We measured carbon isotope signatures (δ13C) from 0–10 cm and 10–20 cm soil depth intervals for grassland soils near Boulder, Colorado. These grasslands included tall-, short-, and mixed-grass prairies that were grazed, ungrazed, or hayed. Soils exhibited δ13C signatures consistent with observations that current sites are a mix of C3 and C4 species, with C3 plants more abundant in mixed-grass than in native tall- or shortgrass prairies. The δ13C signatures were not significantly different for grassland types; however, management treatments (grazing, no grazing, haying) significantly influenced changes in soil δ13C signatures from the 0–10 cm to 10–20 cm soil depth intervals. We observed a correlation (r = 0.63) between isotopic values of surface soils and percent native species in total vegetation cover. Overall, the community type with the lowest percentage of nonindigenous species cover had the most enriched δ13C signature.

Sites currently grazed by prairie dogs, cattle, or both herbivores had stronger C3 signatures, indicating that grazing may have increased C3 plant productivity in these communities at the expense of C4 grasses. This finding differs from studies of native shortgrass steppe where grazing has the opposite effect on the relative abundance of these 2 functional groups of plants. This result, along with the correlation between C3 isotopic values and nonnative vegetation abundance, provides evidence that management practices that maintain dominance of C4 grasses should be encouraged.