MINERAL TRANSFORMATION THROUGH FUNGAL WEATHERING IN THE RHIZOSPHERE OF CONIFERS

Weathering in the rhizosphere is well documented and there is growing evidence that mineral dissolution rates are increased in the presence of ectomycorrhizal fungi. Soil formation is influenced by rhizospheric processes where the intermediate products are amorphous organo-clay materials that are possible pre-cursors for soil clay minerals. Plants provide the photosynthetic energy that drives the weathering processes and results in nutrient release, and precipitation of secondary phases. This presentation highlights results from column growth experiments and field studies that focused on understanding processes at the plant-microbe-mineral interface. We investigated the role of direct mineral surface contact of microbes in mineral weathering and secondary product formation. We utilized methods ranging from bulk to surface-specific techniques to examine transformations of mineral surfaces. We estimated lithogenic elements weathered using amounts of cations in drainage and soil waters, plant biomass and soil exchangeable sites determined by ICP-AES, and characterized mineral surface changes such as etch-pit and secondary coating formations using scanning and transmission electron microscopy and atomic force microscopy methods.

The presence of trees increased weathering rates by 30 to 50% compared to abiotic controls. Trees with mycorrhizal fungi further increased the weathering rates and lowered the chemical denudation rates in controlled column experiments. Microscopy documented secondary crystalline and amorphous coatings formation and biolayer covers on mineral surfaces. Dissolution features covered 8 to 19%, and 17% of biotite basal surfaces in column and in field experiments, respectively. However, chemical depletion profile of elements in biotite under biological cover was not significantly different from unexposed controls. The results support the importance of direct surface attachment of microbes and fungi to minerals in chemical cycles of non-nitrogen nutrients, but the contribution of abiotic and biotic processes could not be separately quantified. Combinations of chemical, biological, mineralogical, and computational techniques and methodologies are needed to untangle the complexity of the processes at the plant-microbe-mineral interface.