WEATHERING UNDERCOVER: MICROBE-MINERAL INTERACTIONS IN THE RHIZOSPHERE OF SCOTS PINE

Silicate minerals are the primary source of base cation nutrients in soils and both bacteria and fungi play a role in mineral dissolution, nutrient acquisition and translocation to plants. Formation of rhizospheric biofilms on soil mineral surfaces in a “three-way symbiosis” suggests an important regulatory tool to increase chemical weathering and decrease chemical denudation (loss) concurrently. Understanding the processes, functions, roles and significance of the rhizospheric biofilms are poor. The goal of this study is to investigate the physical and chemical characteristics of mineral-microbe interactions in biofilm cover on mineral particles that were collected from the rhizosphere of Scots pine. Scots pine (Pinus Sylvestris) seedlings were grown in columns containing silica sand amended with biotite and anorthite, which were the sole sources of potassium, calcium and magnesium in the columns. The seedlings were inoculated with three species of ectomycorrhizal fungi and bacterial colonization was not prevented. Unplanted columns served as controls. At 3, 6 and 9 months, a subset of the columns was destructively sampled. Biotite and anorthite surfaces were analyzed using election microscopy and spectroscopy techniques. Potassium, calcium and magnesium mass-fluxes were also calculated for the columns based on cation concentrations in input and output waters, on exchangeable cation sites of the growth medium, and in plant biomass. Microscopy showed that the fungal partner of the pine is the main interface between plant and minerals, bacteria are present along fungal hyphae and biofilm develop in patchy formation. Percentage of colonization and biofilm coverage of the mineral surfaces increased over the course of the experiment. The output solution pH decreased to about 4 by the 9^th week and remained low for rest of the experiment. The cation mass-fluxes were high, 0.6 mol m^-2yr^-1 for Ca and Mg, and 0.2 mol m^-2yr^-1 for K, at 3 months and decreased by 8 and 5 fold, respectively, at 9 months. Results suggest that biofilms may help to mediate mineral-microbe-root chemical exchanges by isolating them from bulk soil water transport and providing suitable microenvironments. Thus, the “three-way symbiosis” developed in biofilms may hold the keys to regulation of nutrient cycling in ecosystems.