Microscale Cadmium Speciation in Submerged Paddy Soil.

Growing rice under submerged condition substantially reduce Cd concentration in rice grain. It is considered that the insoluble precipitation phase of CdS becomes dominant in soil under reductive condition; thereby Cd absorption by rice root is restricted. However, even the soil matrix is in reductive condition, the boundary soil matrix in the rhizosphere of a rice root is in oxic condition due to the presence of oxygen supplied through the aerenchyma of the rice stems and roots. Then, the redox gradient from the rhizosphere to the soil matrix would affect the speciation of Cd in the soil. The purpose of this study is to identify the effect of the redox gradient around rice roots on the distribution and speciation of Cd in rice paddy field soil.
Rice plant was grown under flooded condition in a pot filled with soil (Aeric Epiaquept) taken from a paddy field. One week after heading, soil Eh was measured and then soil core including rice root was sampled and was immediately frozen by liquid nitrogen. The soil core were freeze-dried, embedded in epoxy resin and made into 100 μm-thick sections. The micro-scale distribution patterns of Cd around the roots were determined by micro-beam X-ray fluorescence at BL37XU, SPring-8. Cd K-edge μ-XANES were measured at spots where Cd had accumulated.
Cd was unevenly distributed on soil particles around the rice root. The μ-XANES analyses showed that Cd was associated with both S and O. On the spot where both Cd and Fe were accumulated, larger fraction Cd tended to be associated with O. Isolated spots where CdS was dominant were also present. Little accumulation of Cd was observed on Fe coating around the rice root. In paddy soil under reductive condition, These results suggested that Cd speciation would strongly depend on micro-scale redox environment in submerged paddy soil.