Under-recognized pathways of N2O production in coastal sediments: Increased fungal and chemo-denitrification in response to elevated N loading

Increasing atmospheric levels of nitrous oxide (N₂O), a greenhouse gas with a 100-year global warming potential more than 300 times that of carbon dioxide, have been strongly linked to human activities – especially the dramatic increase in nitrogen loading to aquatic and marine ecosystems worldwide. While many studies have demonstrated that N₂O is formed through a number of microbially mediated pathways, the factors regulating the emission of N₂O to the atmosphere remain difficult to predict and the global N₂O budget remains poorly constrained. In particular, coastal ecosystems, which bear much of the brunt of anthropogenically-derived nitrogen from watershed inputs and rapidly growing coastal human populations, represent large gaps in our understanding of sources and sinks of atmospheric N₂O.

In large part, these challenges stem from the fact that a diverse number of N₂O production pathways are operative under the dynamic redox conditions encountered in coastal and estuarine sediments, complicating our ability to understand their relative roles in N₂O fluxes. Here, we use whole-core sediment incubations together with a suite of conventional and novel stable isotopic tools to identify both factors influencing N₂O flux as well as those underlying biogeochemical processes responding to those factors. We find that under elevated N loading to coastal sediments, an observed increase in N₂O flux to the overlying water is not mediated by direct bacterial activity, but instead is catalyzed by fungal denitrification and/or abiotic interactions with reduced iron (e.g., chemodenitrification). These findings shed new light on the complexity of nitrogen cycling in coastal sedimentary environments and highlight the need for an improved understanding of eukaryotic and abiotic processes in regulating fluxes of climatically important gases such as N₂O.