Evidence for fungal and chemodenitrification based N2O flux from nitrogen impacted coastal sediments

Although increasing atmospheric nitrous oxide (N₂O) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse N₂O production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on N₂O production mechanisms in intertidal sediments using novel isotopic approaches and microsensors in flow-through incubations. Here we show that during incubations with elevated nitrate, increased N₂O fluxes are not mediated by direct bacterial activity, but instead are largely catalysed by fungal denitrification and/or abiotic reactions (e.g., chemodenitrification). Results of these incubations shed new light on nitrogen cycling complexity and possible factors underlying variability of N₂O fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit N₂O yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in N₂O fluxes, especially in redox-dynamic sediments of coastal ecosystems.