Nature and genesis of spodic horizons characterized by inverted color and organic content in a subalpine podzolic soil (Pyrenees Mountains, France)

Podzolic soils showing an inverted sequence between Bh and Bs horizons have been recently described in the French Pyrenees. This paper deals with the spodic characters of two inverted podzol profiles found in the granitic Estibere valley. Taking into consideration a coherent genesis of these soils, we perform this study through a multidisciplinary approach (e.g., pedology, mineralogy). We describe the evolution of the different forms of free iron and aluminum in relation to organic constituents and the mineralogical characteristics of the soil. Spodic criteria indicate that those soils could be classified as podzolic soils. The study of the spodic horizons show the existence of an ochrous horizon "BP1", rich in immobilized forms of iron (i.e., amorphous and partially crystallized forms). It overlies a darker chocolate colored horizon "BP2" rich in organic-complexed aluminum, aluminosilicate gels and humic constituents, particularly humic acids. We explain the organic accumulation in the deep "BP2" as the result of particular seasonal hydrologic processes related to the geomorphologic and climatogenic situations of the soils profiles.