Plant silicon isotopic signature might reflect soil weathering degree

Plants fractionate Si isotopes which provides a useful Si tracer in the Si soil-plant cycle. This study reports plant Si content and Si-isotopic signatures in mature banana plants grown on soils with different weathering degree, but all developed from basaltic pyroclasts in the Mungo area, Cameroon. The delta Si-30 compositions were determined in various plant parts and soil surface horizons by MC-ICP-MS in dry plasma mode with external Mg doping to a precision of +/- 0.15aEuro degrees (+/- A 2 sigma(SD)). The Si-isotopic compositions in banana plants grown on weathered clayey soils (+0.54 +/- A 0.15aEuro degrees) are heavier than on weakly developed soils rich in fresh ash and pumice (+0.02 +/- A 0.15aEuro degrees). The corresponding bulk soils display lower delta Si-30 value in weathered soil (-1.41aEuro degrees) than in poorly developed soil (-0.41aEuro degrees). We suggest that the dissolved Si source for the plant, governed firstly by dissolution of easily weatherable minerals, was isotopically enriched in heavy isotopes through clay formation over long periods. At seasonal to annual time scale, this source is influenced by a combination of following processes: Si adsorption of light isotopes onto Fe oxides, plant Si uptake and recycling in surface horizons. This would provide an isotopically heavier Si source in the more weathered soil since the Fe oxides content increases with weathering. Plant Si-isotopic signature might thus reflect the soil weathering degree. This study further suggests that in addition to weathering processes, rivers isotopic signatures likely depend on the fate of phytoliths in the soil-plant-river system.