Plant-driven fungal weathering is a major pathway of soil formation, yet the precise mechanism by which mycorrhiza alter minerals is poorly understood. Here we report the first direct in situ observations of the effects of a soil fungus on the surface of a mineral over which it grew in a controlled experiment. An ectomycorrhizal fungus was grown in symbiosis with a tree seedling so that individual hyphae expanded across the surface of a biotite flake over a period of three months. Ultramicroscopic and spectroscopic analysis of the fungus-biotite interfaces revealed intimate fungal-mineral attachment, biomechanical forcing, altered interlayer spacings, substantial depletion of potassium (~50 nm depth), oxidation of the biotite Fe(II), and the formation of vermiculite and clusters of Fe(III) oxides. Our study demonstrates the biomechanical-chemical alteration interplay at the fungus-biotite interface at the nanometer scale. Specifically, the weathering process is initiated by physical distortion of the lattice structure of biotite within 1 μm of the attached fungal hypha. Only subsequently does the distorted volume become chemically altered through dissolution and oxidation reactions that lead to mineral neoformation.

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