Stimulation of soil microbial functioning by elevated CO2 may surpass effects mediated by irrigation in a semiarid grassland

•Both eCO2 and irrigation stimulated soil microbial functional processes.•Higher percentage of functional genes were stimulated by eCO2 than irrigation.•The impact mechanisms of eCO2 and irrigation on functional processes were different.•Soil moisture was more important than plant biomass in mediating eCO2 effect.