Effect of cadmium pollution on soil organic carbon particle size fractions in Hamadan and Lahigan soils treated with wheat straw

Soil pollution by heavy metals like cadmium (Cd) with lowering the activity of microorganism can change carbon sequestration in soil. Plant residues when applied in soil are in free particulate form. They will be covered by clay particle and finally will be transformed to organomineral particles biologically. In this study, the effects of Cd addition (0 and 10 mg kg-1 soil) and time duration (1 day, 1, 3 and 5 months) on particle size fraction of organic carbon (OC) were studied in Hmadan and Lahijan soils in 3 replicates. Sand size fraction (SSF) and silt牖 size fraction (Siड) of soil organic carbon were analyzed by the particle size fractionation and OC determination by wet oxidation. The main part of OC in Hamadan soil was in SSF (14.12 mg kg-1 soil or 40%) but it was in Siड in Lahijan soil (15.80 mg kg-1 soil or 32%) in 1st day of soil incubation. The rate of OC decrease in Hamadan soil was markedly higher than that in Lahijan soil. Soil pollution with Cd decreased OC biodegradation in both of the studied soils. It increased OC retention especially in Lahijan soil. This effect may be related to higher clay content and higher Cd bioavailability and contamination factor in Lahijan soil.