Redox dependence of manganese controls cadmium isotope fractionation in a paddy soil-rice system under unsteady pe + pH conditions

Flooding in paddy soils alters the soil redox of manganese (Mn) and produces elevated concentrations of soluble Mn that can reduce cadmium (Cd) uptake by rice. To better understand the fates of Mn and Cd, along with changes in soil redox conditions, we conducted microcosm incubations in paddy soil covering the reduction to oxidation to re-reduction phases. The extractable Cd concentration decreased rapidly during the reduction phases but increased upon oxidation, and Cd availability largely depended on soil pH, Eh, pe + pH, and the extractable Mn concentration. Exogenous Mn can promote Cd binding with Fe-Mn(oxyhydro)oxides. A trade-off effect between the soil-extractable Cd and Mn concentrations across changes in pH, Eh, pe + pH was identified, and attaining an optimal pe + pH value of 6.8 was targeted. Furthermore, to provide insights into how the redox status of Mn changes to alter Cd mobilization in a paddy soil-rice system, Cd isotope ratios across the paddy soil-rice tissue continuum were investigated using planted rhizobox experiments under different irrigation regimes. The heavy Cd isotopes from the soil to liquid-phase (Δ114/110Cdextract-soil = 0.40-0.82‰) and from the soil to rice grain (Δ114/110Cdgrain-soil = 0.84-0.89‰) were preferentially enriched. Light isotopes were likely to be enriched in Cd bound to Fe/Mn-oxides, a process that was promoted by increased Mn availability. These results suggest that Cd isotopes are systematically fractionated within the paddy soil-rice system, which is caused by the unsteady soil redox, and the stabilization of Cd in the bound soil pool such as Fe-Mn(oxyhydro)oxides-Cd under reducing conditions could be developed as a Cd retention mechanism in paddy soils.