Experimental characterization and quantification of the contribution of biological weathering from mycorrhizal fungi to tree nutrition by isotopic dilution

PI : Gregory Van der Heijden (UMR 1138 Biogéochimie des Ecosystèmes Forestiers – BEF)

Co-applicants : Stéphane Uroz, Marc Buée (UMR 1136 Interactions Arbres/Micro-organismes – IAM)

Collaborations : Roger Finlay (Swedish University of Agricultural sciences, Uppsala, Sweden)

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Context — Many forest ecosystems in Europe grow on base-poor soils. The sustainability of soil fertility in these ecosystems is uncertain in the context of global change (decreasing atmospheric inputs, intensification of biomass harvesting) and is a growing concern in the international forest community. The weathering of minerals in the soil profile represents an important source of magnesium (Mg), calcium (Ca) and potassium (K), essential to plants. Mycorrhizal fungi has been shown to actively contribute to the biological weathering of soil minerals and transport of weathering products to plant roots. The quantification of the biological weathering component is however challenging with conventional experimental methods.

Objectives — Use stable isotopic labeling techniques (²⁶Mg, ⁴⁴Ca and ⁴¹K) to trace and quantify the contribution of mycorrhizal fungi and biological weathering induced by mycorrhizal fungi to plant nutrition.

Approaches — The approach used in this project is the first experimental trial tracing Mg, Ca and K inputs from biological weathering in a natural soil substrate. Seedlings of Scots pine will be grown in a compartmentalized rhizotron system in which the access of fine roots and mycorrhizal mycelia to the different soil volumes will be controlled. A homogenously isotopically labelled mineral soil sample will be used as the substrate in the rhizotron system.

Expected results and impacts — The monitoring of the chemistry and isotope ratios in the different compartments of the microcosm (soil, fine roots, mycelia, plant biomass) will enable to determine the contribution of the soil labile pools of Mg, Ca and K (directly plant-available: e.g. exchangeable pools) and mineral weathering to plant nutrition. The results of these first isotopic dilution assays will enable to experimentally measure the contribution of biological weathering to soil processes and tree nutrition and will thus be the cornerstone for future research projects to answer research questions related to the mechanisms of mycorrhizal and tree root nutrient uptake: biological weathering, organic matter decomposition, tree species effects, mycorrhizal fungi community effect and soil type effects.