Production of rhizosphere reactive oxygen species by root-surface iron plaque during rice growth

The interaction between oxygen released from rice roots and reduced species in the surrounding soil matrix establishes the rice rhizosphere as a hotspot for the production of reactive oxygen species (ROS), thereby driving various critical biogeochemical processes. In this study, we observed that the concentration of dissolved Fe²⁺ in rhizosphere porewater exhibited pronounced temporal and spatial variations during the rice growth period, consistent with the dynamic patterns of ROS production. Iron plaque on the root surface facilitated the oxidation of dissolved Fe²⁺, leading to the production of hydrogen peroxide (H₂O₂) and hydroxyl radical (^•OH) under acidic rhizosphere conditions (pH 5.5). Accordingly, rice roots with a higher content of iron plaque showed a stronger capacity to promote Fe²⁺ oxidation and ROS production. Iron oxides in the iron plaque (e.g., ferrihydrite, lepidocrocite, and goethite) rapidly induced the transformation of dissolved Fe²⁺ into surface-adsorbed and solid-phase Fe(II) and facilitated the oxidation of total Fe(II), thereby increasing the production rate and yield of ^•OH. These findings suggest that dissolved Fe²⁺ in rhizosphere porewater can serve as an electron donor, while iron plaque plays a crucial role in regulating iron redox cycling and ROS production in the rice rhizosphere.