Arsenic Dynamics and Fractions in Silicon-Treated Soil and Its Accumulation in Rice.

Arsenic (As) is a global environmental contaminant and its accumulation in rice (Oryza sativa) grains has increased human’s daily intake. Studies have shown that arsenite is taken up by roots through known silicon (Si) transporters. Two greenhouse experiments (2015 and 2016) were conducted to investigate the role of elevated levels of Si on As adsorption and concentration of different species in soil and As uptake by rice. Treatments included five As rates (0, 10, 20, 40 and 120 mg kg^-1) with or without soil applied-Si. Arsenic sorption isotherms were also evaluated at five levels of added Si (0, 10, 20, 40 and 80 mg kg^-1). All treatments were arranged in a randomized complete block design with four replications. Biomass, roots, and grains Si and As uptake were determined at heading stage and harvest. Soil As species were separated into easily-soluble, aluminum (Al)-bound, iron (Fe)-bound, reducible, acid-soluble, and residual forms. Arsenic application significantly increased soil total As content (P<0.001). Among the As species, increases in Al-bound and Fe-bound As contributed the greatest to raise the total content. Silicon application significantly increased soil Si content from 44 to 102 ug g^-1 at heading with further increase to 200 ug g^-1 at harvest (P<0.001). The concentration of different As species in the soil was not affected by Si fertilization, except for Fe-bound As which reduced from 6.7 to 4.9 mg kg^-1 at harvest 2015. Silicon content in soil slightly increased with incubation time. High levels of As in solution tend to decrease Si content in soil and increase its availability in solution. Application of Si significantly reduced As grains content by 0.3 ug g^-1 compared to untreated rice (P<0.01). The outcomes of these studies suggest that high level of soil Si may enhance food safety through reduction of As content in rice grains.