Variations of delta Si-30 and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture

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. delta Si-30 and Ge/Si ratios were determined in bulk soils (<2 mm), sand (50-2000 mu m), silt (2-50 mu m), amorphous Si (ASi, 2-50 mu m) and clay (<2 mu m) fractions: delta Si-30 by MC-ICP-MS Nu Plasma in medium resolution, operating in dry Plasma with Mg doping (delta Si-30 vs. NBS28 +/- 0.12 parts per thousand +/- 2 sigma(SD)), 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 (delta Si-30 = -0.38 parts per thousand; Ge/Si = 2.21 mu mol mol(-1)), soil clay fractions (<2 mu m) were enriched in light Si isotopes and Ge: with increasing weathering degree, delta Si-30 decreased from -1.19 to -2.37 parts per thousand and Ge/Si increased from 4.10 to 5.25 mu mol mol(-1). Sand and silt fractions displayed delta Si-30 values close to fresh ash (-0.33 parts per thousand) or higher due to saharian dust quartz deposition, whose contribution was evaluated by isotopic mass balance calculation. Si-isotopic Signatures of bulk soils (