Effect of cultivation practices on cadmium concentration in rice grain

Cadmium (Cd) is one of the most toxic heavy metals, polluting the general environment. The application of sewage sludge, wastewaters and Cd-containing fertilizers causes an increase in Cd content in agricultural soils. Cd is easily taken up by plants and then enters the food chain, resulting in a serious health issue for humans. There is increasing concern regarding the occurrence of cadmium in rice, not only in the rice-growing areas of the Far East, but also in Europe. In this work we highlighted that, even when the agricultural soil is unpolluted and the concentration of Cd is low, e.g. 0.96 mg kg⁻¹, the Cd content of rice may still exceed the regulatory limit of 0.2 mg kg⁻¹. To reduce the uptake of Cd by rice, paddy-field flooding and soil amendment with lime and compost were tested in a field trial during 2003 and 2004 in Rosate, near Milan, Italy. We found that submersion was the main factor decreasing the Cd concentration in rice grain, producing maximum concentrations of 0.14 mg kg⁻¹in 2003 and 0.06 mg kg⁻¹in 2004. By comparison, Cd concentrations was at least two times higher for rice cultivated by irrigation only. Moreover, the addition of lime decreased the Cd concentration of rice by about 25% versus control under dry conditions. Lime addition thus appears to be a promising technique to reduce the bioavailability of soil Cd and minimize Cd concentrations in the produced rice. In contrast, the application of compost alone does not produced a significant effect. Differences in uptake over the years, with concentrations up to 40% lower in 2004, can be explained by differences in transpiration. These results shows that the influence of climatic conditions on Cd uptake in plants should not be underestimated. Such agronomic information represents a very helpful tool for rice growers, in particular in the case of cultivation of Cd-polluted soils and production of Cd-contaminated rice grain.