『Abstract
　In soils, silicon released by mineral weathering can be retrieved from soil solution through clay formation, Si adsorption onto secondary oxides and plant uptake, thereby impacting the Si-isotopic signature and Ge/Si ratio of dissolved Si (DSi) exported to rivers. Here we use these proxies to study the contribution of biogenic Si (BSi) in a soil-plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping. δ³⁰Si and Ge/Si ratios were determined in bulk soils (＜2 mm), sand (50-2000μm), silt (2-50μm), amorphous Si (ASi, 2-50μm) and clay (＜2μm) fractions: δ³⁰Si by MC-ICP-MS Nu Plasma in medium resolution, operating in dry plasma with Mg doping (δ³⁰Si vs. NBS28±0.12‰±2σSD), Ge/Si computed after determination of Ge and Si concentrations by HR-ICP-MS and ICP-AES, respectively. Components of the ASi fraction were quantified by microscopic counting (phytoliths, diatoms, ashes). Compared to fresh ash (δ³⁰Si = -0.38‰; Ge/Si = 2.21μmol mol^-1), soil clay fractions (＜2μm) were enriched in light Si isotopes and Ge: with increasing weathering degree, δ³⁰Si decreased from -1.19 to -2.37‰ and Ge/Si increased from 4.10 to 5.25μmol mol^-1. Sand and silt fractions displayed δ³⁰Si values close to fresh ash (-0.33‰) or higher due to saharian dust quartz deposition, whose contribution was evaluated by isotopic mass balance calculation. Si-isotopic signatures of bulk soils (＜2 mm) were strongly governed by the relative proportions of primary and secondary minerals: the bulk soil Si-isotopic budget could be closed indicating that all the phases involved were identified. Microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation. δ³⁰Si and Ge/Si values of clay fractions in poorly developed volcanic soils, isotopically heavier and Ge-depleted in surface horizons, support the occurrence of a DSi source from banana phytolith dissolution, available for Si sequestration in clay-sized secondary minerals (clay minerals formation and Si adsorption onto Fe-oxide). In the soil-plant system, δ³⁰Si and Ge/Si are thus highly relevant to trace weathering and input of DSi from phytoliths in secondary minerals, although not quantifying the net input of BSi to DSi.』

1. Introduction
2. Materials and methods
　2.1. Soil materials
　2.2. Characterization of silicon in soils
　　2.2.1. Total analysis and selective extraction of Si from soil
　　2.2.2. Extraction of the amorphous Si fraction from soil
　　2.2.3. Microscopic counting of ASi components
　2.3. Silicon isotopes analyses
　2.4. Ge/Si measurements
3. Results
　3.1. Si distribution in soils
　3.2. Distribution of ASi components
　3.3. Si-isotopic signatures of soil fractions
　3.4. Ge/Si ratios in soils
4. Discussion
　4.1. Variations of δ³⁰Si and Ge/Si ratios in soils with weathering degree
　　4.1.1. Evolution of δ³⁰Si and Ge/Si in clay fractions with weathering
　　4.1.2. Bulk soil isotopic mass balance calculation
　4.2. Contribution of saharian dust quartz to soil
　4.3. Biogenic Si contribution to soils
　　4.3.1. Impact on clay fractions Si-isotopic signatures and Ge/Si ratios
　　4.3.2. Evaluation of banana BSi input to soil
　4.4. Implications on biogeochemical Si cycle in the soil-plant system
5. Conclusions
Acknowledgments
References