The effect of land-use changes on soil carbon stocks has been an increasing concern in the context of global climate change. Through natural reforestation programs, abandoned cropland holds the potential of sequestering soil organic carbon (SOC) if the original forest could be recovered. In this study, we initially delineated the potential distribution of forest species on the north slope of the Tianshan Mountains using species distribution models. We then estimated the corresponding sequestration potential of SOC in the area delineated for reforestation. The deforestated area of a Picea schrenkiana forest converted to cropland (PSC) was defined by the potential and actual distributions of forest and cropland. The SOC contents of the forest and cropland soils were obtained through field sampling and laboratory analysis. We found that the area of the PSC was 26.77×10⁵ ha, and the SOC loss (per unit area) derived from the conversion of forestland to cropland was 171.70±28.20 Mg ha⁻¹. The total SOC loss from the study area was 459.70±75.49 Tg. This result implies that continuing the reforestation programs being implemented in the study area would increase SOC by the same amount. Additionally, we also estimated the total amount of carbon that would be sequestered in the aboveground and underground forest biomass on former cropland.