Prediction of heavy metals uptake by different rice species in paddy soils near contaminated sites of Taiwan

Food safety and quality becomes a social concerning issue of in our country. It is a very important task of soil environmental scientists around the world to study how to solve the heavy metals contamination of soil by scientific methods for lightening and threatening food safety problem. Eight sites of heavy metals-polluted farmlands that located at Chang-hua city, Ho-mei town and Lu-kang town were selected for cultivation in this project. Most of farmlands contain high concentration of chromium, copper, nickel and zinc. 12 cultivars of rice, that culture is common in Taiwan at present were cultivated, included japanica type; indica type and sticky rice 3 major rice varieties were cultivated. The 1st crop and 2nd crop of 2005 was harvested and the result showed: (1) Comparing 0.1M hydrochloric acid, 0.43M nitric acid and 0.1M EDTA soil extractants, they have similar efficiency and high relationship to extract the heavy metals in soils. (2) The uptake heavy metals rate is different among the rice varieties, the indica varieties have higher potent to uptake heavy metals than japonica varieties, especially cadmium in brown rice; the brown rice have high content of heavy metals by order is zinc, copper, nickel, cadmium, chromium and lead. (3) It is found that soil cadmium concentration in some plots don’t meet the standard level of monitoring, but cadmium concentration in the brown rice of indica varieties are over the rice safety regulation of heavy metals. (4)For the new rice safety regulation in the near future, it is recommended that indica varieties should be cultivated in the soil, which has the concentration of cadmium by aqua regia method less than 0.6 mg/kg and japonica varieties less than 1.2 mg/kg. (5) Cadmium uptake of rice is affected by high concentration of zinc in the soil; they may affect the cadmium uptake of rice plants. In the Chang-hwa project, the result showed that the methods of soil extractants, soil pH and soil organic matter contents or soil CEC would affect the prediction of cadmium uptake by different rice species significantly. Prediction of cadmium uptake by 0.01M CaCl2 extracted method has less relationship with soil pH than that of acids extractants and aqua regia. And 0.01M CaCl2 extract method showed a stable results in prediction of cadmium uptake by different rice species in Taiwan paddy soils. It is recommended that indica type varieties rice do not grow in some part of the soil derived from marine sediments and cadmium polluted soils.