Rice root Fe plaque enhances paddy soil N₂O emissions via Fe(II) oxidation-coupled denitrification

Iron (Fe) plaque, defined as a film of poorly crystalline Fe oxides deposited on the surface of rice roots, potentially mediates paddy-soil N₂O emissions. The aims of this study were to test if, and how, Fe plaque affects N₂O production and reduction within a rice paddy soil. Rice seedlings were grown so that Fe plaque was either present or absent. With Fe plaque present, emissions of both N₂O and N₂ doubled, with the abundance of both Fe-redox bacteria and denitrifying functional genes elevated at the root-soil interface. Under hydroponic conditions, Fe plaque promoted N₂O emissions in the presence of NO₃⁻ but not NH₄⁺. In addition, chelating the Fe(II) eliminated the promoting effects of Fe plaque on N₂O emission while Fe(II) addition to the Fe plaque-free roots increased N₂O emission. These results demonstrate that Fe plaque promotes soil N₂O emission and N loss predominately via Fe(II) oxidation-coupled denitrification. Our results indicate that Fe plaque is a hotspot for both N₂O emission and N loss from paddy soils. Mitigation of N₂O emission and N loss from paddy soils should consider methods to limit Fe plaque effects