Novel insights into effects of silicon-rich biochar (Sichar) amendment on cadmium uptake, translocation and accumulation in rice plants

The effects and mechanisms of biochars with different silicon (Si) contents on Cadmium (Cd) uptake, translocation and accumulation in rice plants are not fully understood. Herein, we report a pot study to disentangle the interaction mechanisms of Si-rich biochars (Sichar RH300, RH700) and Si-deficient biochars (WB300, WB700) with high-Si soil (HSS) and low-Si soil (LSS) on Cadmium (Cd) and Si accumulation in rice (including grains, straw, and roots). Sichar was found to be better than Si-deficient biochars in reducing Cd uptake and accumulation in rice, and RH300 amendment was better than the RH700 treatment. The surface complexation of Cd with carboxyl groups and Si from biochar led Cd immobilization in soil, as portrayed by Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy. The high Si content of biochars indicates a relatively lower bioaccumulation factor and translocation factor of Cd. The Sichar (e.g., RH300) treatment significantly increases the silicon concentration in rice (including grains, straw, and roots), but the Si concentrations of rice grains and roots decrease with WB700-amended LSS. Negative correlations between the concentrations of rice Si and Cd were observed, which could be related to lower expression as observed by Si transport genes (Lsi1 and Lsi3) in rice by Sichar amendment. These findings suggest that the Si released from Sichars can reduce the gene expression of Si transport channel of rice roots and inhibit the transport channel of Si, thus thereby inhibiting the Cd uptake, probably due to the utilization of same channel for Cd and Si. Integrative mechanisms of Sichar (RH300 and RH700) reduced Cd plant accumulation can be proposed by soil immobilization, inhibition of root transport, and prevention of plant translocation.