Effects of the Modified and Compounded Clay Mineral Conditioners on Cadmium Availability in Soil and Cadmium Content in Brown Rice

Pot experiments using a cadmium (Cd)-contaminated agricultural soil (total Cd 0.79 mg·kg^-1, pH 6.71) were carried out to study the influence of three alkaline conditioners, namely mercapto-modified sepiolite (SGP, pH 9.31, application rate 0.5 g·kg^-1), amino-modified attapulgite (ISS-1, pH 9.78, application rate 0.5 g·kg^-1) and humus-sepiolite complex (ISS-2, pH 9.94, application rate 1.3 g·kg^-1) on soil total and 0.1 mol·L^-1 calcium chloride (CaCl₂)-extractable Cd, rice biomass and brown rice Cd content. Soil solution Cd concentrations declined rapidly during the first three weeks after application of SGP, ISS-1 and ISS-2 conditioners by 47.2%, 50.1% and 48.6% at the tillering, jointing and maturity growth stages, respectively. ISS-1 application decreased only soil CaCl₂-extractable Cd at jointing (25.4%). ISS-2 application decreased soil CaCl₂-extractable Cd concentrations at tillering (20.2%) and jointing (25.3%). SGP and ISS-2 applications also decreased the Cd contents in brown rice by 35.2% and 34.7%, respectively, but treatment with ISS-1 had no significant effect. All three conditioners showed no significant effect on rice plant height or yield, ratio of productive tillers, 1 000-grain weight, or soil pH. The decrease observed in soil Cd bioavailability may be related to adsorption and complexation by the conditioners. Controlled application rates of the modified and compounded clay mineral conditioners (SGP and ISS-2) to slightly Cd-contaminated neutral agricultural soil may effectively reduce soil Cd bioavailability and brown rice Cd content without any significant change in soil pH.