Effect of alkalinity application on cadmium uptake by lowland rice [Oryza sativa] under different water managements

This study was conducted to evaluate the effect of successive alkalinity applications on cadmium (Cd) uptake by lowland rice (Oryza sativa L. cv. Koshihikari) under different water managements. The brown rice produced in this study field was slightly polluted with Cd (around 0.4 mg/kg) and the soil pH was low (4.82). In experimental plots managed by intermittent irrigation after midsummer drainage, Cd content in brown rice was decreased with successive alkalinity applications. But the stability of the effect on the Cd content was not satisfactory because mean Cd content +99% confidence interval exceeded 0.4 mg/kg (the standard of Cd content in brown rice distributed by the government). On the other hand, Cd content in brown rice with flooding treatment completely satisfied the standard (0.4 mg/kg), and the effect of alkalinity application was not confirmed. In oxidative soil conditions caused by midsummer drainage and intermittent irrigation the soil pH did not decline with alkalinity application. And in the plots in which Cd content in brown rice decreased with alkalinity application, the average soil pH around heading stage rose to about 6 from 5 in Eh range above 200 mV. The successive alkalinity application hastened the growth of rice plants and resulted in a decrease of yield caused by early lodging in some years. To avoid lodging and decrease of yield caused by alkalinity application flooding treatment was considerably effective. Silicate application accompanied with alkalinity increased Si content in rice straw and was to be expected in adjustment of the canopy structure. In conclusion, it was considered that the combination of alkalinity application accompanied with silicate and flooding treatment was effective to decrease Cd content in brown rice and to stabilize growth and yield of lowland rice while amending soil pH in slightly polluted paddy fields with low soil pH.