『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