Distribution, transfer, and time-dependent variation of Cd in soil-rice system: A case study in the Chengdu plain, Southwest China

Abstract Heavy metal contamination in soil has been a major environmental concern and affected agricultural safety, and it is generally believed that the risk of heavy metals in soil is associated critically with their forms. This study was carried out to identify the distribution of cadmium (Cd) speciation in paddy soil of Chengdu Plain and its time-dependent variation during rice growing process and to determine the controls of soil Cd fractions on their accumulation in rice. The speciation of Cd in soil was analysed by Community Bureau of Reference (BCR) sequential extraction method. The result showed that the Cd concentration range in study fields was 0.54 mg/kg ˜ 1.65 mg/kg and the distribution order of Cd in soil was basically: residua > exchangeable ≈ reducible > oxidizable. Cd accumulation and distribution in rice largely depended on non-residual fraction Cd, rather than the total Cd, especially exchangeable fraction Cd having a linear relationship with total Cd in grain within a use limit value of Cd concentration in soil. In the process of rice growth, the distribution of Cd in rhizosphere soil was changed, and the relatively high percentage of non-residual fraction Cd was reached before heading, which suggested that the amount of Cd took up by paddy roots during the time of tillering was more than at any other period. Overall, this study provided a basis for exploring more appropriate risk assessment and heavy metal contaminated soil remediation.