Suppression of silicates regulation on Cd uptaking of rice in Se-rich paddy soils

Abstract: Cd contamination is serious in some Se-rich paddy soils, which has posed potential human health risks through soil-to-crop transfer of Cd. Being the staple food feeding more than half population of China, rice has exhibited a prominent capability of Cd uptake and accumulation. With arable land per capita being less than half of the world average, it is quite essential to develop strategies to deal with soil Cd contamination for food safety and agricultural sustainability in China. In order to achieve safe utilization of Se-rich paddy soil, it is essential to reduce Cd bioavailability and thus mitigate its accumulation in rice grain. Previous studies have showed that it is difficult to achieve effective mitigation on Cd transfer into paddy rice by using a single ameliorator. The present work studied the effect of different combination of three kinds of silicates, i.e. sepiolite(SP), silica-calcium composite mineral(CS) and water-soluble foliar silicon fertilizer(YS), on bioavailability of Cd and Se in paddy soils and their transport into rice. Firstly, the dynamic changes Cd/Se/Si/Ca in soil pore water were determined with sepiolite(SP), sepiolite-silicon calcium composite mineral(SPC) and sepiolite-silicon calcium composite mineral-water soluble silicon mineral fertilizer(SCY) treatments. Secondly, the Cd and Se concentration of brown rice was investigated to understand exactly the mitigating effect of each treatment on grain Cd accumulation. Finally, the impacts of Si treatments on the availability and uptake of some mineral nutrients were also identified and discussed. The results showed that with SP, SPC and SCY treatment, the pH of rice rhizosphere soil increased by 0.15-0.31 units, which largely favored Cd transformation from easily exchangeable pool to carbonate- and organic-bound fractions while Se from residual fraction to bioavailable pool. In parallel, the concentration of Si, Ca and Se in soil pore water was remarkably enhanced by SPC and SCY treatment. Compared to control(CK), CaCl2-Cd in rhizosphere treated by SP, SPC and SCY was decreased by 19.5%-34.0%, while available Se was enhanced by 17.8%-36.8%. Transport factor(TF stem/white root) of Cd in rice plants from SPC and SCY treatments decreased by 92.1%-96.5% relative to that of CK, suggesting a significantly enhanced Cd sequestration in rice root. Most importantly, Cd concentration in brown rice was decreased from 0.32 mg/kg in CK to 0.15 and 0.02 mg/kg with SPC and SCY amendment, respectively, which were 25% and 90% lower than Chinese food safety standards for total Cd in rice (0.2 mg/kg). Each treatment had no significant effect on the accumulation of Se and other essential mineral elements (Cu, Fe, Zn, Mg and Mn) in brown rice. Taken together, SCY exhibited the most prominent mitigation effect on Cd accumulation in rice grain, which provides a cost-effective pathway for safe utilization of Cd-rich paddy soils without affecting Se enrichment in food chain.