GRASS AND MICROBIOTA EFFECT ON LITHOGENIC ELEMENT MOBILIZATION DURING WEATHERING OF BASALT, GRANITE, RHYOLITE, AND SCHIST

The goal of this project was to measure how plant–microbe interactions affect the initial weathering of different rock types and the extent to which this weathering results in chemical denudation and biomass accumulation. The mesocosm experiment was conducted for a period of twenty months using four porous rock types, basalt, rhyolite, granite, and schist; and four biological treatments: abiotic control, microbial only treatment, Buffalo grass (Bouteloua dactyloides) and Buffalo grass with arbuscular mycorrhiza (Rhizophagus irregular). All treatments have been inoculated with the same microbial consortium derived from basalt, but inoculum for abiotic control was sterilized. Before introduction into the mesocosms all rocks were cleaned, crushed, sieved to 250-500 µm size, and sterilized. They were also characterized for total elemental and mineral composition. Rocks with plants and microorganisms were placed in 30 cm long and 5 cm in diameter Plexiglas columns inside transparent boxes with filtered air to prevent outside contamination. Columns were regularly watered using purified water and pore water solution was collected and analyzed for pH, conductivity, organic and inorganic C, total N, anions, major, trace and REE elements. Every four months, columns were sacrificed and plants harvested. Dry biomass, mycorrhizal infection rate, and lithogenic element content were determined for the plants. Changes in mineralogical and chemical composition of the rock were also evaluated. Fine fraction (particles <250 µm formed as a result of incongruent weathering) was extracted, quantified and characterized using X-ray diffraction. Plants developed large root systems and for some major elements, such as P, Fe, Mn, Al, and Ti, as well as many trace and all rare earth elements, plant biomass presented a greater pool of mobilized metals than soil solution. Biological, both plant and mycorrhiza, effects differed across rock types and individual elements but in general, increase in total element mobilization (in solution and plant) was observed in the presence of biota. However, denudation often decreased when plants were present.