Harnessing the Microbial Iron Redox Chain: Liquid Organic Acid Fertilizer and EDTA-Fe Drive Unexpected Cadmium Passivation in Alkaline Upland Soils

Abstract The immobilization of cadmium (Cd) in weakly alkaline upland soils, a major agricultural system in northern China, remains a challenge. This study examined whether liquid organic acid (OA) fertilizer and EDTA-Fe could stimulate Fe redox to stabilize Cd. OA fertilizer (active ingredients: citric acid, lactic acid, and glucose) induced microbial Fe(III) reduction and acidification within 1 day, shifting Cd from Fe/Mn-oxide and organic fractions to exchangeable and carbonate forms. Iron reduction and acidification contributed 52.0–55.8% and 44.2–48.0% of Cd activation, respectively. Subsequent Fe(II) oxidation over 60 days reimmobilized Cd into Fe/Mn-oxide and residual fractions. Adding EDTA-Fe enhanced Fe(III) reduction (raising Cd activation by 35.4%) while slowing Fe(II) oxidation and increasing Cd passivation by 31.8%. Such passivation was driven by newly formed secondary Fe oxides and Fe–Al–Si minerals, and ceased when Fe(II) oxidation was inhibited. Simulations supported Cd association with Fe–Al–Si phases, and EDTA-Fe enhanced immobilization via surface complexation/coprecipitation. Unlike paddy soils, Cd dynamics in alkaline soils are distinct despite both being Fe redox-driven, underscoring the need for alkaline-specific strategies. This approach works across diverse soils (pH 7.41–8.43; Cd 1.33–30.06 mg kg–1), reducing available Cd by 19.4–51.6%, and provides new insights into Fe redox-controlled Cd stabilization in upland soils.