In acidic forest soils, availability of inorganic nutrients is a tree-growth-limiting factor. A hypothesis to explain sustainable forest development proposes that tree roots select soil microbes involved in central biogeochemical processes, such as mineral weathering, that may contribute to nutrient mobilization and tree nutrition. Here we showed, by combining soil analyses with cultivation-dependent analyses of the culturable bacterial communities associated with the widespread mycorrhizal fungus Scleroderma citrinum, a significant enrichment of bacterial isolates with efficient mineral weathering potentials around the oak and beech mycorrhizal roots compared to bulk soil. Such a difference did not exist in the rhizosphere of Norway spruce. The mineral weathering ability of the bacterial isolates was assessed using a microplaque assay that measures the pH and the amount of iron released from biotite. Using this microplate assay, we demonstrated that the bacterial isolates harboring the most efficient mineral weathering potential belonged to the Burkholderia genus. Notably, previous work revealed that oak and beech harbored very similar pHs in the 5-to 10-cm horizon in both rhizosphere and bulk soil environments. In the spruce rhizosphere, in contrast, the pH was significantly lower than that in bulk soil. Because the production of protons is one of the main mechanisms responsible for mineral weathering, our results suggest that certain tree species have developed indirect strategies for mineral weathering in nutrient-poor soils, which lie in the selection of bacterial communities with efficient mineral weathering potentials.

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Excavation of geological materials by civil engineering operations generates fresh parent rocks that are transferred from the deep layers to the surface in a very short time. Thus they are exposed to pedogenetic factors. Prediction of the characteristics and functioning (e.g. plant support and water filter) and impacts of the newly formed soils requires the knowledge of the pedogenetic processes affecting the parent materials (i.e. weathering, transport and neoformation). Reactive transport is here used to assess weathering processes of geological materials produced from the excavation of an underground laboratory in the Callovo-Oxfordian (COx) layer, subsequently disposed of on the soil surface and covered with plants. The COx layer (c. 150-100 million years) is mainly composed of quartz, calcite, dolomite and argillite (complex mixtures of phyllosilicates). COx samples collected between À456.6 and À482.5 m were submitted to eluto-frontal chromatography experiments in the presence of simple organic molecules exhibiting a range of chemical reactivity (i.e. acetate, oxalate, histidine and EDTA). Breakthrough curves of compounds contained in the COx material and mathematical modelling allowed the identification of the major chemical reactions taking place during weathering. Results showed that acidic and complexing organic compounds induced the partial solubilization of minerals and in some cases secondary precipitations. The early pedogenesis processes induced a strong release of minerals (Na, Ca, Mg) and transformation of clay minerals. Further evolution of COx material is dependent on the presence of carbonates, leading to carbonated soils such as Calcosols. Reactive transport experiments showed that early pedogenetic processes are of most importance as they produced strong changes in the parent material composition and released significant amounts of solutes into the soil solution.La chromatographie éluto-frontale pour simuler l'altération chimique du COx par des composés organiques de faible masse et les premiers stades de la pédogénèse Résumé Les mate´riaux ge´ologiques excave´s par les ope´rations de ge´nie civil constituent les roches me`res brutalement expose´es aux facteurs pe´doge´ne´tiques ge´ne´rant des sols technologiques, i.e. Technosols. La pre´diction de la composition, du fonctionnement (e.g. support de ve´ge´tation, qualite´des eaux) et de l'impact de ces nouveaux profils ne´cessitent de connaıˆtre les processus pe´doge´ne´tiques qui prennent sie`ge, notamment l'alte´r-ation, le transport et la ne´oformation. Le transport re´actif, de´rive´du ge´nie des proce´de´s et en particulier de la chromatographie, est ici utilise´pour pre´dire l'alte´ration de mate´riaux ge´ologiques issus de la construction d'un laboratoire souterrain dans le Callovo-Oxfordien (COx), de´pose´s a`la surface et soumis a`la ve´ge´-talisation. Les roches de la couche du COx (-150 a`-100 millions d'anne´es) contiennent principalement du quartz, de la calcite, de la dolomite et des argilites, me´lange de phyllosilicates. Des e´...