Stimulatory effects of Mn-embedded root plaques on N₂O emissions from paddy soil depend on light intensity

Iron (Fe) plaques on rice roots are naturally occurring redox-active mineral interfaces and recognized hotspots for nitrous oxide (N₂O) production in paddy rhizospheres. However, the role of trace manganese (Mn) naturally co-deposited within Fe plaques remains poorly understood. Here, we used experimentally induced Fe plaques and Mn-embedded Fe plaques as a controlled model to evaluate whether plaque-associated Mn modifies rhizosphere redox conditions and N₂O production. Compared with Fe-only plaques, Mn-embedded Fe plaques showed a greater increase in N₂O emissions and were associated with higher hydroxyl radical (•OH) production. Under illuminated conditions, quenching •OH with terephthalic acid reduced N₂O emissions most strongly in the Mn-embedded Fe plaque treatment, supporting an association between plaque-associated •OH and light-enhanced N₂O accumulation. Despite the shading-induced decrease in rhizosphere O₂, N₂O production remained higher in the Mn-embedded Fe plaque treatment than in the Fe plaque and control treatments. In anaerobic incubations, EDTA addition reduced the Mn-associated response, whereas exogenous Mn(II) increased N₂O production. Metagenomic profiling provided supporting evidence for these responses, showing shifts in denitrification-related taxa and functional genes, including norB and nosZ. Together, these findings identify Mn incorporation into rice root Fe plaques as a previously overlooked factor modulating rhizosphere N₂O production and highlight the need to consider trace-metal composition when evaluating the biogeochemical function of root plaques.