Elucidating the links between N₂O dynamics and changes in microbial communities following saltwater intrusions

Saltwater intrusion in estuarine ecosystems alters microbial communities as well as biogeochemical cycling processes and has become a worldwide problem. However, the impact of salinity intrusion on the dynamics of nitrous oxide (N₂O) and associated microbial community are understudied. Here, we conducted field microcosms in a tidal estuary during different months (December, April and August) using dialysis bags, and microbes inside the bags encountered a change in salinity in natural setting. We then compared N₂O dynamics in the microcosms with that in natural water. Regardless of incubation environment, saltwater intrusion altered the dissolved N₂O depending on the initial saturation rates of N₂O. While the impact of saltwater intrusion on N₂O dynamics was consistent across months, the dissolved N₂O was higher in summer than in winter. The N-related microbial communities following saltwater intrusion were dominated by denitrifers, with fewer nitrifiers and bacterial taxa involved in dissimilatory nitrate reduction to ammonium. While denitrification was a significant driver of N₂O dynamics in the studied estuary, nitrifier-involved denitrification contributed to the additional production of N₂O, evidenced by the strong associations with amoA genes and the abundance of Nitrospira. Higher N₂O concentrations in the field microcosms than in natural water limited N₂O consumption in the former, given the lack of an association with nosZ gene abundance. The differences in the N₂O dynamics observed between the microcosms and natural water could be that the latter comprised not only indigenous microbes but also those accompanied with saltwater intrusion, and that immigrants might be functionally rich individuals and able to perform N transformation in multiple pathways. Our work provides the first quantitative assessment of in situ N₂O concentrations in an estuary subjected to a saltwater intrusion. The results highlight the importance of ecosystem size and microbial connectivity in the source-sink dynamics of N₂O in changing environments.