Unravelling the Activity and Presence of N₂O Reducers on Urban Greening Tree Leaves

Nitrous oxide (N₂O) is a potent greenhouse gas and can be biotically emitted from soils, water, and the less recognised plant leaves. Leaves can produce N₂O and may host N₂O-reducing microbes that use it as a respiratory substrate, potentially mitigating climate warming. This study examines the ecophysiology of N₂O reducers in the plant phyllosphere. Anoxic microcosm experiments, quantification of N₂O reduction kinetics, and analysis of nosZ gene governing N₂O reduction were conducted to assess the activity and presence of N₂O reducers in leaf epiphytes from various canopy positions of Photinia fraseri, an urban greenery plant. Results revealed canopy position-dependent N₂O reduction activity in the leaf microbiota. We identified previously unrecognised atypical Clade II nosZ gene in the phyllosphere microbiome, with its absolute abundance positively correlated with N₂O reduction activity, highlighting its significance in this process. Sequencing of bacterial and archaeal 16S rRNA genes revealed keystone taxa as primary drivers of N₂O reduction activity. These findings underscore the functional potential for N₂O reduction and the presence of the Clade II nosZ group within epiphytic microbes. This work provides insights into the ecophysiology of epiphytic N₂O reducers and underpins the development of leaf-based microbial solutions for N₂O mitigation under future warming.