Biogeochemical Effects of Saltwater Intrusion and Increased Inundation on Everglades Peat Soil

The mangrove wetlands that dominate the coastal Everglades (Florida, USA) overlie >1 m of carbon-rich peat soil and serve as a globally important carbon sink.  With sea level rising at ~3 mm y-1, these intertidal systems are being exposed to increased inundation and potentially higher salinity seawater. Changes in water level and salinity can affect soil microbial activity by altering redox potential, electron acceptor availability, the intensity of osmotic stress, and consequently, the rate of carbon cycling. Twenty-four field replicate intact peat monoliths from lower Shark River Slough (SRS 6) were collected in perforated buckets and transported to an outdoor tidal mesocosm facility in Key Largo, FL. Cores were randomly assigned to treatments: control water level or increased inundation (+7.6 cm); and ambient salinity (15-20 ppt) or elevated salinity (30-35 ppt).  Carbon dioxide production (day and night low tide, rising and falling tide) methanogenesis (low tide), porewater nutrients (DOC, NH4+, NOx, SRP), and redox potential were routinely monitored during the 3-month study.  On average, elevated salinity accelerated CO2 flux by 53%, increased inundation decreased CO2 flux by 12%, and the combined effects of increased inundation and elevated salinity was a 21% decrease in the rate of CO2 flux.  Despite low redox potential (-300 mV), methane production was minimal, but slightly greater in the soils exposed to increased inundation and ambient salinity treatment (27.4 mg CH4-C m-2 d-1).  DOC production averaged 12.5 ± 0.5 mg L-1 for all treatments except the increased inundation, elevated salinity treatment, which produced 57% more DOC than the other 3 treatments.  In terms of total organic C cycling, elevated salinity increased the rate of total C loss by 53%, increased inundation decreased the rate of loss by 8%, and the combined effect of elevated salinity and increased inundation resulted in the lowest rate of C cycling- 18% less than the control.  This study illustrates that both biological and physical changes to the soil may contribute to reduced C storage in peat soils exposed to sea level rise.