Silicon cycle in the soil-plant system : biogeochemical tracing using Si isotopes
Abstract (Summary)Despite the suspected biological imprint on the terrestrial silicon (Si) cycle, plant contribution to the Si continental reservoir is poorly quantified. Within the soil-plant system, aqueous silicon (H4SiO40) can be retrieved from soil solution by plant uptake, clay formation and adsorption onto secondary oxides, or leached out and transferred to stream waters. Two approaches are very promising to trace Si within the soil-plant cycle: the Si stable isotopes and Ge/Si ratio. This thesis aims at quantifying the Si isotopic fractionation induced by plants and soil processes in both controlled (in vitro) and natural conditions (in situ). The model used is a tropical soil-plant system involving a Si-accumulating plant (banana, Musa acuminata Colla, cv Grande Naine) cropped on soils derived from basaltic ash but differing in weathering stage (Cameroon, West Africa). Si isotopic compositions in the different compartments of the soil-plant system were measured by MC-ICP-MS in dry plasma mode with external Mg doping. The analytical method required specific developments, and was validated by an inter-laboratory comparison of reference materials. Plant root uptake of Si, and Si transport within the plant induce an isotopic fractionation quantified in vitro and measured in situ. The plant Si isotopic signature is influenced by soil weathering stage, precisely by soil contents of clay and iron oxide. Soil clay-sized fractions sequestrate light Si isotopes following abiotic fractionating processes of mineral weathering and clay formation, leaving a residual solution enriched in heavy Si isotopes. Biogenic Si, enriched in light Si isotopes, constitutes a Si source for clay formation. Clay-sized Fe-oxides concentrate with increasing weathering and soil development. They selectively adsorb light Si isotopes by surface complexation of monomeric H4SiO40. Silicon transport within the plant produces a Ge/Si fractionation involving Ge sequestration in roots whereas weathering and clay formation induce a selective Ge sequestration in clay minerals, producing an increased Ge/Si ratio with increasing weathering, in excellent agreement with Si isotopic data. In the soil-plant system studied, Si stable isotopes thus trace three major continental processes: plant uptake and phytolith formation, sequestration of Si in clay minerals, and adsorption of Si by Fe-oxides. These biotic and abiotic processes all lead to a progressive enrichment of pore waters in heavy Si isotopes.
Source Type:Master's Thesis
Keywords:si isotopes soil plant cycle
Date of Publication:05/26/2008