Tracing nickel uptake pathways in hyperaccumulator plants using an isotopically enriched soil amendment

Short abstract
Hyperaccumulator plants are capable of acquiring exceptional contents of metals. Yet, the processes enabling Ni mobilisation remain unclear. In this work, 61Ni-enriched saponite was synthesised and employed as a tracer to quantify Ni contributions from a defined mineral phase. In rhizotests with Odontarrhena chalcidica, natural and 61Ni-enriched saponite were added to two ultramafic soils. Ni uptake and bioavailability were assessed by elemental and isotopic analysis of plant digests and diffusive gradients in thin films (DGT) using inductively coupled plasma mass spectrometry (ICP-MS). Isotope pattern deconvolution (IPD) revealed distinct isotopic shifts in plant and DGT samples, enabling source attribution even at tracer levels down to 10 fg 61Ni per g dry biomass. The results provided an indication for increased Ni mobilisation from the amendment by plants grown on soil with lower Ni contents.
Full abstract
Metal hyperaccumulator plants are capable of acquiring element contents in their aboveground tissues that are 100 to 1000 times higher than those found in non-accumulating species, without exhibiting symptoms of toxicity. While understanding the mechanisms of metal uptake in hyperaccumulating plants is essential for advancing sustainable soil management, the details of the rhizosphere processes that govern the increased mobilisation from soil are still a matter of debate. The central objective of this work is to evaluate the use of synthetic 61Ni-enriched saponite as a tracer for nickel (Ni) mobilisation from a defined clay mineral phase.
Saponite amendments containing either natural or 61Ni-enriched Ni were synthesised and applied in a Rhizotest experiment on the model of Odontarrhena chalcidica (Alyssum murale), a plant species well-characterised for its Ni hyperaccumulation properties. The saponites were added to two ultramafic soils with different Ni contents. A control included the amendment of ground natural serpentine rock instead of synthetic saponite. The setup comprised multiple soil treatments with and without plant growth to assess the role of root-induced processes in Ni mobilisation. Ni bioavailability and uptake were assessed by elemental and isotopic analysis of plant digests and diffusive gradients in thin films (DGT) using inductively coupled plasma quadrupole mass spectrometry (ICP-QMS). Isotope pattern deconvolution (IPD) was employed as data reduction tool to determine the contribution of the soil amendments to overall Ni in the plants. The method allowed for deconvolving the pattern from measured isotope ratios without requiring prior knowledge of the quantities of different Ni sources incorporated, even at tracer levels down to 10 fg 61Ni per g dry biomass.
While DGT results revealed soil-dependent differences in labile Ni concentrations, no significant plant-induced mobilisation was observed during the 14-day growth period from the elemental data. In contrast, IPD revealed shifts in Ni isotopic composition both in plant and DGT samples, indicating mobilisation of 61Ni from the spiked saponite dependent on the Ni content of the soil. This is corroborated by the fact that the tracer uptake was higher in the soil with lower background Ni.
The presented methodology supports research on sustainable soil remediation by delivering a highly sensitive analytical tool for tracing plant Ni uptake pathways using a 61Ni isotope spike. The approach enables the differentiation of Ni contributions from various soil sources, allowing for source assignment that would not be achievable with quantification alone. The tracer amounts can be reduced to levels that are negligible with respect to plant physiology and soil chemistry, while still providing sufficient isotopic contrast to resolve enrichment levels below 0.02% in plant tissue.