Impact of saltwater intrusion on microbial community function and organic matter breakdown in tidal freshwater wetlands

Background/Question/Methods

Sea level rise is a climatic stressor that has a unique impact on tidal freshwater wetlands. It causes saltwater intrusion into environments historically dominated by freshwater flows, where even modest increases in salinity can adversely affect plant community composition and productivity, and potentially change ecosystem-scale carbon dynamics. In addition, microbial activity in the wetland soils may also be affected because the influx of sulfate offers a new substrate for anaerobic microbial respiration, shifting redox conditions and changing pathways of carbon mineralization.  The objective of this research was to determine microbial community responses to elevated salinity associated with a long-term field study in a tidal freshwater marsh in South Carolina, where in situ manipulation consistently raised porewater salinities from freshwater to oligohaline levels (~2-5 ppt).  At the end of the three-year field manipulation, soil cores were collected and extracellular enzyme assays (EEA) were performed for several labile (β-1,4-glucosidase and 1,4-β-cellobiosidase) and recalcitrant (β-D-xylosidase, phenol oxidase, and peroxidase) components of the soil carbon pool.  

Results/Conclusions

Saltwater addition did not have a consistent effect on EEA of the labile substrates, but activity decreased dramatically for the more recalcitrant substrates.  For example, the activity of phenol-oxidase and peroxidase enzymes in the saltwater-added plots were 10-20% of the activity in the control plots.  These changes in microbial activity were correlated with rate potential measurements for methanogenesis, methanotrophy, and anaerobic CO₂ production, which were all greater under freshwater conditions.  Further, whole-core incubations showed a similar pattern in that soil O₂ demand was 4-6 times higher in freshwater versus salinized soils.  Combined, the results demonstrate the value in understanding enzyme activity as a possible predictor of the greenhouse gas fluxes, and reveal a community response to global change capable of affecting ecosystem-level processes.  The change in community function due to elevated salinity was coincident with a shift in the population structure of sulfate reducers and methanogens as assessed using whole-community DNA fingerprinting (T-RFLP for each functional group), demonstrating that both microbial community structure and function can be altered by global change stimuli.  This information is important for understanding the potential long-term effects on organic matter decomposition, which may play a role in wetland sediment accretion and net carbon storage