Manganese, Zinc, and pH Affect Cadmium Accumulation in Rice Grain under Field Conditions in Southern China

Very little has been reported on the effects of pH, Zn, and Mn on Cd uptake in rice (Oryza sativa L.) and their levels under field conditions. Rice accumulates a high concentration of Cd in acid soils, even at low soil Cd levels. Lime (CaO) was spread at 1200 kg ha(-1) on the topsoil of a rice field during the tillering stage. Effects of liming on rice Cd, soil pH, and amorphous Mn (MnO2-AM) were then investigated. Slight increases in pH from 5.17 to 5.45 and MnO2-AM from 66.3 to 82.1 mg kg(-1) were observed after liming. The proportion of rice samples with a Cd concentration greater than the Chinese rice Cd standard (0.2 mg kg(-1) dry wt. in grain) decreased by similar to 15%. The pH, which varied from 4.8 to 5.8, did not significantly affect rice Cd, whereas soil Cd and Zn had a significantly positive effect, together accounting for similar to 14% of the variance. Soil Mn had significantly negative effects on rice Cd, accounting for >18% of the variance. For a Cd concentration <0.2 mg kg(-1) dry wt. in rice grain, the critical pH value in paddy soil was similar to 5.7, and that for Mn at pH 4.8 to 5.8 was similar to 300 mg kg(-1). Our findings showed that Cd concentration in rice grain in an acid paddy soil increased with an increase of Zn and a decrease of Mn when these metals were at sufficient levels.