Formation of ferrihydrite induced by low pe+pH in paddy soil reduces Cd uptake by rice: Evidence from Cd isotope fractionation

Ferrihydrite (Fh) is an important iron mineral in paddy soil and is prone to phase transition during dynamic redox condition, which affects Cd distribution and induces Cd isotope fractionation across soil to rice. Here, we conducted rice culture experiments with or not Fh application under different irrigation regimes to study the relationship between Fe species and Cd availability, as well as the isotope ratio of Cd in different Cd pools in paddy soil-rice system. Fh addition under continuous flooding (FL) with the decrease of pe + pH from 9.36 to 3.44 promoted the formation of amorphous Fe oxides as increased by 120.1% and facilitated Cd immobilization along with the increase of Fe/Mn oxides bound Cd by 25.3%, compared with continuous drying (DY) treatment. The isotopically heavy Cd were preferentially enriched from soil to extractable Cd (Δ114/110Cdextractable Cd-soil = 0.39-0.62‰) and from soil to grain (Δ114/110Cdgrain-soil = 0.40-0.66‰) particularly at low pe + pH and with Fh addition, while light Cd were enriched in Fe/Mn oxides (Δ114/110CdFe/Mn oxides bound Cd-extractable Cd = -0.65 ∼ -0.14‰). Besides, the expression of transporters involved in Cd transport in rice like OsNRAMP1, OsNRAMP1, OsHMA3, OsHMA2 and OsLCT1 were suppressed under low pe + pH condition. These findings indicated that low pe + pH facilitated Cd stabilization by the existence of more amorphous iron oxides, which induced the enrichment of heavy Cd isotope in liquid phase and light in Fe/Mn (oxy)hydroxides, respectively.