Genesis and Behaviour of Ultramafic Soils and Consequences for Nickel Biogeochemistry

International audience; Ultramafic outcrops represent less than 1% of the terrestrial surface but their unusual geochemistry makes them a global hotspot for biodiversity. Ultramafic soils are a peculiarity for soil scientists in all climatic zones of the world. These soils lack essential pedogenetic elements: Al, Ca, K, and P. Whereas serpentinites will most likely give birth to Eutric Cambisols with little influence by climate, peridotites will induce an acceleration of weathering processes; this over-expressed weathering is due to their deficiency in Si and Al and lack of secondary clay formation. Soils evolve towards Ferralsols in tropical conditions. Results from isotopic dilution techniques show that Ni borne by primary minerals is unavailable. Secondary 2:1 clay minerals (e.g. Fe-rich smectite) and amorphous Fe oxyhydroxides are the most important phases that bear available Ni. Therefore, smectite-rich soils developed on serpentinite and poorly weathered Cambisols on peridotite (only in temperate conditions) are the soils with highest availability of Ni. Although soil pH conditions are amajor factor in controlling available Ni, the chemical bounds of Ni to containing phases are even more important to consider. Plants take up significant amounts of Ni, and its biogeochemical recycling seems an essential factor that explains Ni availability in the surface horizons of ultramafic soils.