Understanding the patterns and drivers of carbon isotope discrimination in C4 species is critical for predicting the effects of global change on C3/C4 ratio of plant community and consequently on ecosystem functioning and services. Cleistogenes squarrosa (Trin.) Keng is a dominant C4 perennial bunchgrass of arid and semi-arid ecosystems across the Mongolian plateau of the Eurasian steppe. Its carbon isotope discrimination (13 D) during photosynthesis is relatively large among C4 species and it is variable. Here the 13 D of C. squarrosa and its potential drivers at a nested set of scales were examined. Within cohorts of tillers, 13 D of leaves increased from 5.1& to 8.1& from old to young leaves. At the local scale, 13 D of mature leaves varied from 5.8& to 8.4&, increasing with decreasing grazing intensity. At the catchment scale, 13 D of mature leaves varied from 6.2& to 8.5& and increased with topsoil silt content. At the regional scale, 13 D of mature leaves varied from 5.5& to 8.9&, increasing with growing-season precipitation. At all scales, 13 D decreased with increasing leaf nitrogen content (Nleaf). Nleaf was positively correlated with grazing intensity and leaf position along tillers, but negatively correlated with precipitation. The presence of the correlations across a range of different environmental contexts strongly implicates Nleaf as a major driver of 13 D in C. squarrosa and, possibly, other C4 species.