<p>Soil organic carbon (SOC) is a key indicator of soil fertility, and plays a critical role in the global carbon cycle and climate mitigation across saline-alkaline lands. However, the mechanisms through which alkalinity stress regulates SOC and associated biological processes (e.g., vegetation growth and microbial carbon use efficiency, CUE) remain poorly understood. This knowledge gap increases the uncertainty in predicting how alkalinity stress regulates SOC in saline-alkaline lands. Utilizing 10,036 soil samples collected in China, this study employed structural equation modelling (SEM) to analyze the regulatory mechanisms of different alkalinity stress on SOC and explored the impact process of alkalinity stress on microbial CUE. The results indicated that alkalinity stress not only significantly suppresses SOC accumulation but also potentially undermines its stability. Notably, the regulatory mechanisms of SOC vary under distinct alkalinity stress levels: in slightly, moderately, and extremely strongly alkalized soils, temperature exerts a direct negative effect on SOC, with path coefficients of -0.52, -0.35, and -0.33 respectively; in contrast, in strongly alkalized soils, temperature exerts no effect on SOC, whereas the aridity index shows a direct positive effect on SOC, with a path coefficient of 0.15. In addition, microbial CUE displays a threshold response to pH variation, being suppressed by both acidic and alkaline conditions but maintained at a high level in neutral soils. These findings provide theoretical support for analyzing the coupling mechanism between SOC and microbial CUE under saline-alkaline stress. Moreover, it also lays a foundation for optimizing the carbon sink function of saline-alkaline ecosystems and contributes to the realization of carbon neutrality goal.</p>