RHIZOSPHERIC WEATHERING: COLUMN EXPERIMENTS WITH SCOTS AND RED PINE SEEDLINGS

Silicate mineral weathering is a main contributor to soil formation, nutrient uptake in ecosystems, and key to elemental and carbon cycling on geologic time scales. It has been recognized that in soils, plant roots and their associated fungi and bacteria, the rhizosphere, drive silicate mineral weathering both in natural and managed systems. However, quantification of the contribution of rhizospheric weathering to whole weathering budgets of ecosystems is still problematic. The goal of this study was to investigate physical and chemical characteristics of bacteria-fungi-mineral interactions in biofilms of scots and red pine rhizospheres.

In three experiments, scots or red pine seedlings were grown in columns containing silica sand amended with biotite and calcium-feldspar as the sole sources of potassium and calcium. The seedlings were inoculated with pure culture of ectomycorrhizal fungi or a soil slurry. Uninoculated seedlings and unplanted abiotic columns were controls. The columns were irrigated with deionized water, and periodically received potassium- and calcium-free nutrient solutions. After 9 months the columns were destructively sampled. The minerals and the roots were investigated with light, scanning electron, and atomic force microscopy. Potassium and calcium release rates by weathering were determined from cation concentrations of input and output waters, initial and final seedling biomass and exchange sites, then normalized to geometric surface area of minerals in each column.

The results showed evidence that various ectomycorrhizal fungi species stimulate silicate weathering, but biofilm formation was not enhanced as a result of potassium and calcium limitation. Direct surface attachment and etching by fungal hyphae were observed, but remained a minor contributor to overall weathering. Our data suggest that the main benefit of ectomycorrhizal fungi in the rhizosphere of pine may come from regulating water utilization and indirectly affecting the nutrient uptake. However, new experiments are needed to fully evaluate this and potential species-specific interactions of trees and fungi in weathering.