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Transfer of trace elements, such as toxic metals, from soil to plant is a corner stone for risk assessment. Rare earth elements (REE) are frequently used as environmental tracers to understand biogeochemical processes in the soil–plant system. In this study, we combined trace element and REE measurements in the soil–plant continuum to evaluate the element transfer between different compartments. We specifically aimed at: (1) assessing the geochemical relevance and representativeness of intermediate compartments (soil solution and soil water-extract as a proxy of the bioavailable soil fraction) by comparing the REE normalized patterns; and (2) characterizing the environmental conditions that control the trace element transfer by quantifying the REE indices. For that purpose, we compared geochemical signatures in an intercropping cover crop (bean, Persian clover, and spelt) in Belgium, including soil, root, shoot, soil solution, soil water-extract, earthworm, and snow samples. Evaluation of the element mobility was performed using both soil extractability and transfer factors. The main result showed dissimilar REE patterns between soil/plant samples and soil solution/soil water-extract samples, indicating that the intermediate compartments (i.e., soil solutions or soil water-extracts) do not chemically represent the bioavailable fraction of elements without obvious propensity to biological accumulation (unlike Cd, Cu, or Zn). Compared to light REE, heavy REE were more extractable and thus transferred to plants unlike what is observed in the literature. According to their different extractabilities, Ce and Eu allowed to highlight distinct transfer from soil to plant due to possible adsorption or organic matter complexation that should be further confirmed by studying contrasted soils.

Purpose Transfer of trace elements from soil to plant is a corner stone for risk assessment. Rare earth elements (REE) are frequently used as geochemical tracers for element transfer to plant. In this study, we evaluated biogeochemical behaviors of trace elements and REE in the soil–plant continuum to assess to what extend the REE monitoring allows identifying preferential pathway from soil to plant. Methods We quantified 11 trace elements and 14 REE in soil, root, shoot, soil solution, and soil water-extract samples of an intercropping cover crop. Evaluation of the element mobility was performed using both soil extractability and transfer factors. Results Results showed distinct soil–plant transfer of trace elements according to their biological roles or substitution potential. Soil, root, and shoot REE patterns were similar, indicating a large influence of soil REE composition on plant one. Compared to light REE (LREE), heavy REE (HREE) were more extractable and thus transferred to plants. We also observed differences in Ce and Eu behavior concerning soil extractability and transfer from root to shoot. Conclusion Soil solution and soil water-extract samples are not chemically representative of the fraction that can be uptaken by plants. However, LREE/HREE and Ce/Eu ratios in soil solution and soil water-extract samples can be used as indicators of environmental conditions.

Rapid permafrost thaw, exposing organic matter (OM) to decomposition, is also responsible for mineral alteration and nutrient release from previously perennially frozen materials. Ice-rich permafrost thaw creates local landscape degradations (subsidence) known as thermokarst structures, resulting in development of distinctive landforms. Two different types of structures are studied: an Active Layer Detachment (ALD) which is a one-time event, and a Retrogressive Thaw Slumps (RTS) which repeats annually during summer months. In the Cape Bounty Arctic Watershed Observatory (Canada), the total elemental content and mineral nutrient released from an ALD formed in 2007 and a RTS still very much active were compared. For the disturbed areas, results show an increase in total elemental content (for Al, Ca, Fe, K) with depth as compared to the undisturbed site, as well as an increase in plagioclase content. The mineral nutrient concentration released is also several times higher (Ca : 5 ; Na : 8 ; Mg : 5) for the RTS disturbed site compared to the undisturbed site. For ALD sites, there was no significant difference between the disturbed and the undisturbed areas. We hypothesize that the one-time ALD event was followed by a seasonal depletion of the total mineral nutrient content as well as the soluble mineral nutrient concentrations in disturbed and undisturbed soils since 2007. In contrast, RTS structures expose perennially frozen materials every summer after thaw and degradation. This study suggests that RTS development in the Arctic might constitute an important driver for mineral nutrient release by permafrost upon thawing, which might affect local to regional ecosystem chemistry. The data are also considered in the context of potential changes in organic carbon stability upon disturbance