The effect of pedological conditions on the sources and sinks of silicon in the Vertic Planosols in south-western Ethiopia

Abstract

As a dynamic source and sink of elements, the secondary Si-bearing solid phases in soil partly govern transfer of elements and pollutants. Terrestrial plants affect silicate mineral weathering and make Si readily soluble through the formation of phytoliths, a biogenic form of amorphous silica, which return on topsoil through litterfall. Besides phytoliths, other readily-soluble Si components, such as pedogenic opal, volcanic glass, adsorbed Si onto Fe oxides and poorly crystalline aluminosilicates take part in the regulation of the Si balance. The aim of this paper is to assess the impact of pedological conditions on the sources of dissolved Si and its dynamics in a Vertic Planosol characterized by contrasting bleached and vertic horizons. The Si-containing readily soluble phases were quantified by specific extractions (Na2CO3: Sialk, oxalate: Siox, DCB: Sidcb and CaCl2: SiCaCl2) performed on samples of the bleached and vertic horizons. Moreover, amorphous silica (ASi) from the soil (including phytoliths and volcanic glass) was extracted by gravimetric separation. In the bleached horizon, phytoliths are the main Si-bearing readily soluble phases of the silt-sized fraction, while Si in the sand-sized fraction is largely controlled by volcanic glass and feldspars. The higher SiCaCl2 concentration and the significant increase of (Si/Al)CaCl2 ratios in soil solution of the vertic horizon attest for a marked difference in terms of Si dynamics. The activity of dissolved Si as H4SiO4 in the aqueous phase of the Vertic Planosols in south-western Ethiopia seems to be controlled in the bleached E horizon by the balance between the dissolution of feldspars and volcanic glass, active uptake by grasses (formation of phytoliths), neoformation of kaolinite and intense lateral leaching. In the vertic Bw horizon the sources and sinks of Si are governed by the dissolution of phytoliths, the adsorption on and desorption from Fe oxides and slow leaching. Our results clearly indicate that the contrasting pedological conditions in the bleached and vertic horizons impact the sources and sinks of Si in the Vertic Planosols in south-western Ethiopia, and as such could partly control terrestrial Si fluxes.