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.