Trace Elements Uptake in Alfalfa Influenced by Rhizobial Inoculation in Soils Naturally High in Nickel

Nickel (Ni) is an essential micronutrient for plants. It is of particular importance in legumes, but it can be toxic in high concentrations. In this research, the uptake and translocation of Ni and other potentially toxic elements (PTEs) were analyzed in alfalfa (Medicago sativa L.) inoculated with thirteen rhizobial strains (Ensifer spp.) grown in two soils that are naturally high in Ni (58 mg/kg and 195 mg/kg and other PTEs (Cr and Pb) over two years. Four cuttings were used in the pot experiment. The values of all tested elements (As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in alfalfa shoots mainly fit within the common concentrations for plants. Significantly higher concentrations of all elements were retained in the alfalfa roots, and the translocation factor (TF) was below 1, with the exception of Cr and Mn (for which the TFs were 1.56 and 1.14, respectively). The Ni concentrations in alfalfa shoot and root varied depending on the strain, soil and cut and ranged from 0.86 to 7.64 mg/kg in shoot dry weight, and from 3.57 to 62 mg/kg in root dry weight, while the TF values for Ni were 0.08–0.81. In the early phase of growth (the first cut in the first year) in both soils, all tested strains reduced the Ni concentrations in alfalfa shoot. Overall, for the whole experimental period, inoculation with strains 4193cs, G-nov, 224, 218, L4 and 4148ss decreased the Ni concentrations in shoots compared to uninoculated plants. The results indicate that rhizobia have the potential to improve alfalfa growth with preferential Ni accumulation in alfalfa roots and, potentially, low translocation in shoots in soils with elevated Ni and other PTE concentrations.