Silicon isotopic composition of boreal forest vegetation in Northern Sweden

Thorough characterization of the Si isotopic composition of the terrestrial biogenic pool could potentially allow Si isotope information to be used when assessing the relative contributions from biogenic and mineral sources to soil water, plants and Surface waters. In the present Study, the Si isotopic compositions of major biomass components ill a boreal forest in Northern Sweden were investigated, along with the relative contributions from exogenous Si incorporated in the plant structure. This was achieved using chemical purification and high-resolution multi-collector inductively Coupled mass spectrometry (MC-ICP-MS) for the precise and accurate determination of the Si isotopic composition of plants. The technique, which relies on multi-elemental analysis of plant ashes and sample-specific HF-dissolution followed by strong-anion exchange chromatography, allows efficient separation of Si from matrix and interfering elements, while recovering in excess of 99% Si. The long-term instrumental reproducibility, expressed as two standard deviations (2 sigma), for the isotopic reference material NBS28 (n=12) was 0.06%. for delta(29)Si and 0.12%. for delta(30)Si. Results for the analyses of composite plant samples for the eight most prolific species in the boreal forest yielded a Surprisingly homogenous Si isotopic composition, expressed as delta(29)Si (expanded combined uncertainty) and delta(30)Si, ranging from (-0.15 +/- 0.11)parts per thousand to (0.13 +/- 0.06)parts per thousand and (-0.31 +/- 0.08)parts per thousand to (0.22 +/- 0.13)parts per thousand , respectively. Isotopic and elemental analyses of local airborne particulate material suggest that the exogenous Si contribution varies between <1% and >70%. indicating that the potential surface contribution must be considered during Si Uptake studies. The present Study thus provides evidence that thorough appreciation of the forms of Si ill plants is all absolute requirement when assessing the plant impact oil the Si cycle via the difference in dissolution kinetics for phytoliths and lithogenic Si. (C) 2008 Elsevier B.V. All rights reserved.