Individual and combined effects of increased temperature and salinity on nitrate-cycling processes and microbial community structure in a tidal freshwater wetland

Tidal, freshwater wetlands (TFWs) play a crucial role in carbon storage and nitrogen removal. However, their ability to provide these essential ecosystem services is threatened by sea-level rise and climate change. While the impacts of various environmental changes on nitrogen cycling are well-studied, the combined effects of ecologically relevant stressors such as increased temperature and salinity remain unclear. Using a flow-through reactor approach, we investigated nitrate transformation rates from multiple stressors in a New England tidal, freshwater wetland. We show that denitrification (DNF) is likely to continue as the primary mechanism for nitrate removal even under the pressure of rising salinity and temperature, but increased salinity may decrease future rates of DNF. Nitrous oxide (N2O) production also increased with increased salinity, with implications for the future N2O budget of these systems. Unlike DNF and N ₂ O production, which were only affected by increased salinity, dissimilatory nitrate reduction to ammonium (DNRA) increased when both stressors were applied. Furthermore, the overall and active microbial communities significantly changed in all treatments with the largest changes occurring in the increased salinity treatment. These findings imply that DNF will continue to be the dominant nitrate transforming pathway despite the simultaneous rise of salinity and temperature, however, TFWs may become small sources of N2O under saltwater intrusion. This work illustrates the need for more multiple stressor field experiment to better predict ecosystem nitrogen cycling under a changing climate.