Factors controlling Si export from soils: A soil column approach

•We performed leaching tests on A and B horizons of forest and cropland soils to evaluate Si export.•A horizons released higher amounts of Si than B horizons.•Forest soils released higher amounts of Si than cropland soils.•The amount of biogenic Si, soil acidity and water flux controls the Si release of soils.

The release of dissolved silicon (DSi) from A and B horizons was investigated with leaching tests on unsaturated columns. As forest A horizons have larger biogenic Si (BSi) pools than arable lands, we compared the Si release from a forest and a cropland from the same geographical region developed on a Luvisol in Belgium and a Cambisol in Sweden. The A horizons released a quickly dissolving Si fraction in contrast to the B horizons, which did contain no or only little amounts of BSi and released lower Si concentrations. Our experiments show that Si export from forest soils is high because of the presence of a large reservoir of soluble BSi as well as due to the acidity of the soil (pH < 4). Leaching at two different water fluxes revealed that export in forest soils was transport controlled while cropland soils were in equilibrium.