Salinization alters nitrate reduction in coastal freshwater wetlands

Abstract:

Coastal wetlands are effective sinks of nitrate, but global change stressors, such as sea level rise, may disrupt this vital ecosystem service. The increase in salinity associated with sea level rise has the potential to affect coastal wetlands and their functioning by modifying soil physiochemistry and altering plant and microbial communities. To better understand how freshwater microbial communities will respond to this disturbance, we implemented a soil transplant study wherein soils from a coastal freshwater wetland were transplanted to wetlands of greater salinity for in situ salinization. Sampling occurred after 7, 10, 19, and 22 months of incubation. Soil physiochemical conditions were monitored throughout the duration of the experiment. Nitrate reduction rates (denitrification and DNRA, using stable isotope enrichment) and microbial community characterization (16s rRNA sequencing and functional gene abundance) were performed to assess response to increased salinity. Results showed a rapid change in nitrate reduction rates, wherein soils exposed to elevated salinity had decreased denitrification and increased DNRA. Microbial community responses varied based on salinity level. Freshwater communities exposed to intermediate levels of salinity were relatively stable and very similar to the freshwater control soils for the duration of the study. In contrast, soils exposed to high levels of salinity quickly developed a unique transition community, and it took two full growing seasons before its composition matched the community native to the more saline site. The abundance of functional genes associated with denitrification generally showed a positive correlation with rates. Taken together, these results suggest that freshwater soil will show suppressed denitrification, and increased DNRA, which could, in turn, exacerbate eutrophication of coastal aquatic ecosystems.