Processes controlling silica concentration in leaching and capillary soil solutions of an acidic brown forest soil (Rhone, France)

Chemical analysis of leaching and capillary soil solutions collected at different soil depths was performed on a monthly basis for several years at the Vauxrenard site (Rhone, France). The seasonal variations in dissolved silica (Si) indicated considerable differences whether contained in leaching or capillary soil solutions. In capillary solutions, the maximum and minimum Si concentrations occurred in the summer and winter, respectively, while the opposite trend was observed with leaching soil solutions. In both solutions types, significant relationships may be obtained between Si concentration and soil temperature (T) and, to a lesser extent, W concentration. Evaporation or evapotranspiration had little effect on Si in capillary solutions, limited to the upper soil layers. An inverse relationship between Si and T found in leaching solutions indicated that weathering did not control Si concentration. In contrast, a positive relationship between Si and T found in capillary solutions was consistent with this process. This was reinforced by a significant relationship obtained between logSi and pH, which was consistent with surface-controlled and proton-promoted weathering. Calculated apparent activation energy and reaction order with respect to pH were both consistent with muscovite at the laboratory scale. It is suggested that Si concentration in leaching solutions was controlled mainly by diffusion of aqueous silica (essentially orthosilicic acid) from capillary solutions in relation to soil drainage. Thermodynamic calculations showed that the temperature-dependence of the solubility of Si-containing secondary phases did not significantly control Si concentration in both soil solution types. However, it was calculated that the reversible formation of some hypothetical siliceous phases (Si/Al > 1) proceeded at relatively slow rates, thus limiting their impact on Si concentration. Kinetic calculations showed excellent results by correlating Si concentration in capillary solutions to specific weathering rate for primary soil silicates. In agreement with most of the statistical analysis, soil temperature appeared to be the main driving force for chemical weathering. Protons (H+) had a significant influence in the deeper soil horizons as well as in the seasons corresponding to lesser soil temperature variations. An important effect of organic ligands and particularly of low molecular weight compounds on weathering may explain larger Si concentration observed in the upper soil layers. (C) 2002 Elsevier Science B.V. All rights reserved.