effect of sewage sludge, clay content and time on the fractionation of nickel and cadmium in selected calcareous soils

abstract a study was conducted to evaluate the effect of incubation time, sewage sludge and clay content on the distribution of nickel and cadmium in different fractions of selected calcareous soils. to meet this purpose, an experiment with two different soils (4 and 30 percent caco3), two different textures (8 and 40 percent clay), two levels of nickel (0 and 100 mg.kg-1), two levels of cadmium (0 and 25 mg.kg-1) and organic matter in the form of sewage sludge in two levels (0 and 50 t ha-1) was carried out. two incubation times (0 and 60 days) and three replications in a completely randomized design were mentioned. the samples were incubated for two months at temperature of 25±2 °c and soil moisture condition close to field capacity. distribution of cd and ni in fractions soluble+exchangeable, organically bound, carbonate bound and residual fractions was determined by a sequential extraction method. the results illustrated that, in both soils, residual form of ni and carbonate form of cd were dominant and showed a significant change with sewage sludge treatment and time. sewage sludge application increased the amount of organic and carbonate bound of ni and cd significantly. the proportion of organic and carbonate bond cd and ni was higher in soil with 33 percent lime. in contrast, in soil with 4 percent lime, the percentage of ni and cd in the residual fractions was higher. residual fraction of ni and cd in clay soil was significantly higher than sandy loam soil. increasing the amount of ni and cd increased all fractions of these metals significantly. the organic forms of ni and cd decreased significantly in the course of time. in general, based on the results, it may be concluded that calcium carbonate, clay percent and sewage sludge treatment play a key role in the distribution of ni and cd in different fractions of soils.