Moisture regulation induced nitrite-mediated Fe(II) oxidation for cadmium immobilization in paddy soils

Cadmium (Cd) contamination in paddy soils poses significant risks to human health and food security, while traditional remediation methods may cause secondary pollution through external amendments. Nitrite oxidized Fe(II) to lepidocrocite and goethite, which immobilized Cd through adsorption and structural incorporation, effectively decreasing Cd mobility. Building on this mechanistic insight, a moisture regulation strategy was applied to enhance nitrite accumulation in paddy soil, and its role in Fe(II) oxidation for Cd immobilization was investigated. Metagenomic analysis identified nitrogen transformation pathways, showing that nitrite accumulation was limited by a higher abundance of ammonia-oxidizing bacteria (AOB, 19.2%) compared to nitrite-oxidizing bacteria (NOB, 11.7%), and predominance of nitrite consumption pathways. Maintaining 50% soil moisture selectively enhanced AOB activity while suppressing NOB, resulting in nitrite accumulation up to 9.56 mg/kg after 90 days incubation, reducing bioavailable Cd from 35 to 16% and increasing residual Cd from 7 to 48%. This study is original in integrating moisture regulation with microbial nitrogen cycling to enhance nitrite accumulation, thereby promoting Fe(II) oxidation and Cd immobilization. It provides a sustainable and chemical-free strategy for Cd management in paddy soils.