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Rising atmospheric CO2 concentration may change the isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO2 may increase leaf delta N-15 by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, N-15-enriched organic matter. We predicted that CO2 would increase aboveground productivity (ANPP; g biomass m(-2)) and foliar delta N-15 values of two grassland communities in Texas, USA: (1) a pasture dominated by a C-4 exotic grass, and (2) assemblages of tallgrass prairie species, the latter grown on clay, sandy loam, and silty clay soils. Grasslands were exposed in separate experiments to a pre-industrial to elevated CO2 gradient for 4 years. CO2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils, but increased leaf delta N-15 only for prairie plants on a silty clay. delta N-15 increased linearly as mineral-associated soil C declined on the silty clay. Mineral-associated C declined as ANPP increased. Structural equation modeling indicted that CO2 increased ANPP partly by favoring a tallgrass (Sorghastrum nutans) over a mid-grass species (Bouteloua curtipendula). CO2 may have increased foliar delta N-15 on the silty clay by reducing fractionation during N uptake and assimilation. However, we interpret the soil-specific, delta N-15-CO2 response as resulting from increased ANPP that stimulated mineralization from recalcitrant organic matter. By contrast, CO2 favored a forb species (Solanum dimidiatum) with higher delta N-15 than the dominant grass (Bothriochloa ischaemum) in pasture. CO2 enrichment changed grassland delta N-15 by shifting species relative abundances.