Spatial Variability in Deep SOC Storage in Georgia (USA) Salt Marshes

Abstract Coastal Georgia's (USA) salt marshes function as soil organic carbon (SOC) sinks because of their high primary productivity and slow decomposition rates. Quantifying SOC stocks is essential to establish a baseline for long‐term SOC trends and to highlight their capacity for long‐term carbon (C) sequestration. From 2021 to 2023, 28 soil cores (25–165 cm deep) were collected from six Georgia marshes and analyzed for SOC%, total nitrogen (N), bulk density (BD), and particle size by horizon. Stable C isotopes were examined in three marshes to identify C sources, and a small subset of samples was analyzed for radiocarbon. Surface SOC (0–5 cm) ranged from 1.4% to 8.5% (SE = 0.28), while SOC across all depths ranged from 0.08% to 8.8% (mean = 3.7%, SE = 0.07). Mainland sites exhibited significantly higher surface SOC than island sites and were dated to be younger with radiocarbon testing. Stable isotopes indicated shifts in C 3 and C 4 vegetation dominance over time. Georgia salt marshes store an estimated 16,777,000 Mg C to 70 cm depth. Calculated to 60 cm, the mean SOC density was 88.5 Mg C ha −1 , approximately 55% of the total U.S. national average for tidal salt marshes. Variations in SOC content were primarily explained by clay content, depth, and BD. These results provide SOC estimates for Georgia's salt marshes and help address a regional data gap for southeastern U.S. coastal wetlands, underscoring the importance of broad scale assessments for improving national C accounting and understanding spatial variability in C storage.