Silicon Amendment Reduces Soil Cd Availability and Cd Uptake of Two Pennisetum Species

Silicon (Si) plays important roles in alleviating heavy metal stress, but the migrating effects and mechanisms, especially for Pennisetum, are not well studied. In this study, Pennisetum glaucum and Pennisetum glaucum × P. purpureum were used to explore the impacts of Si application on alleviating cadmium (Cd) toxicity and its possible mechanism. Treatments consist of four levels of Cd (0, 10, 50, and 100 mg·kg(−1)) with or without 2.0 mM Si amendments. Under Cd stress, Si application significantly increased plant biomass and Si content, reduced Cd content, and decreased the enrichment factor in shoots and roots. Si treatment also increased soil pH and soil residual Cd, while reducing available/oxidizable/reducible Cd content in soil at 50 and 100 mg·kg(−1) Cd levels, thereby leading to a reduction of the soil’s available Cd. These findings indicate that Si application is effective in alleviating Cd phytotoxicity of Pennisetum, mainly through reducing plant Cd uptake and increasing soil pH and Cd immobilization, thereby reducing Cd bioavailability.