Nickel transformation in two calcareous soil textural classes as affected by applied nickel sulfate

Sequential extraction has been used as a suitable method for identification of chemical forms of trace elements and their plant availability. However, in calcareous soils of Iran little attention has been paid to studying the capacity of soil components for nickel (Ni) retention and the change in its chemical forms as affected by time. The objective of this study was, therefore, to determine the effect of incubation time, soil texture, and application of different rates of Ni on chemical forms of Ni. The textural class of a clay loam calcareous soil [Fine, mixed (calcareous), mesic Typic Calcixerepts] was converted to sandy loam by adding acid-washed pure quartz sand and both the original clay loam and the produced sandy loam were treated with 0-60 mg kg(-1) Ni as nickel sulfate. The treated soils were placed in plastic containers and incubated at 25 +/- 2 degrees C near field capacity moisture. After 0, 1, 2, 4, 8, and 16 weeks, a sequential extraction scheme was used to fractionate Ni of incubated samples into soluble + exchangeable (Sol+Ex), carbonate-bound (Car), organically-bound (ONi), Mn-oxide-bound (MnOx), amorphous Fe-oxide-bound (AFeOx), crystalline Fe-oxide-bound (CFeOx), and residual (Res) forms. Results showed that, although soils under study were calcareous in nature, the amounts of Car were negligible in both native and treated soils. This can be attributed to the lack of sufficient dissolution capacity of 1 M NaOAc (pH = 5), relatively low affinity of Ni for carbonate and the role of metal nature in distribution of its chemical forms. The conversion of applied Ni to Sol+Ex and Car in the sandy loam was higher than in the clay loam, whereas the reverse was true for other Ni forms. Increasing the Ni rate increased all forms of Ni. Iron oxides showed the highest capacity for Ni retention. Incubation time had a significant effect on Ni forms. At start of the experiment, added soluble Ni appeared in the more bioavailable fractions like Sol+Ex, Car, and ONi, but as the incubation proceeded, Sol+Ex, and Car fractions transformed into oxides and, to a lesser extent, into Res with relatively lower bioavailability. The fact that a major part of native soil Ni was extracted by the more aggressive reagents, and the conversion of added soluble Ni into forms such as oxides and Res, suggest that the bioavailability of Ni in soils under study might be low. (C) 2007 Elsevier B.V. All rights reserved.