Chemical forms of cadmium in two calcareous soil textural classes as affected by application of cadmium-enriched compost and incubation time

Abstract

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 cadmium (Cd) retention and the change in 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 enriched compost on chemical forms of Cd. 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 30 g kg − 1 of municipal waste compost (MWC). The compost had been enriched with different amounts of CdSO 4 to obtain Cd concentrations ranging from 5 to 60 mg Cd kg − 1 in treated soils. The treated soils were placed in plastic containers and incubated at 25 ± 2 °C near field capacity moisture. After 0, 1, 2, 4, 8, and 16 weeks, a sequential extraction scheme was used to fractionate Cd of incubated samples into soluble + exchangeable (Sol + Ex), carbonate-bound (Car), organically bound (OCd), Mn-oxide-bound (MnOx), amorphous Fe-oxide-bound (AFeOx), crystalline Fe-oxide-bound (CFeOx), and residual (Res) forms. Results showed that, depending on the textural class, about 82–88% of Cd was converted to Sol + Ex, Car, and OCd forms. In both soils, carbonate fraction was dominant. The conversion of applied Cd to Sol + Ex and Car in the sandy loam was higher than in the clay loam, whereas the reverse was observed for other Cd forms. The CFeOx was less than the detection limit of atomic absorption in both textural classes. Increasing the Cd rate increased all forms of Cd but the % increase depended on the capacity of different soil components for Cd retention. Carbonate showed the highest capacity for retention. Incubation time had a significant effect on Cd forms. However, in almost all treatments, more than 80% of added Cd was converted to Car and OCd immediately, and the proportion approximately stayed the same throughout the course of experiment. Although the presence of a large amount of cadmium in the first three steps of extraction procedure shows a relatively high Cd bioavailability in soils under study, it can be concluded that the calcareous nature of these soils plays a key role in cadmium retention because a major portion of the soluble Cd entered carbonate fraction immediately after addition to soils.