Blue carbon stocks in temperate salt marshes (North Carolina, USA) vary in response to accelerated sea-level rise, salinity gradients and hydraulic modifications.

Organic-rich sediment accumulation in coastal wetlands mitigates atmospheric CO₂ pollution by sequestering carbon, potentially for thousands of years. Continued net carbon burial requires salt marshes to keep pace with sea-level rise while maintaining high sedimentary organic matter content. Over the last century, accelerated sea-level rise and human modifications such as ditching drove changes in the vegetation communities and sediment properties of estuarine Spartina alterniflora and Juncus roemarianus marshes in Core Sound and southern Pamlico Sound, North Carolina. The sites we studied range from sheltered, low-salinity high marsh, to more exposed and/or more saline low marsh, and also compared ditched and unditched areas. Surface vegetation and sediment patterns generally reflect increases in average salinity (from ~12 to ~33 psu) and mean tidal range (from < 20 to > 40 cm) from north to south, respectively over the 40 km length of our study region.

We measured sediment bulk density and organic carbon content in samples at 2-cm depth intervals in 34 sediment cores from throughout the study region. Cores range in depth from 50 to >250 cm depending on marsh sediment thickness. These data allow determination of buried carbon stock per unit marsh area. Analyses of stable carbon isotopic composition of organic matter provide information on shifting carbon sources over time, e.g., from C3 Juncus to C4 Spartina marsh vegetation in response to increased tidal inundation.

We find the highest organic carbon concentrations (>35%) in sheltered, unditched, low-salinity, low tidal-range sites dominated by Juncus. However, many marshes throughout the study area, both Juncus and Spartina, have much lower average carbon contents of ~10%. The blue carbon stock calculated for the upper 100 cm of the sediment column varies across our study region from >430 MgC/hectare to less than 280 MgC/hectare. Analyzing carbon density by depth shows a general upward pattern toward lower carbon stocks. This trend is more pronounced at ditched, exposed, and higher salinity sites. The trend toward decreasing below-ground organic carbon storage in response to faster sea-level rise at most of our sites implies reduced CO₂ uptake by blue carbon ecosystems similar to our study area without significant new tidal marsh restoration.