The role of phytolith formation and dissolution in controlling concentrations of silica in soil solutions and streams

By integrating the results of recent studies on the concentration and flux of Si in European forest soils (podzols and acid brown soils) with published evidence on the amounts of Si cycled through forest vegetation, we conclude that phytoliths must be the principal immediate source and sink of silica in soil solution, although mineral weathering is the ultimate source. During the growing season, forest vegetation takes up most of the Si made available by weathering of soil minerals and by phytolith dissolution, and converts it into phytoliths. Phytoliths will be the principal source of Si leached from the soil during winter rains and spring snowmelt. This work accounts for the plateau concentrations of Si observed in laboratory equilibrations of Bs horizon soils, for the maximum Si concentration reported for B horizons in the field, for a deep sink for Si in Bs horizons, and for the fact that the concentrations of Si in soil drainage water and in streams all fall in, or below, the range expected for phytolith solubility. Concentrations of Si in surface horizons can exceed these levels during droughts. The common assumption that the concentration of Si in soil solution is a result of a dynamic balance between Si released by mineral weathering and that lost by leaching is untenable for Bs horizons. (c) 2004 Published by Elsevier B.V.