Assessment of phytoavailability of nickel in soils

Methods are needed for the rapid characterization of the phytoavailable fraction of trace elements in soils to assess the risk of contamination of the food chain and evaluate the impact of waste management practices. This investigation was undertaken to study the phytotavailability of Ni in soils using the isotopic exchange method. Isotopic exchange of Ni-63(2+) ions was studied in two soils, a silt loam and a clayey muck, and the isotopic composition of Ni in the soil solution was determined. Plant tests were conducted on the same soils amended with Ni-63(2+), and the isotopic composition of Ni in the plant tissues was also measured. The isotopic composition of Ni in soil extracted by DTPA at the end of the culture was also determined. Results showed that the isotopic composition of Ni in the soil solution was identical to the isotopic composition of Ni taken up by plants during the same time. The Ni in the plant came exclusively from the pool of the isotopically exchangeable Ni of the soil. Also, under these experimental conditions, DTPA extracted mainly isotopically exchangeable Ni, reinforcing the validity of this chemical to assess the phytoavailability of Ni. Besides, the phytoavailable soil Ni could be characterized with the intensity, quantity, and capacity factors deduced from isotopic exchange kinetics. The kinetics for which parameters were obtained from short-term experiments were successfully extrapolated to longer times of exchange corresponding to plant growth which demonstrated that isotopic exchange kinetics is an appropriate tool to assess the truly phytoavailable Ni in soils. This technique requires more data from many different soils before it can be used for routine measurements.