Effect of Si soil amendments on As, Cd, and Pb bioavailability in contaminated paddy soils

Numerous greenhouse and laboratory experiments have shown that the application of Si-rich materials may affect the mobility and transport of hazardous elements in the soil–plant system. The effects of three types of Si-based soil amendments on As, Cd, and Pb in paddy soil and their translocation into the rice roots, leaves, and grain were evaluated in four field tests. The Si-based soil amendments had positive influence on the rice productivity and crop quality: increased yield and reduced the total content of pollutants in leaves and grain by 2.2–54.0% for As, 11.4–51.9% for Cd, and 3.2–51.4% for Pb. The rice yield was mostly related to the water-extractable monosilicic acid from fresh soil sample (r = 0.60–0.96) as compared with the 0.1 M HCl-, 0.01 M CaCl2- or 0.5 M acetic acid-extractable Si from dry soil sample. Several mechanisms were proposed to be responsible for the Si-material-assisted reduction in the soil contaminant mobility: (a) pollutant sorption by the Si material applied; (b) reaction of the newly formed monosilicic acid with pollutants followed the sorption or precipitation of the silicates on soil particles; (c) primary sorption of newly formed monosilicic acid by soil solid components and then interaction between the sorbed monosilicic acid and pollutants. Additional plant Si nutrition activated physiological mechanism of reduction in the root-to-grain transport of the hazardous elements. Physiological mechanisms were more significant than soil mechanisms in decreasing grain accumulation of the tested hazardous elements under application of the Si-rich soil amendments.