Effects of source and placement of phosphorus on concentration of cadmium in the grain of two durum wheat cultivars

Cadmium is toxic to humans. Because it enters the food chain through plant uptake, application of phosphorus fertilizer carrying Cd as an impurity may increase the Cd concentration of cereal grains above proposed threshold values. Field experiments were conducted to determine the effect of two phosphorus fertilizers on Cd concentration of two durum wheat (Triticum turgidum L. var. durum) cultivars, a low Cd accumulator (Arcola) and a high accumulator (Kyle) grown in two soils of southwestern Saskatchewan, Canada. Phosphorus was applied as monoammonium phosphate (15 mg Cd kg-1) and as triple superphosphate (75 mg Cd kg-(1)) at a rate of 18 kg P ha-(1). The fertilizer was applied in a mid-row band or in the seed row. Nitrogen was applied as urea or as ammonium sulphate. Variability in Cd concentration was attributed primarily to genotype and environment, which accounted for 41% and 29% of the explained variability, respectively. The Cd concentration of Arcola averaged 80 μg kg-(1), while that of Kyle averaged 143 μg kg-(1). In contrast, fertilization accounted for only a small proportion of the explained variability, with phosphorus treatments accounting for 3% of the model sums of squares, and N sources accounting for less than 1%. Phosphorus fertilization increased the Cd concentration of Arcola, regardless of source or placement, but the proportion of samples with Cd concentration above 100 μg kg-(1) remained unchanged at 17%. In the case of Kyle the Cd concentration increased only when the fertilizer was seed-placed, but in this case the proportion of samples with Cd concentration above 100 μg kg-1 increased from 67 to 90%. The effect of P appeared to result from the effects of fertilizer on plant density and vigour rather than from Cd impurities in the fertilizer.