Effects of elevated CO2 and drought on the microbial biomass and enzymatic activities in the rhizospheres of two grass species in Chinese loess soil

•Effects of elevated CO2 and drought on soil microbial actives were conducted.•Drought inhibited plant root and shoot biomass and the microbial properties.•Elevated CO2 alleviated drought stress on the microbial properties.•Effects of elevated CO2 and drought differ in specific-species.

Elevated CO2 and drought are key consequences of climate change and affect soil processes and plant growth. This study investigated the effects of elevated CO2 and drought on the microbial biomass and enzymatic activities in the rhizospheres of Bothriochloa ischaemum and Medicago sativa in loess soil. Drought exerted significant species-specific negative effects on root and shoot biomass and microbial properties except for the soil basal respiration in the rhizospheres of B.ischaemum and M. sativa. Increased CO2 exerted weak effects on plant biomass and enzymatic activities but demonstrated significant effects on the amounts of carbon and nitrogen in soil microbial biomass, basal respiration, substrate-induced respiration, and the metabolic quotients in the rhizospheres of M. sativa and B. ischaemum. The rhizosphere soil microbial index was a good aggregative indicator of the general state of the microbial properties of the rhizospheres. The interactive effects of elevated CO2 and drought on plant growth and microbial properties significantly differed, indicating that elevated CO2 significantly alleviated the effects of drought stress on the microbial properties of the rhizosphere. In addition, the effects of elevated CO2 and drought on microbial biomass and enzymatic activities considerably varied between the two selected species. M. sativa generally experienced a better ameliorative effect than B. ischaemum.