Selenium geochemistry of paddy soils in Yangtze River Delta

Selenium (Se) content both in total and bioavailable forms were very low (25-35 and 9.5 μg/kg, respectively) in waterlogged paddy soils in the central Yangtze River Delta. This was due to changes in geochemistry and the inherent properties of the parent materials of these lowland paddy soils. The very low Se content (23 μg/kg) in the parent material resulted in low total Se content in the soil. The main chemical changes under long-term waterlogged conditions are depletion of molecular oxygen, decrease of redox potential, and reduction of Fe (III) to Fe (II) and SeO₃^2- to Se⁰. This led to low availability of Se in soils, and subsequently low Se content (29 μg/kg) in brown rice grain produced in this region. It has been suspected that low Se content in staple food might be one of the major reasons for a high infection rate of the intestine and stomach cancers and the higher death rate caused by these diseases in the region. Foliage spray of Na₂SeO₃ at early heading stage of rice plant growth improved the Se content of brown rice grain, hull, and straw, and would improve human and animal health.