Morphological features and temporal variations of Fe(II) presence in Armorican Massif soil covers. Example of two hillslopes developed on granite andschist.

Knowledge of the bottomland hydric functioning in humid zones is important in order to estimate denitrification that takes place in anaerobic conditio ns. The objective of this study was to point out the temporal dynamics of s oil cover redox conditions for different geological substrates and to compa re these dynamics to soil morphological features. The survey of the seasona l variations of Fe(II) presence along loamy hillslopes developed on granite and schist, the two main substrates in the Armorican Massif, was achieved over two years. Results evidenced in both situations a preferential localiz ation of Fe(II) volumes in the lower parts of hillslopes within the 'Alluvi osols', 'Luvisols reductic' and 'Luvisols redoxic' domains. The extension o f reduced volumes, which presented stronger temporal variations on granite, was maximal within the surface organo-mineral horizons and saprolites. The contact of these volumes with the channel network was marked by a fringe w ithout Fe(II) more developed on granite. The temporal variations of reducin g conditions were higher on granite than on schist. In addition, in order t o better interpret morphological features of hydromorphic soils, we examine d the duration of reducing conditions. (C) 2000 Academie des sciences / Edi tions scientifiques et medicales Elsevier SAS