We experimentally increased salinities in a tidal freshwater marsh on the Altamaha River (Georgia, USA) by exposing the organic rich soils to 3.5 yr of continuous (press) and episodic (pulse) treatments with dilute seawater to simulate the effects of climate change such as sea level rise (press) and drought (pulse). We quantified changes in root production and decomposition, soil elevation, and soil C stocks in replicated (n = 6) 2.5 × 2.5 m field plots. Elevated salinity had no effect on root decomposition, but it caused a significant reduction in root production and belowground biomass that is needed to build and maintain soil elevation capital. The lack of carbon inputs from root production resulted in reduced belowground biomass of 1631 AE 308 vs. 2964 AE 204 g/m 2 in control plots and an overall 2.8 AE 0.9 cm decline in soil surface elevation in the press plots in the first 3.5 yr, whereas the control (no brackish water additions) and the fresh (river water only) treatments gained 1.2 AE 0.4 and 1.7 AE 0.3 cm, respectively, in a 3.5-yr period. There was no change in elevation of pulse plots after 3.5 yr. Based on measurements of bulk density and soil C, the decline of 2.8 cm of surface elevation resulted in a loss of 0.77 AE 0.5 kg C/m 2 in press plots. In contrast, the control and the fresh treatment plots gained 0.25 AE 0.04 and 0.36 AE 0.03 kg C/m 2 , respectively, which represents a net change in C storage of more than 1 kg C/m 2. We conclude that, when continuously exposed to saltwater intrusion, the tidal freshwater marsh's net primary productivity, especially root production, and not decomposition, are the main drivers of soil organic matter (SOM) accumulation. Reduced productivity leads to loss of soil elevation and soil C, which has important implications for tidal freshwater marsh persistence in the face of rising sea level.

The long‐term stability of coastal wetlands is determined by interactions among sea level, plant primary production, sediment supply, and wetland vertical accretion. Human activities in watersheds have significantly altered sediment delivery from the landscape to the coastal ocean, with declines along much of the U.S. East Coast. Tidal wetlands in coastal systems with low sediment supply may have limited ability to keep pace with accelerating rates of sea‐level rise (SLR). Here, we show that rates of vertical accretion and carbon accumulation in nine tidal wetland systems along the U.S. East Coast from Maine to Georgia can be explained by differences in the rate of relative SLR (RSLR), the concentration of suspended sediments in the rivers draining to the coast, and temperature in the coastal region. Further, we show that rates of vertical accretion have accelerated over the past century by between 0.010 and 0.083 mm yr−2, at roughly the same pace as the acceleration of global SLR. We estimate that rates of carbon sequestration in these wetland soils have accelerated (more than doubling at several sites) along with accelerating accretion. Wetland accretion and carbon accumulation have accelerated more rapidly in coastal systems with greater relative RSLR, higher watershed sediment availability, and lower temperatures. These findings suggest that the biogeomorphic feedback processes that control accretion and carbon accumulation in these tidal wetlands have responded to accelerating RSLR, and that changes to RSLR, watershed sediment supply, and temperature interact to determine wetland vulnerability across broad geographic scales.

A paradigm in carbon cycling science predicts that sea-level rise will enhance carbon accumulation in an apparent negative carbon-climate feedback1,2. However, ecosystems exposed to combinations of stressors and subsidies – such as saltwater intrusion and sea-level rise – may adapt, transition to an alternative state, or experience a decline in functions, such as carbon storage, thereby altering their response trajectories to environmental changes3,4. Climate change is increasing salinity in coastal ecosystems worldwide yet the effects on ecosystem metabolism remain uncertain4-8. Here, we synthesized gross ecosystem productivity (GEP), ecosystem respiration [CO2 and CH4 (ERCO2 and ERCH4)], and net ecosystem productivity (NEP) from diverse coastal marshes exposed to experimental additions and observational gradients in salinity. Increases in salinity generally caused decreases in median GEP, ERCO2, and ERCH4 but increases in GEP and NEP from ~5 to 10 ppt. Increased saltwater intrusion can stimulate or stress wetlands based on relative exposure and acclimation to increased salinities, and we detected positive NEP where salinity increases had greater negative effects on ERCO2 and ERCH4 than GEP. Although increases in NEP are detectable at low salinities, saltwater intrusion and climate-driven disturbances may reduce carbon storage capacity of coastal ecosystems as productivity declines toward higher salinities.

Deltaic wetlands in coastal Louisiana are experiencing widespread changes in vegetation dynamics and distribution due to rising sea level and long-term modifications in hydrology and sediment supply. Using field and remote sensing data, we investigated how aboveground biomass (AGB) and C stocks change in response to seasonality along salinity and soil nutrient gradients across different wetland habitats in two coastal basins with active (Atchafalaya - AB) and inactive (Terrebonne - TB) hydrological regimes. The highest seasonal changes in AGB and C stocks across both basins occurred in saline (SL) sites (AGB range: 343 ± 101 to 1214 ± 210 g m− 2) in early growing and peak biomass season, respectively. Biomass productivity rates varied across basins, with SL sites being the most productive, albeit less species-diverse. Foliar nutrient uptake was higher in the mineral-rich soils of AB freshwater (FW) site. In contrast, Terrebonne FW plants (as well as brackish and SL) had lower tissue nutrients and higher biomass allocation, indicating greater nutrient use efficiency with increasing salinity stress. Seasonal variation in AGB was positively correlated with porewater salinity and with soil nutrients (total nitrogen (N) and phosphorus). As hypothesized, changes in plant and soil isotopic signatures in both basins paralleled the spatiotemporal patterns in environmental stressors (e.g., elevated salinity and nutrient availability). Our findings show that in both active and inactive coastal delta basins, herbaceous wetlands maintain high biomass and C stocks by developing adaptive strategies in response to the distribution of environmental stressors and availability of resources.