Prediction of Cadmium Uptake by Brown Rice and Derivation of Soil-Plant Transfer Models to Improve Soil Protection Guidelines.

Transfer of metals from soil into food crops is an important pathway leading to elevated exposure of human beings to metals. One of the most important food chains where metals pose a major problem is human exposure to cadmium (Cd) due to consumption of rice. Rice is a staple food for a large part of the human population in the world, especially in South and Southeast Asia. In Taiwan, for example, the daily intake of rice equals 50% of the total intake of fibers with an annual intake per person of 49 kg yr^-1. Total background concentrations of Cd in soils in Taiwan usually are below 0.4 mg kg^-1, but Cd concentrations in soils of paddy fields in the vicinity of electroplating industries have increased dramatically to levels as high as 30 mg kg^-1. Hence, predictive models or suitable soil tests are needed to evaluate the quality of these soils for rice cropping. Levels of Cd in soil and rice grains of 12 different rice cultivars were measured in 19 paddy fields across the western plains in Taiwan. Total Cd concentration in soil ranged from less than 0.1 mg kg^-1 to 30 mg kg^-1. Measured Cd levels in brown rice were predicted very well (R² > 0.8) based on Cd and Zinc concentrations measured in a 0.01 M CaCl₂ extract or a soil–plant transfer model using the reactive soil Cd content as estimated by extraction with 0.43 M HNO₃, pH, and cation exchange capacity. In contrast to the current soil quality standard used in Taiwan for arable land, which is based on the total Cd concentration as determined by extraction with Aqua Regia, such models are effective in identifying soils where Cd in rice will exceed food quality standards.