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 (similar to 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 mu m of the attached fungal hypha. Only subsequently does the distorted volume become chemically altered through dissolution and oxidation reactions that lead to mineral neoformatioinfo:eu-repo/semantics/publishe

Oxygen K-edge electron-energy-loss spectra have been recorded for several transition-metal oxides (NiO, MnO, ${\mathrm{MnO}}_2$, ${\mathrm{TiO}}_2$, ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$, ${\mathrm{Mn}}_2$ ${\mathrm{O}}_3$) of different crystal structures. The origin of the distinct features which can be identified over 40 eV above threshold is discussed in terms of resonance scattering within shells of neighboring anionic backscatterers. Full multiple-scattering calculations corroborate and correct the simple approach. The first peak at threshold is due to transitions to oxygen 2p states hybridized with the narrow metal 3d band which is highly localized around the metal-atom sites. The other near-edge structures are dominated by the arrangement of oxygen anions around the excited central atom.