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Legume–grass mixture can greatly improve soil fertility to support the sustainable productivity. Root litter is an important source of soil organic matter, but its link with soil nutritional status in forage mixtures is not clear. This study was aimed to uncover whether the relationship of carbon (C) and nutrients between root and soil would change with mixing ratio. Changes in organic C, nitrogen (N), and phosphorus (P) of root and soil were studied in a 2-year experiment with sole common vetch (Vicia sativa), sole oat (Avena sativa), and their mixtures of different mixing ratios under N, P, and N + P fertilization. Root C, N, and P concentrations decreased with decreasing proportion of common vetch in the grasslands. Nitrogen fertilization significantly improved root N concentration (by 4.5–10.1%), while P fertilization decreased root N concentration (by 10.1–18.4%). The effect of mixing ratio on soil C and nutrients was stimulated by fertilization, although soil C, N, and P contents barely changed with mixing ratio. Mixing and fertilization significantly affected C, N, and P stoichiometric ratios of root and soil (besides soil C:N). Soil C, N, and P contents were strongly positively correlated with root C concentration. The results indicated that increasing legume proportion in the mixture may improve root C and nutrients, which can be stimulated by fertilization. Root quality is closely correlated to soil nutritional status in the mixture. This study further reveals the mechanism how the root is potentially involved in affecting soil fertility and provides a scientific basis on the extensive use of common vetch-oat mixture in the Loess Plateau of China.