Identification and distribution of the readily soluble silicon pool in a temperate forest soil below three distinct tree species

In forest ecosystems, the silicon (Si) mass-balance at the watershed scale can be strongly influenced by readily soluble Si components, such as dissolved Si, adsorbed Si, amorphous silica (biogenic and pedogenic opal) and short-range ordered aluminosilicates. The aim of the present study is to (a) identify the components of the readily soluble Si pool in the Cambisol found below three tree species, under homogeneous soil and climate conditions, and (b) study the impact of Si recycling by tree species on the Si pools. We therefore measured the concentrations of Si extracted by Na2CO3 (Si-alk), oxalate (Si-ox) and CaCl2. The Si-alk concentration decreased from the humus layer to 15 cm depth and then slightly increased until a depth of 75 cm. In the humus layer, the Si-alk concentration consisted mainly of phytoliths and differed significantly between tree species (expressed as mg SiO2 g(-1)): Douglas fir (14.5 +/- 0.65) > European beech (11.8 +/- 0.30) > Black pine (5.4 +/- 0.31). Below 7.5 cm, the Si-alk content did not differ significantly between tree species, and the Si-ox content, increasing significantly, was mainly comprised of Si adsorbed onto amorphous Fe oxides. These results suggest that (a) tree species can impact the readily soluble Si content in the topsoil, through different rates of Si uptake and phytolith restitution by the vegetation, and (b) the soil's readily soluble Si pool is mainly comprised of phytoliths and adsorbed Si. Here, the readily soluble Si pool is thus controlled by both the iron dynamics and Si biocycling.