Contrasting effects of alkaline amendments on the bioavailability and uptake of Cd in rice plants in a Cd-contaminated acid paddy soil

Abstract

Reducing cadmium (Cd) concentrations in rice grains is important for food safety, particularly in acid paddy fields in South China where the soils have been previously contaminated with Cd. A field experiment was conducted to evaluate the effects of four alkaline amendments, i.e., lime, compost, biochar, and carbide slag on soil bioavailability and uptake of Cd in plants of two rice cultivars (Oryza sativa L.) in a Cd-contaminated acid paddy soil. The addition of these amendments significantly decreased the concentrations of CaCl₂-extractable Cd by 13–41%. Cd in the acid-soluble fraction was decreased in these amended soils while it increased in the residual fraction. The amendments also decreased the uptake of Cd in the plants at the tillering and mature growth stages. The concentrations of Cd in plant tissues at maturity were in the order: root > shoot > bran > polished rice > husk. The amendment of carbide slag decreased Cd concentration in rice grains the most, followed by lime, biochar, and compost. The increases in soil pH and the decreases in the acid-soluble fraction of Cd (F1-Cd) indicated that these amendments can directly transform the highly availability fraction of Cd to a more stable fraction (residual Cd fraction) in soils. Furthermore, the Cd concentrations in polished rice grains of the two rice cultivars used were reduced by 66–67% by treatment with carbide slag. Our study suggests that carbide slag has a great potential to reduce the bioavailability and uptake of Cd in rice plants in Cd-contaminated acid paddy field soils.