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 oxyÀhydroxide 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 Nibearing mineral phases. Ni hyperaccumulation (above 1,000 mg kg À1 ) 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.
Nickel, a potentially toxic metal, is present in all soils with an average concentration of 20 to 30 mg/kg, sometimes exceeding 10,000 mg/kg (e.g., ultramafic soils). The ecotoxicological risk of Ni in soils to organisms is controlled by its availability. It is therefore essential to identify an efficient and reliable method for the evaluation of this risk. This paper presents a complete study of the effect of Ni origin, localization, and soil properties on its availability as assessed with the isotopic exchange kinetics (IEK) method and compares plant response to isotopically exchangeable properties of Ni in soils. We performed IEK on 100 soil samples representing a worldwide range of Ni fate, and concentrations showed that pH was the main influencing parameter and that labile Ni (i.e., isotopically exchangeable Ni, Et) could be reasonably well assessed by a single diethylene triamine pentaacetic acid extraction. The identification of the soil mineral phases that bear Ni (bearing phases) in 16 Ni-rich samples selected among the 100 soils showed a strong effect of the mineralogy of the bearing phases on Ni availability (IEK). Plants with different Ni accumulation strategies all took up Ni from the same labile pool of Ni in four contrasting soils, and the amount taken up by hyperaccumulator plants could be anticipated with the IEK parameters, thus confirming the usefulness of isotopic dilution methods for risk assessment.
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