Effective methods to reduce cadmium accumulation in rice grain

Contamination of cadmium (Cd) in paddy soil is a serious environmental problem threatening food safety in some parts of southern China, where rice grain Cd concentration often exceeds the Chinese limit (0.2 mg kg-1). We tested the effect of CaCO3 liming combined with growing low Cd cultivars on Cd accumulation in rice grain in 2-year field trials. A liming model was used to predict the doses of lime required. Lime (2.25 - 7.5 t ha-1) was applied in 2016 only and the effect monitored in both 2016 and 2017. Soil pH was increased from the initial 5.5 to the target value 6.5 by 7.5 t ha-1 CaCO3. Liming greatly reduced CaCl2-extrable Cd in the rhizospheric soil. Grain Cd concentration in the control exceeded the limit by 2 - 5 times. Liming at 7.5 t ha-1 decreased grain Cd concentration by 70 - 80% in both seasons without affecting grain yield. Grain Cd concentration was below the limit in the 7.5 t ha-1 liming treatment in both seasons. Grain Cd concentration correlated closely with CaCl2-extractable Cd in rhizospheric soil at the grain maturing stage. Seasonal difference in grain Cd concentration was attributed to the soil water status at the grain maturing stage. Liming had no significant effect on grain arsenic concentration or speciation. A single application of CaCO3 to raise soil pH to 6.5, combined with low Cd cultivars and delayed drainage of paddy water during late grain filling stage, was highly effective at reducing Cd accumulation in rice grain.