Soil microbial communities in coastal wetlands rely on anaerobic metabolisms to breakdown organic material. In the absence of oxygen, many use nitrate as a terminal electron acceptor for respiration following one of two major pathways: denitrification (nitrate removal) or DNRA (nitrogen retention). Coastal wetlands are usually effective sinks of nitrate, but global change stressors such as sea level rise and saltwater intrusion may disrupt this important ecosystem service. This study specifically examined the effect of salinization on microbial nitrate reduction in tidal freshwater wetlands, considering both biogeochemical and microbial community responses. We utilized a soil transplant approach to simulate salinization by encasing freshwater wetland soils in nylon mesh bags and relocating them to a low-salinity (oligohaline) and moderate-salinity (mesohaline) wetland for in situ incubation. Transplanted soils were collected after 7, 10, 19, and 22 months (2 growing seasons). We performed qPCR of key function genes involved in denitrification, DNRA, and sulfate reduction. In addition, we employed amplicon sequencing of the 16srRNA gene to determine changes in overall prokaryotic community composition. We performed this genetic analysis in tandem with stable isotope tracing (15N) to measure rates of both denitrification and DNRA.
Results/Conclusions
Salinity levels increased 10-fold in soils incubated at the oligohaline site and up to 100-fold in soils incubated at the mesohaline site. By the end of the 22-month incubation, soil organic matter significantly decreased in the fresh soil exposed to mesohaline conditions. Rates of denitrification were also lowest in these soils, and DNRA rates were highest. While the abundance of the denitrification nirS gene correlated with denitrification rates, no correlation was evident between DNRA nrfA gene abundance and DNRA rates. Due to the tight cycling of sulfur and nitrogen in anaerobic soils, we also quantified the abundance of a key sulfate reduction gene (dsrA). Initially low in freshwater soil, dsrA abundance increased to match each host site by the end of the incubation period. Amplicon sequencing showed distinct shifts in community composition in the transplanted soils, becoming more similar to the host environment. Taken together, these results suggest salinization of freshwater soils will promote nitrogen retention and suppress nitrate removal, potentially exacerbating coastal eutrophication, and significantly alter microbial community structure.