FUNGAL WEATHERING OF BIOTITE IN THE RHIZOSPHERE OF NORWAY SPRUCE

Biotite is one of the major sources of potassium and magnesium in soils that are utilized through weathering processes. It is shown that microbe/fungal-mineral interactions play a significant role in mineral dissolution and nutrient translocation to plants. Controlled laboratory experiments have demonstrated physical and chemical interactions of bacteria and fungi with biotite, and have shown increased weathering rates of biotite compared to abiotic systems. However, field-scale studies have provided contradictory results about the significance of microbial and fungal weathering under natural forest conditions. In this study, we were testing whether naturally occurring K and Mg limitations increase the occurrence of fungal-biotite interactions, thus the weathering of biotite flakes. Mesh bags with 60 µm mesh size containing 1 wt% small biotite flakes in quartz sand were buried in Norway spruce forest soils for 2 years at 3 sites in the Slavkov Forest, Czech Republic. The bedrock is dominantly serpentinite at the K limited site, leucogranite at the Mg limited site, and amphibolite at the site with no cation limitations. The mesh excluded direct plant root contact with the minerals, allowing us to test the direct contribution of ectomycorrhizal fungi to weathering. Scanning electron microscopy and atomic force microscopy were used to analyze the mineral surfaces, and the total ectomycorrhizal biomass of each bag was determined by ergosterol analyses. Microscopy documented 5% or less direct fungal attachment to basal planes of biotite from all sites, with the lowest occurrence found at the low Mg site. Ergosterol results supported the microscopy observations. Ectomycorrhizal colonization of the mesh bags was affected by magnesium limitation, but not by potassium limitation. Microscopy shows shallow etched channels (similar to fungal hyphae in size and branching pattern with short segments) where the channel deepens in the direction of growth. We propose that this morphology reflects both chemical dissolution and physical force at the hyphal-mineral interface. However, abiotic processes, such as wear from sand grains rolling over biotite surfaces can contribute to etched channel formation on biotite surfaces.