Cornelis,J.-T., Delvaux,B., Cardinal,D., Andre(eの頭に´),L., Ranger,J. and Opfergelt,S.(2010): Tracing mechanisms controlling the release of dissolved silicon in forest soil solutions using Si isotopes and Ge/Si ratios. Geochimica et Cosmochimica Acta, 74, 3913-3924.

『ケイ素同位体とGe/Si比を用いて森林土壌溶液における溶存珪素の放出をコントロールするメカニズムを突き止める』


Abstract
 The terrestrial biogenic Si (BSi) pool in the soil-plant system is ubiquitous and substantial, likely impacting the land-ocean transfer of dissolved Si (DSi). Here, we consider the mechanisms controlling DSi in forest soil in a temperature granitic ecosystem that would differ from previous works mostly focused on tropical environments. This study aims at tracing the source of DSi in forest floor leachates and in soil solutions under various tree species at homogeneous soil and climate conditions, using stable Si isotopes and Ge/Si ratios. Relative to granitic bedrock, clays minerals were enriched in 28Si and had high Ge/Si ratios, while BSi from phytoliths was also enriched in 28Si, but had a low Ge/Si ratio. Such a contrast is useful to infer the relative contribution of silicate weathering and BSi dissolution in the shallow soil on the release of DSi in forest floor leachate solutions. The δ30Si values in forest floor leachates (-1.38‰ to -2.05‰) are the lightest ever found in natural waters, and Ge/Si ratios are higher in forest floor leachates relative to soil solution. These results suggest dissolution of 28Si and Ge-enriched secondary clay minerals incorporated by bioturvation in organic-rich horizons in combination with an isotopic fractionation releasing preferentially light Si isotopes during this dissolution process. Ge/Si ratios in soil solutions are governed by incongruent weathering of primary minerals and neoformation of secondary clays minerals. Tree species influence Si-isotopic compositions and Ge/Si ratios in forest floor leachates through differing incorporation of minerals in organic horizons by bioturbation and, to a lesser extent, through differing Si recycling.』

1. Introduction
2. Materials and methods
 2.1. Experimental site
 2.2. Sampling and physico-chemical characterization
  2.2.1. Soil and plant samples
  2.2.2. Solution samples
 2.3. Si and Ge analyses
 2.4. Si isotope analyses
3. Results
 3.1. Soil fractions
 3.2. Chemical composition of solutions
 3.3. Ge/Si ratios
 3.4. Si-isotopic compositions
4. Discussion
 4.1. δ30Si and Ge/Si ratios in solid samples
 4.2. δ30Si and Ge/Si ratios variations in solutions
 4.3. Implications on biogeochemical Si cycle in forest ecosystem
5. Conclusions
Acknowledgments
References


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