Effect of Mycorrhizospheric Fungal/bacterial/root/mineral Interactions on Chemical Weathering and Nutrient Partitioning in Pine Growth Experiments

It has long been recognized that chemical weathering is driven by geochemical disequilibrium and mediated by biology, and that rhizospheric microbial associations with plant root systems play a key role. In natural soils these systems both enhance the release of nutrients from minerals via chemical weathering, and take up those nutrients for plant growth, in the process partitioning mineral mass into soil solution and other ecosystem pools. Much previous work in geochemistry and agriculture has investigated the relevant physico-chemical processes at interfaces between pairs of mineral, solution, microbe, and root phases; interaction of these processes in natural systems has been difficult to study due to complexity and variability.

We studied the effects of ectomycorrhizal fungus and bacteria associations with a growing pine seedling root system, on mineral surface alteration, weathering fluxes, and the partitioning of weathering products, in simple replicated flow-through column systems. Tree-system soil waters exhibited 20-60% lower concentrations than controls without trees, and the tree systems showed correspondingly smaller drainage losses. Mineral weathering fluxes, which were largely diverted into soil and biomass pools, were greatest in fungus + bacteria treatments. Microscopy showed widespread microbe-mineral attachment via extracellular polysaccharides (EPS) which blanketed etched mineral surfaces. The same phenomena, including suppression of soil-water nutrient concentrations, were observed in the five-order-magnitude larger Hubbard Brook “sandbox” mesocosms growing 15-20 yr old pine trees.

These results are consistent with a mycorrhizosphere chemical-weathering model in which mineral, microbe, solution, and root interfaces coexist in micron to sub-micron proximity inside bacterially generated EPS biofilms. Within these biofilms, acid and ligand generation, mineral dissolution, and plant uptake mass transfers can occur a) over very small distances, and b) in relative isolation from bulk soil water, thereby increasing plant nutrient acquisition efficiency and reducing drainage loss.