Interaction of rice root Fe plaque with radial oxygen loss enhances paddy-soil N₂O emission by increasing •OH production and subsequently inhibiting N₂O reduction

Abstract

Amorphous iron (Fe) oxides are commonly deposited on rice roots, forming a distinctive Fe plaque, which has been observed as a notable hotspot for increased nitrous oxide (N₂O) emission from paddy fields. However, the precise mechanisms underlying this phenomenon have remained uncertain. We hypothesized that the interaction between Fe plaque and rice root radial oxygen losses (ROL) leads to the generation of hydroxyl radical (•OH), thereby inhibiting N₂O reduction and increasing N₂O emission. To test this hypothesis, we transplanted rice seedlings with induced Fe plaque coating alongside those without Fe plaque coating into the same paddy soil. The results showed that paddy soil with Fe plaque-coated rice seedlings exhibited significantly higher concentrations of •OH and increased N₂O emission rates, but a reduced abundance of the microbial N₂O reduction gene (nosZ), compared to paddy soil with Fe plaque-free rice seedlings. These observed differences in N₂O emission disappeared when we introduced an •OH scavenger into the paddy soil or shaded the rice leaves to reduce ROL. Furthermore, the supplementation of Fe (II) into the paddy soil with Fe plaque-free rice seedlings markedly increased •OH concentrations and N₂O emissions. Our results demonstrate that Fe plaque, in conjunction with rice root ROL, facilitates •OH generation through the Fenton reaction. This •OH production effectively inhibits microbial N₂O reduction, ultimately leading to increased N₂O emissions from paddy soils. Our study uncovers a new mechanism whereby Fe plaque enhances N₂O emissions from paddy soil. Minimizing Fenton reactions on Fe plaque may offer a promising strategy for reducing N₂O emissions from paddy fields.