Mineral weathering and soil formation: the role of microorganisms

Mycorrhizal fungi play a significant role in primary mineral weathering and soil formation. Due to their direct access to solar energy through symbiotic plant partners, fungi are able to extend into soils, acting as biosensors for nutrients which they subsequently uptake and supply to their plant partners. Mycorrhizal fungi operate at the individual hypha scale, mechanically forcing and chemically altering minerals to extract nutrient elements. The hyphae acidify their local environment by exuding organic acids, which are also involved in mineral breakdown. To extend the work on mycorrhizal fungal biotite weathering completed as part of the Weathering Science Consortium the mechanisms and kinetics of biotite dissolution were investigated. This was done by characterising the biotite surface as a function of fluid composition and measuring dissolution rates. During contact with dilute solutions, the chemical composition of the biotite surface changed dramatically as a function of pH. The rapid release of elements during these experiments was not stoichiometric but was highly pH dependent. A combination of electrokinetic measurements and potentiometric titrations further highlighted the variable composition of the biotite surface by yielding two values for zero points of charge, separated by ~7 pH units. Abiotic dissolution of biotite progressed by the formation of a dissolution front depleted in K Mg, Fe and Al, the extent of which varies spatially and with pH. The presence of the organic ligands, citric acid, oxalic acid and DFOB (desferrioxamine B) slightly enhanced the overall biotite dissolution rate in lightly acidic and near neutral pH conditions. The growth rate of mycorrhizal fungi over the surface of biotite was quantified at two levels of atmospheric CO2, 350 ppm and 1500 ppm. Initial growth rate calculations in the 1500 ppm experiments revealed hyphae to grow at an average of 10 μm d-1. Finally, changes in the biochemistry of fungal hypha were observed using μ-FTIR. Results suggested that biochemical changes present could be related to changes in fungal functionality spatially in future work.