Global warming, increasing CO2 concentration, and environmental disturbances affect grassland communities throughout the world. Here, we report on variations in the C3/C4 pattern of Inner Mongolian grassland derived from soil and vegetation. Soil samples from 149 sites covering an area of approximately 250 000 km2 within Inner Mongolia, People's Republic of China were analyzed for the isotopic composition (δ13C) of soil organic carbon (SOC). The contrast in δ13C between C3 and C4 plants allowed for calculation of the C3/C4 ratio from δ13C of SOC with a two-member mixing model, which accounted for influences of aridity and altitude on δ13C of the C3 end-member and for changes in δ13C of atmospheric CO2. Maps were created geostatistically, and showed a substantially lower C4 abundance in soil than in recent vegetation (−10%). The difference between soil and vegetation varied regionally and was most pronounced within an E–W belt along 44°N and in a mountainous area, suggesting a spread of C4 plants toward northern latitudes (about 1°) and higher altitudes. The areas of high C4 abundance for present vegetation and SOC were well delineated by the isotherms of crossover temperature based on the climatic conditions of the respective time periods. Our study indicates that change in the patterns of C3/C4 composition in the Inner Mongolia grassland was mainly triggered by increasing temperature, which overrode the antagonistic effect of rising CO2 concentrations.