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Changes in DTPA extractability of added cadmium in two different soil types treated with wheat straw in sterile and unsterile conditions

Heavy metals in soluble form have the highest bioavailability and toxicity in soil. DTPA-extractable Cd was investigated in two different soil types treated with wheat straw (5%) under sterile and unsterile conditions. Soils were located in Hamadan (Iran, 48o 28' 23" E and 34o 56' 48" N), a fallow cropland with a semi-arid climate, and Lahijan (Iran, 50o 1' 51" E and 37o 11' 59" N), a tea plantation with a temperate climate. DTPA-extractable Cd in contaminated soils (10 mg Cd kg-1) was measured from 1 minute to 3600 hours. During the soil incubation period, DTPA-extractable Cd was higher in the Lahijan soil, but at the end of the soil incubation period it was higher in the Hamadan soil. The positive effect of wheat residue on DTPA-extractable Cd was higher in the Lahijan unsterile soil at the end of the soil incubation period. The decrease of DTPA extractability of the added Cd was lower in the Lahijan soil incubated under unsterile conditions compared to that under sterile conditions. In contrast, Hamadan sterile soil treated and untreated with wheat residue had the highest DTPA-extractable Cd at the end of the soil incubation period. The decrease in DTPA extractability of the added Cd in soils was exponential with 3 steps. In the 1st step the highest and the lowest rates of DTPA decrease were observed in Hamadan sterile and Lahijan unsterile soils treated with wheat residue. In the 3rd step it was reversed, and the decrease in DTPA extractable Cd was lower in the Hamadan soil compared to the Lahijan soil.