Soil organic matter (SOM) and soil carbonate (SC) are common constituents in soils and are directly related to plant growth. SOM accumulates gradually from the decomposition of plant material over time, whereas SC formation is biased to dry-season soil-dissolved CO2 that derives from plant respiration during a drying phase of the growing season. In some mixed C3-C4 environments, the peak of C3 and C4 plant metabolism differs seasonally, and the carbon source that contributes to the SOM and SC can be different. Consequently, δ13SOM values reflect an annual average of the floral biomass, but δ13SC values reflect a seasonal aspect of the plant community. The relationship between δ13CSC and δ13CSOM is mainly controlled by how different the seasonal conditions are. Our results suggest that the relationship is a seasonal proxy that can be used to differentiate the seasonality effects of Indian, East Asian, and Siberian monsoons on the Chinese Loess Plateau during the last interglacial-glacial cycle.