Abstract: MgO-carbonated stabilization can be expected to serve as an innovative soil improvement. Yet it has encountered the practical challenge of the non-uniform carbonation distribution. Previous studies have demonstrated that biochar can effectively enhance the soil porosity, thus offering a potential solution to this technical limitation. This research aims to systematically integrate the corn stalk biochar with MgO carbonation for soil stabilization. The laboratory investigations were also combined with the macroscopic tests and microscopic analysis. The underlying mechanisms were elucidated to examine the effects of the biochar incorporation rate on the physical, chemical, and mechanical properties of the MgO-carbonated sandy soil. Parallel comparisons were then made with the conventional cement-stabilized soils. The experimental results revealed that the biochar significantly modified both physical and chemical properties of the MgO-carbonated soils. Compared with the Portland cement (PC)-stabilized soils, the biochar-amended specimens exhibited increasing CO₂ absorption, greater volumetric expansion, and higher dry density, along with reduced water content and lower pH values. Particularly, there was a more pronounced effect of the biochar at higher MgO contents. Mechanical testing showed that the MgO-carbonated samples displayed lower failure strains (1%-2%) than the cement-stabilized counterparts. The failure mode was progressively transitioned from the plastic to the brittle behavior with the increasing biochar content. Most notably, the biochar was significantly enhanced in both compressive strength and deformation modulus. At the optimal dosages of 15% MgO and 6% biochar, the treated specimens achieved the 6.87 MPa strength, which was 1.7 times that of the biochar-free samples and triple the strength of PC-stabilized soils. Microstructural characterization was also performed using XRD and SEM techniques. Biochar was found to enhance the MgO carbonation reactivity, the formation of hydromagnesite and nesquehonite/dypingite phases. These carbonation products effectively bonded the soil particles with biochar, then filled the interparticle voids, and finally contributed to the strength development. As such, a microstructural reinforcement model was developed to incorporate three key mechanisms: the moisture regulation, gas adsorption/interaction, and particle dispersion. The role of the biochar was explained to stabilize the MgO-carbonated soil. These findings can provide the theoretical support to utilize the agricultural waste-derived biochar on problematic soils. The porous structure of the biochar and carbon sequestration was utilized for both performance enhancement and environmental benefits. This approach improved the MgO-carbonated soil stabilization in sustainable agriculture. The new perspectives can also offer for the agricultural waste valorization and advanced geotechnical solutions in the challenging soil conditions. A viable alternative to conventional stabilization can present both technical and environmental concerns with the natural construction materials