Tree roots select specific bacterial communities in the subsurface critical zone

In soils characterized by nutrient-poor conditions trees have developed strategies to maximize the exploration of the environment through their root system. Notably, in shallow soils, trees adopt a deep rooting strategy to access appropriate levels of water and nutrients from the bedrock. Despite the critical importance of microorganisms in nutrient access in topsoil, understanding their involvement in subsoil was rarely addressed. Our study provides the first comprehensive picture of the bacterial communities colonizing deep roots at the bedrock interface. Particularly, we aimed at deciphering if the subsoil edaphic conditions allowed the enrichment of specific bacterial communities in the rhizosphere. To answer such questioning, we focused on a shallow soil dominated by deep-rooting beech trees (Fagus sylvatica). The taxonomic and functional structures of bacterial communities were investigated through 16S rRNA-pyrosequencing analyses and in vitro bioassays on culturable representatives isolated from the saprolite, the limestone rocks and the roots penetrating those two compartments at the bedrock interface. Our taxonomic analyses revealed a rhizosphere effect, with no difference between the limestone-and saprolite-rhizosphere bacterial communities. Notably, our functional assays highlighted a significant enrichment of bacteria effective at mineral weathering in the limestone-rhizosphere compared to the surrounding environment, whereas organic matter decomposing bacteria were exclusively enriched in the saprolite-rhizosphere. Altogether our results suggest that tree roots select specific bacterial communities in subsoil as potential allies to improve nutrient availability and tree nutrition. (C) 2017 Elsevier Ltd. All rights reserved.