Root-induced changes to cadmium speciation in the rhizosphere of two rice (Oryza sativa L.) genotypes

Our aim was to investigate rhizosphere effects on the chemical behavior of Cd. This was done in a glasshouse experiment, where two rice cultivars (Zhenong54 and Sixizhan) were grown in soil spiked with cadmium (Cd) at two levels, 3.9±0.5 and 8.3±0.5 mg kg−1 soil, placed in a rhizobox until ripening stage. Chemical forms of cadmium near the root surface were then assessed using a sequential extraction procedure (SEP). There were significant differences in Cd species, especially exchangeable Cd (EXC-Cd) between the two rice cultivars as affected by rice roots. The lowest EXC-Cd with Zhenong54 appeared in the near-rhizosphere area with little difference between tillering stage and ripening stage while Sixizhan had its lowest EXC-Cd concentration in the root compartment. Both cultivars had slight changes in the Fe/Mn oxide-bound fraction of Cd (FMO-Cd) at the grain ripening stage while the control treatments without plants had a significant increase in FMO-Cd at the same time, indicating a transformation from a less bioavailable form (FMO-Cd) to more bioavailable forms (EXC-Cd). Soil microbial biomass in the vicinity of the root surface had opposite trends to some extent with EXC-Cd, partly because of the root-induced changes to bioavailable Cd. Unlike Zhenong54, Sixizhan had a higher Cd concentration in the root, but only a small proportion of Cd translocated from the root to grain.

Research highlights
► We investigated genotypic effects on Cd speciation in the rhizosphere of rice.
► Zhenong54 (ZN) and Sixizhan (SX) were grown in rhizobox to show root-induced changes.
► Lowest exchangeable-Cd of ZN was in near-rhizosphere while SX in root compartment.
► Soil microbial biomass had opposite trends with exchangeable-Cd in both cultivars.
► Unlike ZN, SX had higher Cd content in roots, but lower Cd content in shoots.