Intensive cycling of nickel in a New Caledonian forest dominated by hyperaccumulator trees

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Adrian Paul, Peter Erskine, Alan Baker and Antony van der Ent, Sustainable Minerals Institute, University of Queensland, Brisbane, QLD, Australia, Sandrine Isnard, AMAP, Université de Montpellier, Montpellier, France, Sandrine Isnard, AMAP, Herbier de Nouméa, Nouméa, New Caledonia, Christine Wawrick and Jason Kirby, Land and Water Business Unit, CSIRO, Urrbrae, SA, Australia, Guillaume Echevarria, Alan Baker and Antony van der Ent, Laboratoire Sols et Environnement, Université de Lorraine, Nancy, France, Alan Baker, School of BioSciences, The University of Melbourne, Parkville, VIC, Australia

Background/Question/Methods
New Caledonia is a biodiversity hotspot with a high level of endemism resulting from the local broad range of ecological niches. Nickel hyperaccumulator plants represent 3.8% of the archipelago dicotyledonous plant diversity and include Pycnandra acuminata, an endemic ultramafic rain forest tree known to have the highest concentration of Ni in any living organism (25 wt% in its latex). Rain forests in New Caledonia evolved on some of the most nutrient impoverished soils globally and are some of the slowest-growing tropical forests known. The study aimed to determine the extent to which large Ni hyperaccumulator trees can alter the biochemical cycling of trace elements. All trees and soil profiles in a 0.25-hectare permanent plot were sampled to assess the biogeochemical compartmentalisation of trace elements (Co, Mn, Ni and Zn) in a dense stand of Pycnandra acuminata trees. Two approaches were used; the first was based on the interpolation of soil and plant analytical data to evaluate the influence of P. acuminata on the ecosystem; and the second used the stable Ni isotope ratios to examine the distribution and fate of Ni in this soil-plant system.
Results/Conclusions
Nickel stable isotope analyses identified the biotic origin of the nickel in the soil upper layers with P. acuminata shoots enriched in lighter nickel isotopes. The δ60Ni latex signature suggests that long-distance transport, radial xylem and phloem loading are at play in P. acuminata. Elemental allelopathy does not appear likely in this ecosystem. We proposed the existence of an extremely tight Ni biogeochemical cycling suggesting a possible side effect of efficient recycling of K and P. Further studies should focus on a mechanistic understanding of Ni fluxes in similar systems by measuring additional parameters via sophisticated sampling regimes and the use of advanced analytical methods including stable isotope techniques.