Speciation analysis of nickel in soil solutions and availability to oat plants

Soil solutions were collected for speciation analysis of nickel from a pot experiment with oats. Oat plants (Avena sativa L.) were grown on 3 soils differing in total amount and origin of nickel (Ni) (Luvisol, LS with 28 mg kg^-1; sludge amended Luvisol, LS+SS with 32 mg kg^-1; Cambisol, CS with 95 mg kg^-1). Results were compared with those for soil solutions obtained from corresponding unplanted pots. Separation methods were used for characterization of size and charge distribution and stability of the Ni species. In addition, short-term experiments were performed on the uptake rates of Ni by oat plants from the different soil solutions as well as from nutrient solutions with increasing concentrations of a synthetic chelator.The Ni concentrations in the soil solutions of unplanted soils increased in the order: LS5000 g mol^-1) was the predominant form, whereas in the other soils the low-molecular-size cationic and chelated Ni species (500–1000 g mol^-1) dominated in the soil solution. In the short-term uptake studies, the uptake rates of Ni from the solutions decreased in the order: nutrient solution > soil solutions, and in the latter in the order: LS>LS+SS>CS, which was inversely related to the concentrations of dissolved organic carbon in the soil solutions.The results demonstrate that Ni availability to plants is not only affected by total concentration of Ni in the soil solution and the rate of replenishment from the solid phase, but also by Ni species, which can differ considerably between soil types.