Nickel bioavailability in an ultramafic toposequence in the Vosges Mountains (France)

A serpentinised harzburgite outcrop located in the Vosges Mountains hosts a population of the Ni-hyperaccumulator Thlaspi caerulescens J. & C. Presl. A complete study was undertaken to relate the variability of Ni availability along the ultramafic toposequence to pedogenesis, soil mineralogy and functioning with X-Ray Diffraction, Transmission Electron Microscope observations coupled with Isotopic Exchange Kinetics and diethylenetriamine pentaacetic acid extraction of Ni. The soil profiles ranging from Dystric Cambisol to Hypermagnesic Hypereutric Cambisol were distributed unevenly along the toposequence probably due to geochemical variability of the bedrock and also complex quaternary erosion features. The richest soils were characterised by slight mineral weathering leading to Ni, Cr and Fe accumulation in the B horizons whereas the lowest saturated soils had very low-metal contents. Most soil minerals were inherited from the parent materials and there were only few traces of formation of secondary minerals. Primary minerals ( e.g. serpentine, chlorite) contained low Ni concentrations ( 0.2%) whereas neoformed goethite, mainly in the B horizons of the richest soils, contained up to 4.3% Ni. Ni was probably sorbed onto amorphous Fe oxy-hydroxide particles ( oxalate extraction) rather than incorporated within the crystal lattice of goethite. Ni availability in the B horizon of Hypereutric Cambisols was extremely high and so was the oxalate extractable Fe. At the toposequence level, there was a high level of Ni availability in the least weathered soils and a very low-availability level in the more intensively weathered soils ( strongly acidic pH). Ni availability was unexpectedly positively correlated to pH and was controlled by soil mineralogy and Ni-bearing mineral phases. Ni hyperaccumulation ( above 1,000 mg kg(-1)) by native T. caerulescens was only reached in the Ni-rich soils as a consequence of the local edaphic factors. Ni uptake by T. caerulescens is strongly regulated by Ni availability in soils and therefore related to pedogenesis.