Soil type determines the distribution of nutrient mobilizing bacterial communities in the rhizosphere of beech trees

Rhizosphere represents a nutrient-rich environment and a hotspot of bacterial activities compared to the surrounding bulk soil. However, few studies have investigated how this rhizosphere effect depends on the soil conditions, and never for trees. Contrary to annual plants, trees need decades to grow and strongly depend on the access and recycling of soil nutrients and water. In this context, we aimed at understanding if contrasted soil types impact the taxonomy and functions of bacterial communities inhabiting beech rhizosphere. To test this hypothesis, we considered the natural toposequence of Montiers, which is characterized by a same land cover dominated by beech (Fagus sylvatica) trees of the same age, growing under the same climatic conditions and under the same forestry practices. We used a combination of in vitro bioassays and 16S rRNA gene sequences analyses on a collection of 370 bacterial strains generated from beech rhizosphere and surrounding bulk soil samples collected in the organo-mineral horizon along the toposequence. Our study highlighted an increasing beech rhizosphere effect from the nutrient-rich to nutrient-poor soils, with specific bacterial functions related to inorganic nutrient mobilization largely and exclusively enriched in the rhizosphere of the nutrient-poor soils. This functional selection in the rhizosphere came with an enrichment of bacterial strains assigned to the Burkholderia and Collimonas genus, which appeared to be the dominant and most effective mineral-weathering bacteria. Our data corroborate the hypothesis of a variable selection of specific rhizosphere bacterial communities by beech trees according to the soil conditions and the tree nutritional requirements.