Influence of Ni-hyperaccumulating trees on nickel biogeochemical cycle in a soil-plant system of New-Caledonia

Ultramafic (UM) soils are of particular interest due to their high content in metals for example Fe, Mn but also in Ni, Co, or Cr up to the ore grade (Butt and Cluzel, 2013). Those high metal contents combined with low contents of plants essential nutrients (Ca, K and P) imply particularly stressful conditions for the vegetation. To take advantage on these specific edaphic conditions, few plant species growing on UM soils have developed ecophysiological strategies including metal hyperaccumulation (Reeves, et al. 2018). Hyperaccumulation implies efficient metal mobilization at the soil-plant interface, i.e. roots, and the transfer to the different aerial organs of plants, which can lead to significant concentrations of metal in stems, sap, latex, and leaves. As example, for Ni, these concentrations can reach up to the percent level, while most plants contain less than 15 µg/g (dry mass) of Ni in their tissues (Brooks et al., 1977). This behaviour is expected to increase Ni-phytobioavailability by litter degradation and complexation of metal with organic ligands in the upper horizon of UM soils (Boyd and Jaffré, 2001; Zelano et al., 2020). This physiological process is also suspected to modify Ni isotope ratios due to absorption, transport and storage in the plant. However, the extent of Ni isotope fractionation in UM soils due to hyperaccumulators remains unclear and debated. While Zelano et al. (2020) suggested that the Ni sequestration by hyperaccumulators and its redistribution in the aerial organs of the plant could hinder Ni isotope fractionation in old individuals, Ratié et al. (2019) reported a preferential uptake of light isotope by roots in soils and Ni fractionation during translocation to the aerial part of the plants leading to heavier isotopic composition in soils.

The present study focuses on Ni-hyperaccumulation Pycnandra acuminata tree, endemic to New Caledonia. To understand the impact of Ni-hyperaccumulating plants on the Ni biogeochemical cycle, twelve soil profiles have been identified in the rainforest of Grande Terre including six profiles developed in the close vicinity of Ni-hyperaccumulating trees P. acuminata and six other profiles developed in the close vicinity of Pycnandra fastuosa, a non-hyperaccumulating tree also endemic in New Caledonia. Nickel concentrations found in hyperaccumulator-soil systems are higher relative to the non-hyperaccumulator-soil systems revealing the influence of P. acuminata and the associated leaves degradation on Ni redistribution in ultramafic soils. Ni isotope compositions and XAS spectroscopy of soil samples will help us to reveal the biogeochemical processes controlling the Ni isotopic signature in UM soils. Although focalized on New Caledonia, our study can be considered representative of the influence of hyperaccumulating trees on the biogeochemical cycle of Ni in UM soils systems worldwide.

 

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