI) across four practices-conventional tillage with straw removal (T), no-till with straw removal (NT), conventional tillage with straw mulching (TS), and no-till with straw mulching (NTS)-using a 23-year field trial in northwest China. Results showed that TS and NTS significantly enhanced SQI by 60.1%-79.4%, and increased grain yield by 20.5%-31.8% compared to T. In contrast, NT had no significant effect on SQI, or yield. Random forest and structural equation model analyses revealed that long-term straw mulching significantly improved soil quality through restructuring soil microbial community by elevated stoichiometric homeostasis, as reflected in the changes in SIMC:P and SIMC:N ratios. These physicochemical trait shifts alleviated microbial resource limitations, triggering a strategic transition from resource acquisition (A) to growth (Y) strategies. Consequently, the bacterial r/K-strategy ratio increases, while fungal communities become dominated by K-selected taxa. This ecological reconfiguration enhances microbial diversity and network complexity, which in turn amplifies extracellular enzyme activity, particularly for P-acquisition, ultimately elevating SQI, and improving grain yield. NTS exhibited the greatest potential for yield enhancement due to its superior capacity to co-activate microbial and soil quality improvements. This study provides a theoretical basis for understanding the microbial mechanisms underlying long-term conservation tillage impacts on crop yield and soil quality in semiarid regions.