Abstract :
[en] Abstract : In terrestrial ecosystems, silicon (Si) uptake by higher plants induces biogenic silica (BSi) deposits in leaves, which contribute to the amorphous silica (ASi) pool in soil through litter-fall. In forests, the ASi pool, including BSi, is ubiquitous and a substantial component of soils, which might influence the Si mass-balance at watershed scale. Here, we examined the distribution of ASi pool, estimated by alkaline dissolution (alkali-extractable Si), in an acid brown soil under three common European tree species in identical soil and climate conditions in order (i) to study how the Si recycling by tree species impacts the ASi pool in soil, and (ii) to identify the different constituents of the alkali-extractable Si pool in soil. We therefore quantified the ASi concentration with alkaline extraction (Na2CO3, 0.1M), the Si adsorbed onto poorly crystalline Fe oxides by oxalate extraction and the “plant-available Si” by CaCl2 extraction. In humus layer, the alkali-extractable Si concentration (mg SiO2 g-1) significantly decreases in the sequence: Douglas fir (14.5±0.65) > European beech (11.8±0.30) > Black pine (5.4±0.31). Below 15 cm soil depth, the alkali-extractable Si concentration is not significantly different between tree species. For each tree species, the alkali-extractable Si concentration in soil decreases from the humus layer to 15 cm depth and then slightly increases from 15 to 75 cm depth. Our data clearly show that tree species can impact the ASi content in topsoil (humus layer - 15 cm) through different Si uptake rates. Indeed, various Si recycling by forest vegetation imply different rates of BSi accumulation in leaves and then, different rates of BSi restitution on topsoil. In mineral layers, pedogenic processes play an important role in the ASi distribution given the alkali-extractable Si pool is mostly influenced by BSi dissolution, stable BSi preservation/translocation and secondarily by Si adsorption onto active amorphous Fe oxide surface.
