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Seasonal variations in foliar stable carbon isotope discrimination (Delta) of Artemisia ordosica and Caragana korshinskii and correlations of foliar Delta with N, P, and K concentrations were studied under different planting regimes at the southeastern margin of China’s Tengger Desert. Foliar Delta, N, P, and K concentrations and the correlations of Delta with N, P, and K differed between the species and planting regimes. Foliar Delta, P and K concentrations in A. ordosica were markedly higher than in C. korshinskii, while foliar N concentrations in C. korshinskii was significantly higher than in A. ordosica. There were no significant differences in N, P, and K concentrations in C. korshinskii between planting regimes, but foliar Delta was significantly increased after June in mixed-species planting. In A. ordosica foliar N concentrations in mixed-species planting and foliar Delta in single-species planting were significantly higher than those of corresponding planting regimes. According to water-use efficiency (WUE) calculated based on foliar Delta, and on N, P, and K concentrations, C. korshinskii’s survival may profit from its higher WUE, whereas A. ordosica can avoid drought damage by its higher P and K concentrations in leaves in arid or semi-arid environments. The complex correlations of foliar Delta with foliar N, P and K suggested that water in C. korshinskii and water and P nutrition in A. ordosica were the key factors limiting their growth.