# The Ratio of S2−/SO42− Induces the Transference of Cadmium in Rhizosphere Soil, Soil Pore Water and Root Iron Plaque

## TL;DR

This study shows how sulfur redox processes affect cadmium mobility in rice paddies, reducing cadmium availability and uptake by rice plants.

## Contribution

The study reveals that the S2−/SO42− ratio controls cadmium transference in soil, pore water, and root iron plaque under waterlogged conditions.

## Key findings

- Sulfur addition reduced cadmium in soil pore water at all growth stages.
- Sulfur application increased cadmium sequestration in rice root iron plaque by 32–67%.
- Sulfur altered the rhizosphere microbiome, increasing sulfate-reducing and sulfur-oxidizing bacteria.

## Abstract

Rice (Oryza sativa L.) readily accumulates cadmium (Cd), posing dietary exposure risks in populations dependent on rice-based diets. This study investigated how sulfur (S) redox processes regulate Cd mobility in S-deficient, Cd-contaminated paddy soil under waterlogged conditions. A pot experiment was conducted with two S treatments (−S and +S, 30 mg kg−1) throughout the rice growing season. S addition markedly increased pore water S2− concentrations during early growth (tillering) and mid-season (booting) and suppressed the diffusion of SO42− from non-rhizosphere to rhizosphere at later stages (filling–maturity). Consequently, Cd in soil pore water was significantly lower in +S than −S treatments at all stages. Sulfur-amended soil showed a redistribution of Cd from labile fractions (exchangeable and carbonate-bound) to more stable fractions (Fe/Mn oxide-bound). Sulfur application also altered the rhizosphere microbiome: the relative abundance of sulfate-reducing bacteria (SRB) increased at the booting and filling stages, while sulfur-oxidizing bacteria (SOB) became more dominant at maturity. Additionally, +S enhanced Cd sequestration on rice root iron plaque by 32–67% during the grain-filling and maturity stages compared to −S. Throughout the rice growing period, redox-driven shifts in the S2−/SO42− ratio emerged as a key control on Cd behavior, with low pe + pH (strongly reducing conditions) promoting Cd sulfide precipitation and high pe + pH (more oxidizing conditions) causing Cd remobilization.