EFFECTS OF BIOTA ON BIOTITE WEATHERING AND POTASSIUM FLUXES: A MULTI-SCALE APPROACH

Vascular plants and associated microbes affect the nutrient resources of ecosystems by enhancing chemical weathering, lowering nutrient losses and regulating nutrient uptake. This study investigated biotite dissolution and K fluxes from micro- to meso-scales through batch, column and field lysimeter experiments. We hypothesized that biota respond to K limitation by promoting the dissolution of K bearing silicate minerals when no other K sources are available. We employed two approaches K mass-balance, and microscopic observation of mineral surfaces and microbial features. Fluxes were estimated from K concentrations in input and output solutions, and K changes in microbial/plant biomass and exchangeable cation sites of the growth medium. Mineral surface changes and root/microbe-mineral interfaces were studied by scanning electron microscopy, atomic force microscopy and image analysis techniques. Mass balances showed increased K weathering fluxes in ectomycorrhizal/microbial treatments compared to abiotic controls in all experiments, and the addition of vascular plants further increased the K weathering fluxes in the column and field studies. However, without vascular hosts, loss of K to solution and drainage nearly equaled the weathering fluxes, and only a small portion of weathered K partitioned into growth medium and fungal/microbial biomass. Vascular plants in both column and field studies used and retained their K resources more effectively than reference non-vascular systems. Microscopy revealed that surface attachment of fungal hyphae was not important in the batch studies, but mineral-microbe and/or root interactions were essential in regulating the K fluxes in both the column and the field studies. The semi-quantitative estimates of weathered biotite basal surface areas positively correlated to the increased K weathering fluxes, but the morphology changes of flake edges could not be quantified. Estimated areas of biofilm coverage, with embedded fungal hyphae and root hairs, were negatively correlated with K losses in the vascular systems. These results suggest that biota enhance weathering of K bearing minerals under K limitations by utilizing biofilms that localize weathering and nutrient uptake through isolation of the root/microbe-mineral interface from soil solution.