PS 22-19 - Determining Tipping Points of Soil Salinity and Water Level on Primary Productivity and Greenhouse Gas Emissions in Tidal Freshwater and Low-salinity Wetlands

Background/Question/Methods

Tidal freshwater forested wetlands (TFFW) play a critical role in wetland blue carbon budgets. However, TFFW are vulnerable to interactions between climate change and rising sea levels. For example, drought-induced saltwater intrusion threatens the carbon sequestration and storage capacity of these systems. Changes in carbon dynamics and biogeochemical processes in TFFW due to hydroperiod and salinity regime shifts remain unclear, especially across a wide range of settings. In this study, we examined thresholds of soil water salinity and water level for annual net primary productivity (NPP) and greenhouse gas emissions (CO₂, CH₄, and N₂O) in TFFW. We used a process-driven wetland biogeochemistry model that was developed for salinity-impacted tidal freshwater wetlands, the Tidal Freshwater Wetland Denitrification and Decomposition (TFW-DNDC) model. We selected tidal forest and oligohaline marsh sites along the Savannah River and Waccamaw River, USA, that are impacted by saltwater intrusion with soil salinities in the range of 1.2-4.9 psu. TFW-DNDC simulated wetland response to a series of soil salinities and water levels to determine specific tipping points of significant change in NPP, as well as CO₂, CH₄, and N₂O fluxes.

Results/Conclusions

Salinity thresholds for NPP (when NPP reduced > 50%) were approximately 5 psu regardless of soil water levels for forest sites, ~7 psu when soils were inundated to a depth >15 cm, and 10 psu when soils were dry for oligohaline marsh sites. Soil and live root respiration (CO₂ emission) decrease by >50% at ~7 psu when forest and marsh soils are inundated. For moderately salinity-impacted forest sites, maximum CH₄ emissions decrease by >50% at 0.5 psu when soils are inundated, and at 2 psu when soils are dry. For highly salinity impacted forest and oligohaline marsh sites, maximum CH₄ emissions decrease by >50% at 2 psu when soils are inundated and CH₄ uptake, rather than emission, occurs when soils are dry. Soil N₂O emissions decrease by >50% at 6 psu when soils are inundated by >10 cm of water. Our results indicate that thresholds of soil salinity and water level for primary productivity and greenhouse gas emissions vary in predictive ways with site-specific conditions in low salinity (≤10 psu) wetlands, suggesting wide-application of the TFW-DNDC model to similar forest-to-marsh transitions along the Atlantic and Gulf of Mexico coasts of the southeastern United States.