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

Knowledge of the bottomland hydric functioning in humid zones is important in order to estimate denitrification that takes place in anaerobic conditions. The objective of this study was to point out the temporal dynamics of soil cover redox conditions for different geological substrates and to compare these dynamics to soil morphological features. The survey of the seasonal 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 localization of Fe(II) volumes in the lower parts of hillslopes within the 'Alluviosols', 'Luvisols reductic' and 'Luvisols redoxic' domains. The extension of 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 without Fe(II) more developed on granite. The temporal variations of reducing conditions were higher on granite than on schist. In addition, in order to better interpret morphological features of hydromorphic soils, we examined the duration of reducing conditions. (C) 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS