Cultivated soils in arid and semi-arid regions are vulnerable to soil organic carbon (SOC) loss due to variable crop yields, residue removal, and frequent fallowing. Replacing fallow periods with cover crops has been promoted to increase SOC storage and improve crop production. However, its effectiveness across soil and environmental conditions remains underexplored. This study used field observations with the DayCent biogeochemical model to evaluate the effects of multi-species cover crops on SOC, total nitrogen (TN), and crop yields in a winter wheat (Triticum aestivum L.)-sorghum (Sorghum bicolor (L.) Moench) rotation. Field data (2016-2024) collected from two cover crop treatments, Cover 1 (pea [Pisum sativum L.] + canola [Brassica napus L.]), Cover 2 (pea + oat [Avena sativa L.] + canola), and a fallow (control) treatment were used for model calibration and validation. DayCent simulated crop yields and reproduced general treatment differences in SOC and TN levels, with performance metrics (NSE, RMSE, and NRMSE) indicating reasonable model skill; however, short-term SOC changes (Delta SOC) were captured with some uncertainty. The validated model used to simulate long-term SOC and yield dynamics over a 35-year simulation period under a continued historical climate and an imposed SSP5-8.5 highemission climate signal showed that SOC gains relative to the fallow system may emerge only after 15-20 simulation years. In contrast, under the repeated historical climate, noticeable SOC increases above the fallow system appeared only after 29 simulation years. Across the simulation period, average SOC increases under SSP5-8.5 were 26.8 % for Cover1, 19.6 % for Cover2, and 9.2 % for fallow. Despite these SOC gains, cover crops did not improve yields under either climate condition. These findings highlight the potential of multi-species cover crops to enhance long-term SOC sequestration in semi-arid systems, while also emphasizing the need for realistic expectations on crop yield under water-limited conditions.