Coastal wetlands store immense amounts of carbon (C) in vegetation and sediments, but this store of C is under threat from climate change. Accelerated sea level rise (SLR), which leads to saltwater intrusion, and more frequent periods of droughts will both impact biogeochemical cycling in wetlands. Coastal peat marshes are especially susceptible to saltwater intrusion and changes in water depth, but little is known about how exposure to salinity affects organic matter accumulation and peat stability. I investigated freshwater and brackish marsh responses to elevated salinity, greater inundation, drought, and increased nutrient loading. I used coupled mesocosm and field experiments to test drivers and mechanisms of peat collapse.
Results/Conclusions
Elevated salinity pulses in a brackish marsh increased CO2 release from the marsh but only during dry-down. Elevated salinity increased root mortality at both a freshwater and brackish marsh. Under continuously elevated salinity in mesocosms, net ecosystem productivity (NEP) was unaffected by elevated salinity in a freshwater marsh exposed to brackish conditions (0 to 8 ppt), but NEP significantly increased with P enrichment. Elevated salinity led to a higher turnover of live to dead roots, resulting in a ~2-cm loss in soil elevation within 1 year of exposure to elevated salinity. When exposing a brackish marsh to more saline conditions (10 to 20 ppt), NEP, aboveground biomass production, and root growth all significantly decreased with elevated salinity, shifting the marsh from a net C sink to a net C source to the atmosphere. Elevated salinity (10 to 20 ppt) did not increase soil elevation loss, which was already occurring under brackish conditions, but when coupled with a drought event, elevation loss doubled. My findings suggest these hypotheses for the drivers and mechanisms of peat collapse. When freshwater marshes are first exposed to elevated salinity, soil structure and integrity are negatively affected through loss of live roots within the soil profile, leaving the peat vulnerable to collapse even though aboveground productivity and NEP may be unaffected. Subsequent dry-down events where water falls below the soil surface further accelerate peat collapse. Although saltwater intrusion into freshwater wetlands may initially stimulate primary productivity through a P subsidy, the impact of elevated salinity on root and soil structure has a greater deleterious effect and may ultimately be the factors that lead to the collapse of these marshes.