Iron plaque in paddy: Formation, properties, functions, and applications

Abstract

Iron (Fe) plaque is a ubiquitous feature formed on root surfaces of wetland plants (e.g., rice), resulting from the oxidation of Fe2+ to Fe3+ driven by radial oxygen loss from roots. Fe plaque formation is primarily driven by abiotic pathways: influx of water with dissolved Fe2+ from bulk soil to roots and rhizosphere, wherein Fe2+ is oxidized by O2 released from aerenchyma, and by reactive species (e.g., ∙HO, ∙NO2, ∙NO) produced by electron transport from Fe2+ within Fe plaque. Biotic pathways, mediated mainly by Fe-reducing and Fe-oxidizing bacteria in rhizosphere, regulate Fe plaque formation. Fe plaque is mainly composed of ferrihydrite (Fe2O3∙nH2O), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), but may include siderite (FeCO3), and vivianite (Fe3(PO4)2). Soil properties, plant species, developmental stages and redox fluctuations substantially influence Fe plaque composition and formation rate, as well its dissolution. As a microbial and biogeochemical hotspot in paddy ecosystems, Fe plaque interacts extensively with nutrients and contaminants, influencing their bioavailability and plant uptake. With extensive reactive surface area and abundant functional groups, Fe plaque functions as both a barrier and reservoir for nutrients and contaminants. We developed the concept of “Fe circuit” to describe its dual functions on elemental cycling in rice rhizosphere. Fe plaque can be utilized for in-situ immobilizing or removing contaminants in paddy soil. This review offers a comprehensive perspective on Fe plaque and its potential to remediate contaminants in paddy soil and other wetlands.