Heavy metal pollution of farmland in China poses a serious threat to food security and public health, which has resulted in widespread public concern. Therefore, it is an urgent task to control heavy metal pollution of farmland to reduce the risk of excessive heavy metal in crops.

Among all kinds of remediation technologies of heavy metal contaminated soil, in-situ passivation of heavy metal has become a research hotspot because of its low cost, easy operation, high remediation efficiency and reducing the absorption of heavy metals by crops without affecting agricultural production. 

Under the guidance of Prof. LI Zhian and MAO Peng, a doctoral student from Research Group for Soil Ecology in South China Botanical Garden of the Chinese Academy of Sciences, investigated the effect of wollastonite with or without phosphorus on cadmium accumulation in rice through field plot experiment. 

The effect of P and wollastonite on soil pH and CEC was better than that of wollastonite alone. After treated with wollastonite alone, the content of cadmium in brown rice decreased by 71%, but when wollastonite was mixed with different phosphorus modifiers, the content of cadmium only decreased by 29-39%.

The reason for this phenomenon is that the bioavailability of manganese in the iron film on the surface of rice root decreased significantly with the addition of phosphorus, and the molar ratio of iron to manganese increased. Mn was negatively correlated with Cd in rice plants and Fe/Mn was positively correlated with Cd in rice plants. It was revealed that the decrease of Mn in soil and iron film under phosphorus stress hindered the accumulation of Cd in rice. 

The application of wollastonite in silicon deficient paddy soil can not only increase rice yield but also effectively reduce the accumulation of Cd in rice. However, the combination of phosphorus and wollastonite showed negative effect on reduction of Cd, so it is worth noting in practical application.

In addition, this study revealed the relationship between Cd and the absorption of related mineral nutrients, which will help to develop more effective measures to reduce Cd in rice grains. 

The relevant results of this study has recently been published in Science of The Total Environment entitled "Phosphate addition diminishes the efficacy of wollastonite in decreasing Cd uptake by rice (Oryza sativa L.) in paddy soil".

Mechanism of cadmium uptake by rice under the combined application of phosphorus and wollastonite. (Image by SCBG)