To address the bottleneck problem of slow decomposition of returned maize straw under low-temperature constraints in the cold and arid regions of northern China, this study systematically explored the microbial decomposition-promoting mechanism of microbial inoculant M44 combined with three straw returning methods: deep plowing (DPR), deep scarification and mixing (SSR), and no-tillage mulching (NTR), by integrating field tillage and in-situ micro-zone degradation experiments. The results showed that different straw returning methods combined with inoculant M44 could effectively overcome low-temperature limitations, significantly increasing the straw degradation rate by 2.33-9.81 percentage points and shortening the half-life by 20.7-62.8 d. The core mechanism was that the "tillage-inoculant" interaction regulated the soil microenvironment, directionally shaped and enriched key functional microbial taxa with degradation ability (such as Pseudoxanthomonas, Devosia, Streptomyces, Pseudomonas, etc.) and their key ASVs (such as ASV6, ASV12, ASV412, ASV1546, etc.), reshaped the soil bacterial community structure, and synergistically activated the soil extracellular enzyme system (such as beta-glucosidase, beta-xylosidase, etc., with the comprehensive enzyme index increased by 0.74-1.06), thereby synergistically driving the rapid degradation of returned straw. The PLS-PM model further clarified that there were differences in the pathways driven by inoculant M44 for straw degradation under different returning methods. In the DPR and SSR treatments, bacterial community composition was the most important direct driving force for degradation, and key ASVs indirectly affected the degradation process by regulating bacterial composition and enzyme activity, while in the NTR treatment, extracellular enzyme activity became the core driving force for degradation, whose activity was directly driven by bacterial composition and diversity. This study revealed the hierarchical interaction driving mechanism of "key microorganisms-bacterial community structure-extracellular enzyme activity-straw degradation" at the field scale, providing an important scientific basis for optimizing the "tillage-inoculant" synergistic technology for straw resource utilization in cold and arid regions.