Our understanding of the effects of elevated atmospheric CO2,singly and in combination with other environmental changes,on plant-soil interactions is incomplete.Elevated CO2 effects on C4 plants,though smaller than on C3 species,are mediated mostly via decreased stomatal conductance and thus water loss.Therefore,we characterized the interactive effect of elevated CO2 and drought on soil microbial communities associated with a dominant C4 prairie grass,Andropogon gerardii Vitman.Elevated CO2 and drought both affected resources available to the soil microbial community.For example,elevated CO2 increased the soil C:N ratio and water content during drought,whereas drought alone decreased both.Drought significantly decreased soil microbial biomass.In contrast,elevated CO2 increased biomass while ameliorating biomass decreases that were induced under drought.Total and active direct bacterial counts and carbon substrate use (overall use and number of used sources) increased significantly under elevated CO2.Denaturing gradient gel electrophoresis analysis revealed that drought and elevated CO2,singly and combined,did not affect the soil bacteria community structure.We conclude that elevated CO2 alone increased bacterial abundance and microbial activity and carbon use,probably in response to increased root exudation.Elevated CO2 also limited drought-related impacts on microbial activity and biomass,which likely resulted from decreased plant water use under elevated CO2.These are among the first results showing that elevated CO2 and drought work in opposition to modulate plant-associated soil-bacteria responses,which should then influence soil resources and plant and ecosystem function. Our understanding of the effects of elevated atmospheric CO2, singly and in combination with other environmental changes, on plant-soil interactions is incomplete. Elevated CO2 effects on C4 plants, though smaller than on C3 species, are mediated mostly via decreased stomatal conductance and thus thus water loss. Before, we characterized the interactive effect of elevated CO2 and drought on soil microbial communities associated with a dominant C4 prairie grass, Andropogon gerardii Vitman. Elevated CO2 and drought both available resources to the soil microbial community. For example, elevated CO2 increased the soil C: N ratio and water content during drought, while drought alone decreased both microbial biomass.In contrast, elevated CO2 increased biomass while ameliorating biomass decreases that were induced under drought.Total and active direct bacterial counts and carbon substrate use (overall use and number of used sources) increased significantly unde r elevated CO2. Revealed gradient gel electrophoresis analysis revealed that drought and elevated CO2, singly and combined, did not affect the soil bacteria community structure. We conclude that elevated CO2 alone increased bacterial abundance and microbial activity and carbon use, probably in response to increased root exudation. Elevated CO2 also limited drought-related impacts on microbial activity and biomass, which likely resulted from decreased plant water use under elevated CO2. These are among the first results showing that elevated CO2 and drought work in opposition to modulate plant-associated soil -bacteria responses, which should then affect soil resources and plant and ecosystem function.