Organo-metallic complexes in an Andosol: a comparative study with a Cambisol and Podzol

Current soil-forming processes were studied in a nonallophanic Andosol from the Vosges mountains (north-eastern France). For this purpose, we conducted comparative studies on the organo-metallic fraction dynamics of selected profiles of an Andosol and other typical soils from this region, Cambisol and Podzol. These soils are derived from old volcanic rocks under mesic soil temperature and udic soil moisture regimes. The Andosol profile displayed the characteristics of nonallophanic Andosols: large accumulation of humus, active Al mostly complexed with humus, and low pH. Special attention was given to physical and microbial stability of these organo-metallic associations, respectively by disaggregation tests and incubation experiments. The results indicated that these associations showed a strong resistance against disaggregation by water and against biodegradation. Moreover, the very low concentration of DOC, Al and Fe in soil solutions collected by zero-tension lysimeters, coupled with very low amounts of water-dispersed clays obtained after dispersion tests, suggested that no significant translocations of these components occurred in this Andosol profile. Low mobility together with a strong resistance against mineralization may account for the large accumulation of carbon in this Andosol.