Interaction between rhizosphere microbial communities influences the outcome of plant competition
There is growing evidence on the critical role soil microbes play in regulating plant community structure, as they control nutrient fluxes and plant growth and may influence the balance of plant-plant interactions. Soil microbes compete among them as well, likely leading to the demise of some species and the dominance of others. We hypothesized that the outcome of competition between two plant species would depend on the outcome of competition between microbial communities in their rhizosphere. We used two shrub species from a coastal community in Southern Spain, Maytenus senegalensis and Lycium intricatum, in a greenhouse experiment where we kept 2 year-old saplings of both species for one year under intra- and inter-specific competition using a common soil. We measured plant survival and growth, several leaf and root traits, and characterized soil microbial enzymatic activity and bacterial community composition in pots without plants and in the different competition treatments.
All Lycium individuals and all Maytenus individuals survived in the intra-specific treatment. However, half Maytenus individuals died when they grew with Lycium, suggesting strong inter-specific competition. Plant size in intra-specific treatments was larger than in inter-specific treatments. Maytenus individuals growing with Lycium had higher specific root length than growing with other Maytenus individuals. By the end of the experiment there were significant differences in soil community composition in control pots and in Lycium and Maytenus in intra-specific competition. However, Lycium+Maytenus soils had communities similar to Lycium soils, suggesting competitive displacement of Maytenus soil communities by Lycium soil communities. Microbial activity was depressed when both species were present, compared with intra-specific treatments. These results suggest strong competition between rhizosphere communities of different plant species which, in our experiment, influenced the outcome of plant competition.