Evidence for fungal and chemodenitrification based NO flux from nitrogen impacted coastal sediments.

Although increasing atmospheric nitrous oxide (NO) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse NO production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on NO 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 NO 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 NO fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit NO yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in NO fluxes, especially in redox-dynamic sediments of coastal ecosystems.