Coupling nickel chemical speciation and isotope ratios to decipher nickel dynamics in the Rinorea cf. bengalensis-soil system in Malaysian Borneo

Abstract: Aims. Rinorea cf. bengalensis is a Ni hyperaccumulator which occurs in Sabah (Malaysia), on Borneo Island, that is able to accumulate considerable amounts of Ni and influences the Ni cycle in surface soil layers, both in terms of Ni concentration and Ni isotopic composition. In this study, the biogeochemical processes underpinning Ni isotopic fractionation in the soil-plant system and the mechanisms regulating Ni homeostasis in R. cf. bengalensis plants were elucidated. Methods: Two specimens of R. cf. bengalensis of different ages and associated surface soils were collected from ultramafic soils in Sabah. Soil mineralogy, Ni concentrations, speciation and isotopic signatures were subsequently determined in plant and soil samples. Results: Nickel in R. cf. bengalensis leaves is mainly complexed with citrate. Soil Ni available fractions have different δ⁶⁰Ni values depending on the Ni bearing phases. Rinorea cf. bengalensis specimens take up lighter Ni isotopes and a pronounced isotopic fractionation within the plant is observed, especially in the young specimen. Conclusions: The results suggest that the observed fractionation in the young plant can be attributable to kinetic effects (lighter isotopes move faster), which become less evident in the older specimen, as Ni is redistributed and homogenized through phloem loading and unloading processes.