The influence of organic complexation on Ni isotopic fractionation and Ni recycling in the upper soil layers

Abstract The quantification of Ni isotopic fractionation induced by Ni binding to organic acids is a preliminary step to better constrain the mechanisms determining Ni isotopic fingerprint observed in surface soils, waters and plants, as well as the contribution of metal recycling during plant litter degradation. In this study, Ni isotopic fraction induced by reaction with small organic acids, e.g. citric and oxalic acids, and with soil purified humic acids (PHA) was investigated at different Ni-L ratio and pH conditions. The Donnan Membrane Technique was used to separate Ni bound to organic ligands from the free metal. Obtained results highlighted that Ni binding with carboxylic groups produces, in the adopted experimental conditions, a Δ 60 Ni bond-free Alyssum murale , the degradation process did not induce any observable fractionation. On the contrary, during Rinorea bengalensis degradation experiment, a fractionation between Ni leached out in the first 10 days and between 10 and 30 days was observed (Δ 60 Ni 10–30day  = 0.20 ± 0.05‰). The observed fractionation evidenced a heterogeneous distribution of Ni within the leaves, and/or distinct chemical bonding to the leaf cells, and finally suggested the influence of the chemical bonding on Ni isotopic signature. Although a precise quantification of plant contribution on Ni isotopic signature in surface soils and waters is still not reached, our results produced important progress to elucidate the role of organic matter in regulating Ni isotopic fingerprint in surface layers.