Changes in ecosystem carbon responses to saltwater exposure: Implications of sea level rise in the Florida coastal Everglades

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Coastal wetlands, which have immense potential to store carbon (C) in vegetation and sediments, are a vital part of the global C cycle. However, how C storage in coastal wetlands will be affected by accelerated sea level rise as a result of a warming climate is uncertain. In oligotrophic wetlands such as the Everglades in the southeastern USA, saltwater intrusion will bring ions (Cl^-, SO₄^2-) and phosphorus (P), a limiting nutrient for ecosystem productivity. It is hypothesized that shifts in stressors and subsidies can shift the soil carbon balance from a net C sink to a C source, stimulating peat collapse, which will in turn accelerate the effects of sea level rise. Previous research on mangrove peat soils found an increase in CO₂ efflux with elevated salinity, suggesting sea level rise may make soils susceptible to peat collapse. The objective of this study is to investigate how simulated saltwater intrusion into freshwater and oligohaline wetlands will change net ecosystem productivity and affect the soil C balance. Using coupled field and mesocosm experiments, we are examining how plant gross primary production, plant respiration, ecosystem respiration, microbial C processing, and net ecosystem exchange in freshwater and oligohaline wetlands will change when exposed to saltwater and an increase in P loading.   

Preliminary results from our mesocosm experiment show that cores exposed to elevated saltwater had higher CO₂ efflux compared to ambient salinity cores when soils were not inundated. Ecosystem respiration, which takes into account both plants and soils, was lower under elevated salinity conditions compared to ambient. For net ecosystem exchange, inundated cores were more of a C sink than exposed cores, while elevated salinity caused more C to be lost.  At both our fresh and brackish water field sites, gross ecosystem exchange was lower in plots manipulated with saltwater. Saltwater had no effect on ecosystem respiration at the freshwater site but was much lower at the brackish water site. Therefore, net ecosystem exchange at the freshwater site was lower in salt exposed plots but was not different across treatments at the brackish water site. Results from this study will reveal how the soil C balance in freshwater and oligohaline wetlands changes with saltwater intrusion due to sea level rise.