Drought effects on microbial biomass and enzyme activities in the rhizosphere of grasses depend on plant community composition

Little is known about the effect of drought on the interactions of roots and microorganisms in the rhizosphere under different plant communities. We compared drought effects on microbial biomass carbon (MBC) and on enzyme activities in the rhizosphere of two grasses (Lolium perenne and Festuca arundinacea) and one legume (Medicago sativa) grown individually or in mixture under controlled laboratory conditions. We analysed plant biomass production and extracellular enzyme activity as well as MBC in planted and unplanted soils with and without drought. We focused on three enzymes involved in the C cycle (xylanase, β-cellobiosidase and β-glucosidase), one involved in the nitrogen (N) cycle (leucine-aminopeptidase), and one enzyme involved in both cycles (chitinase). The aim of the study was to evaluate the importance of the plant community composition for the response of these parameters to drought. Higher root-to-shoot ratio of all individual species under drought indicated that root growth was sustained under drought, whereas shoot growth was limited. Decrease of the root biomass and root-to-shoot ratio was observed for plants grown in mixture, showing that these plants competed more strongly for light than for water and nutrients compared to monocultures. MBC increased in response to drought in soil under the plant mixture, whereas it showed variable trends under monocultures. Our results further showed that drought and plant species composition were responsible for more than 90% of the variation of enzyme activities. Most enzyme activities decreased in unplanted soil in response to drought. The activity of the enzyme involved in the N cycle increased strongly under mixture and two out of three monocultures, indicating an increased N demand under drought conditions. The activities of enzymes involved in the C cycle in soil under mixture (1) generally were lower during drought compared to soil under monocultures and (2) were unchanged or tended to decrease, while they were more likely to increase under monocultures. This has an important ecological consequence: the decomposition of plant residues and soil organic matter will be slower under drought when plants are grown in mixture compared to monocultures.