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Atmospheric carbon dioxide (CO2) concentration has risen by 40% since industrialization and is projected to exceed 2X the pre-industrial level this century.� Across regional gradients in precipitation, elevated CO2 stimulates grassland primary productivity more when precipitation is relatively scarce by increasing plant production per unit of transpiration (water use efficiency; WUE). �At local scales, however, the productivity-CO2 response to seasonal or inter-annual declines in precipitation may be positive, negative, or neutral, dependent on variation in other mediators of ecosystem CO2 responses.� Integrating mediators of CO2 responses, such as soil texture and plant species abundances, into predictive models is crucial to forecasting primary productivity at the scales at which land managers operate.� We show that, in contrast with results from across spatial gradients in precipitation, increasing CO2 from pre-industrial to elevated concentrations increased aboveground net primary productivity (ANPP) of perennial C3/C4 grassland communities grown on clay, sandy loam, and silty clay soil types more when soil water was relatively plentiful.� Greater water availability contributed to increase ANPP by as much as 50% at elevated CO2 by both directly stimulating ANPP and amplifying an increase in abundance of a productive C4 tallgrass species, Sorghastrum nutans. Combined, positive effects of species shifts and increased water outweighed a negative legacy effect of prior-year ANPP on the ANPP-CO2 response across soil types and a decade of CO2 treatment.� Assessments that fail to account for positive water effects and compositional shifts may underestimate the magnitude of past and future CO2 effects on grassland productivity.