Silicate weathering mechanisms determined using soil solutions held at high matric potential

We present evidence of surface-controlled and proton-promoted chemical weathering of primary silicates in a brown acidic soil (Vauxrenard, Rhône, France). We used aqueous silica (Si) in soil solutions held at high matric potential (180–1600 kPa), which are representative of solutions reacting with soil solids. Si concentration was well correlated with H+ concentration and to a lesser extent with dissolved organic carbon (DOC), which showed a significant affect (P<0.05) only in the surface layer (0–15 cm). Significant negative linear relationships were obtained between log(Si) and pH at the profile scale, at each soil depth and for most sampling dates. We found no significant influence of soil temperature (P>0.05). Geochemical modelling showed that primary silicates dissolved under far-from-equilibrium conditions, and that organic ligands (modelled with a triprotic analogue) may have a weak but significant effect on the variations in log(Si) at the profile scale and at both 15–30 and 30–45 cm depths. Comparison of Si/Al ratios to literature data and observed soil mineralogy demonstrated that significant linear relationships found in the activity diagram between log[Al3+]+3pH and log[H4SiO4°] may not have been caused by the reversible formation of secondary Al–Si phases in the soil. Instead, the apparent trends may arise from relationships between log(Si) and pH and the control of Al-mobility by the reversible formation of Al-hydroxides in the vermiculite interlayer. All these results indicated that active, in situ chemical weathering of silicates may be surface-controlled and mostly proton-promoted. Mineralogy suggested that K-feldspar weathered much faster than albite and white mica, in contrast to the weathering gradient inferred from the mass balance between unweathered and soil material. This was certainly caused by differential changes in mineral reactive surfaces with time.