Soluble and colloidal translocation of Al, Fe, Si and Mn in an artificially drained French Retisol

Abstract

Soil evolution results from both solute and colloid/particle translocation, but their relative importance in terms of soil formation is still under debate. The genesis of albic material in Retisols represents an ideal case to quantify the relative contribution of these two forms of transport in soil evolution since it results from i) clay eluviation, ii) reductive solubilisation of Fe oxides, and possibly iii) silicate hydrolysis. This issue seems particularly challenging in cultivated Retisols in which detailed analysis of the soil solid phases has demonstrated that artificial drainage and liming alter the genetic pathway of albic material both qualitatively and quantitatively.In the present study, we characterised the chemistry (pH, Eh, Al, Si, Fe and Mn) and dynamics of soil water and its evolution with increasing distance from the drain. Rainfall water, soil solution at water saturation in the E&Bt horizon and drainage water were sampled by an open rainfall collector, piezometers located at different distances from the drain, and an automatic water collector located at the main drain outlet, respectively.An element budget at the profile level revealed that only Si was lost from the soil profile while the leaching losses of Al, Fe and Mn were overcompensated by deposition from the atmosphere. The constant Fe/Al concentration ratios in piezometers and drain waters as well as the breakthrough curves at the drain outlet for individual rainfall events suggested that Fe and Al were exported together and mainly in colloidal form. While this was also partly true for Si, a longer tailing of the Si than the Fe and Al breakthrough curves indicated that Si was additionally transported in soluble form. Lastly, the temporal variation of the Mn concentrations was unrelated to that of the other elements, pointing to the predominance of translocation in soluble form. All these results demonstrated that: i) particle/colloidal translocation is more important than translocation in soluble form in a drained Retisol; ii) reductive solubilisation of Fe and Mn oxides still occurred, in spite of the artificial drainage; iii) processes such as wet and dry deposition, dissolution of quartz or ferrolysis also contribute to element budgets in Retisols, at least at some microsites and in some years.