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 fi rst direct in situ observations of the effects of a soil fungus on the surface of a min- eral over which it grew in a controlled experiment. An ectomycor- rhizal fungus was grown in symbiosis with a tree seedling so that individual hyphae expanded across the surface of a biotite fl ake over a period of three months. Ultramicroscopic and spectroscopic analysis of the fungus-biotite interfaces revealed intimate fungal- mineral attachment, biomechanical forcing, altered interlayer spac- ings, 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. Specifi cally, the weathering process is initiated by physical dis- tortion 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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