Bioaccumulation of Cadmium in Grain Crops As Affected by Inorganic Amendments

Cadmium (Cd) pollution is of concern in many industrialized communities around the globe, particularly where untreated waste water is released from industry and cities. The prediction of bioavailable proportion of Cd is of crucial importance for the assessment of environmental quality of Cd contaminated soils. In consonance to this objective the first pot study was conducted on two different textured soils, collected from raw sewage irrigated areas to compare the efficiency of different extractants to extract the bio-available fraction of Cd from soils and to compare the phyto-available Cd concentration extracted by different plants. Both the soils were impregnated with Cd @ 20 mg kg-1 soil using CdCl2 salt. The soils were allowed to equilibrate for 30 days at moisture level just equal to field capacity. Four crops viz. wheat, canola, barseem and rice were grown during the present studies. Different extractants viz. AB-DTPA, 0.005 M DTPA, 0.05 M EDTA, 0.01 M CaCl2, 1 M NH4NO3 and 0.1 M NaNO3 were used to extract Cd from soils and the metal concentration was determined with AAS. Correlation analysis between the extractable and plant bioavailable Cd was done to evaluate the best extractant to represent phytoavailable Cd in soil. Results are quite consistent where AB-DTPA extractable Cd had positive and significant correlation with plant Cd concentration both in straw and grain harvested from sandy clay loam soil. Results are inconsistent for the loamy sand soil as the highest correlation of Cd concentration in canola straw and wheat grains was found with CaCl2-Cd, wheat straw and canola grains with DTPA-Cd, Cd in rice straw with NaNO3- Cd and Cd in rice grains with AB-DTPA extractable Cd. Cadmium concentration in barseem straw significantly correlated with total concentration of Cd in both the soils. Overall, ABDTPA gave more comparable results than other extractants. It is obvious that soil properties play a significant role in the efficiency of any single extraction procedure. Chemical immobilization of Cd with soil-applied inorganic amendments seems important for Cd contaminated soils. To evaluate the effectiveness of inorganic amendments at different rates of application, a pot study was conducted in the wire house, during 2005-06. The experiment included nine amendments viz. lime (4, 6, 8 %), gypsum (5, 10, 15 mmolc Ca 100 g- 1), calcium oxide (5, 10, 15 mmolc Ca 100 g-1), rock phosphate (500, 1000, 1500 ppm of P), diammonium phosphate (100, 500, 1000, 2000 ppm of P), potassium dihydrogen phosphate (100, 500, 1000, 2000 ppm of P), ammonium sulphate (60, 120, 180 ppm), elemental sulfur (5, 10, 15 g kg-1) and sulfuric acid (≈ 5, 10, 15 g S kg-1). Loamy sand and sandy clay loam soils used in the first study were used in this study. Both the soils were impregnated with Cd @ 20 mg kg-1 soil by spraying and mixing CdCl2 in solution equal to 75% of saturation percentage. The results showed that Ca and P based inorganic amendments significantly insolubilized the Cd in soils, as Cd 17 uptake by plants decreased statistically. Increasing application rates of both Ca- and Pamendments performed better compared to their lower rates. The S-based amendments solubilized Cd and increased AB-DTPA extractable Cd significantly. Maximum yields of wheat straw and grain were harvested with ammonium sulfate while values of these yields of rice were the highest with DAP and rock phosphate. Elemental sulfur remained promising regarding the yield of both the crops in both the soils. Minimum wheat grain, rice paddy and straw yields were obtained with the H2SO4 treatment. It solubilized Cd in soils to result in maximum Cd concentration in grain/paddy and straw of both the wheat and rice crops. The amendment effectiveness regarding AB-DTPA extractable Cd, after the harvest of two crops (wheat, rice), was in the decreasing order of H2SO4 > elemental sulfur > ammonium sulphate > rock phosphate > CaO > KH2PO4 > DAP > gypsum > lime for sandy clay loam soil. In loamy sand soil, the order was H2SO4 > elemental sulfur > rock phosphate > ammonium sulphate > CaO > DAP > KH2PO4 > gypsum and lime. The effectiveness of these amendments under field conditions and their economics was evaluated in a field experiment in the suburbs of Multan city. Ten treatments viz. contol, MAP @ 0.2 %, MAP @ 0.4 %, MAP @ 0.8 %, gypsum @ 0.2 %, gypsum @ 0.4 %, gypsum @ 0.8 %, elemental sulfur @ 0.2 %, elemental sulfur @ 0.4 % and elemental sulfur @ 0.8 % were applied in four replicates following randomized, complete block design. The results showed that MAP and gypsum insolubilized Cd in both the surface (0.00-0.15 m) and subsoil (0.15-0.30 m). Higher rates of both the amendments performed significantly better compared to their lower rates. Elemental sulfur solubilized Cd and increased AB-DTPA extractable Cd significantly. Maximum straw, wheat grain and rice paddy yields were obtained with MAP followed by gypsum. A similar trend was obtained for cadmium concentration in wheat grain, rice paddy, and straw for both the crops. Minimum Cd concentration was recorded in soil amended with MAP and gypsum while it was maximum with sulfur except its lower rate of application. Economic analysis showed that there is negative net benefit with the use of phosphate amendments owing to its high cost while gypsum proved an economical soil amendment which decreased Cd concentration below the critical limit in plant (< 0.2 mg kg-1). Among different rates of application, gypsum @ 0.4 % can safely be recommended to farmers to increase yield of crops which will be safe regarding Cd concentration for the health of human, who will be consuming produce of these plants.