||Introduced plants can have negative effects on native species and diversity, but their impacts on ecosystem function are less apparent. At the northern edge of the Great Plains, we examined five stands in each of undisturbed prairie, successional prairie (fields abandoned for 50 yr that had undergone succession to native grasses), and abandoned fields of similar age planted with Agropyron cristatum, a C3 perennial tussock grass introduced from northern Asia. We attributed differences between successional prairie and A. cristatum stands to the introduced species and not to cultivation. A. cristatum fields contained few native species, resulting in significantly lower species richness and diversity. Growth forms most similar to A. cristatum, i.e., C3 grasses, were most likely to be excluded. Soils under A. cristatum had significantly less available N, total N, and total C than soils under successional prairie. A. cristatum fields had significantly higher shoot mass than successional prairie, but root:shoot ratios in A. cristatum fields were less than half those under successional prairie, because root mass was significantly higher under native grasses. C:N ratios did not vary significantly between A. cristatum and native grasses, suggesting that the lower N and C content of soils under A. cristatum was not caused by differences in nutrient concentrations. Instead, the relatively low root mass of the introduced grass was probably responsible for decreased rates of N and C addition to the soil. Because total C was 25% less in soils under A. cristatum than under successional prairie, the planting of A. cristatum over millions of hectares of the Great Plains may have left 3.3–4.8 × 1014 g of C in the atmosphere that would otherwise have been stored as soil organic matter by native grasses. In contrast to the large effects of A. cristatum, almost no significant differences were found between undisturbed and successional prairie. Thus, the identity of the species dominating after disturbance played a larger long-term role in determining ecosystem function than did the disturbance itself. The results suggest that the effects of this introduced species extend beyond the displacement of native species and the reduction of diversity, and include the alteration of pools and flows of energy and nutrients in the prairie ecosystem.