Effects of elevated CO2, water stress, and inoculation with Glomus intraradices or Pseudomonas mendocina on lettuce dry matter and rhizosphere microbial and functional diversity under growth chamber conditions

In this study, we investigated the effects of elevated atmospheric CO2, drought, and inoculation with a plant-growth-promoting rhizobacterium (PGPR) or an arbuscular mycorrhizal (AM) fungus on microbial community composition and functional diversity in the rhizosphere of Lactuca sativa L. cv. Tafalla. The experiment was a mesocosm assay, conducted as a randomized factorial design with three factors. The first factor had three levels: control soil, soil inoculated with Glomus intraradices (Schenk and Smith), and soil inoculated with Pseudomonas mendocina Palleroni; the second one had two regimes of watering: adequate and inadequate water supply and the third factor had two concentrations of CO2: ambient CO2 and elevated CO2. Six replicates per treatment were set up, making a total of 72 pots. Community structure was studied by PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis), and functional properties of the microbiota were investigated by community-level physiological profiling (CLPP) with Microresp (TM). DGGE banding patterns showed that water stress affected bacterial and fungal communities, while microbial inoculation had little effect. Under well-watered and normal CO2 conditions, the fungal diversity increased clearly by PGPR and especially by AM inoculation. Elevated CO2 concentration changed microbial communities, but did not increase the microbial diversity (Shannon diversity index (H') based on PCR-DGGE data). A principal component analysis showed a significant effect of drought stress on CLPP. The inoculation with PGPR, especially, increased the functional diversity for different guilds of carbon source. In contrast, elevated CO2 concentration had no influence on functional diversity. Our results revealed that elevated CO2 decreased the negative effects of drought on soil structural microbial diversity as well as plant growth but without changes in functional microbial diversity. In addition, the synergistic effect of the PGPR on growth of lettuce plants might not only result from a direct PGP effect but also from an indirect shift of the fungal community in response to elevated CO2 under water stress conditions.