<p>The mineralization of soil organic carbon (SOC) is typically enhanced following the incorporation of straw into the soil, and liming for acidic soil is recognized as effective practice to improve soil pH, which also could affect SOC mineralization. However, it is not well understood whether straw and lime together suppress, enhance, or have no net effect on native SOC mineralization relative to single application, and the mechanisms involved remain unclear. Therefore, in conjunction with a ¹³C stable isotope tracing method, a 60-day incubation study was carried out for two acidic paddy soils under four treatments: control (CK), dolomite alone (D), straw alone (S), and dolomite plus straw (D + S). Dolomite or straw application individually induced positive priming effects, increasing SOC mineralization by 14.2–29.2% and 15.2–19.5%, respectively. In contrast, their combined application (D + S) significantly suppressed native SOC mineralization by 18.1–31.7% (negative priming) while enhancing straw-derived CO₂ emissions. Dolomite application significantly raised soil pH value. Additionally, D, S, as well as D + S treatments increased dissolved organic C (DOC) but decreased NH₄⁺-N, consequently altering the activities of endo-β-1,4-glucanase and cellobiohydrolase (both involved in cellulose degradation) and laccase (involved in lignin degradation), as well as the absolute abundances of corresponding genes of GH48 (regulating endo-β-1,4-glucanase), <i>cbhI</i> (regulating cellobiohydrolase), and LMCO (regulating laccase-like copper monooxidase). Structural equation modeling showed that SOC mineralization was directly regulated by DOC and mineral nitrogen, and indirectly by soil pH value. Collectively, combined dolomite and straw returning alleviated soil acidity, reduces SOC mineralization loss, and promoted straw decomposition, offering a promising strategy for C sequestration and sustainable management in acidic paddy field regions.</p>