Historic and current carbon storage in four marsh habitats of coastal Louisiana: Insight into potential responses to future global climate change and coastal restoration

The vegetation community and environmental characteristics (e.g., salinity, soil organic matter, soil chemistry) of marsh habitats influence how carbon is produced, decomposed, and stored in soils. In coastal Louisiana, marsh habitats have historically been classified as fresh, intermediate, brackish, and saline based on their position along the salinity gradient. Changing environmental conditions, such as sea-level rise and coastal restoration activities, may change the relative extent of the four marsh habitats and may ultimately influence carbon storage. To assess historic and current carbon storage, a 100-cm deep soil cores was collected at each of 24 sites within the four marsh habitats in two coastal Louisiana basins, Terrebonne and Barataria. Each core was sectioned into 2-cm depth intervals and analyzed for % moisture, bulk density, organic matter, carbon, and radionuclide geochronology (¹³⁷Cs and ²¹⁰Pb). In addition, historic maps (1949 to present) of the four marsh types at all 24 sites were used to help determine if marsh types have transitioned over time. Fresh and saline marshes tended not to experience marsh type transitions over a ~60 year period, whereas intermediate and brackish marshes had frequent transitions. Initial results indicate that fresh marshes tend to have the highest soil carbon content (mean = 30%, n = 63), while saline marshes have the lowest carbon content (mean = 10%, n = 66). Soil carbon content in intermediate and brackish marshes (mean = 20% and 18%, n = 72 and 66, respectively) represented mid-values between those of fresh and saline marshes and were similar to each other, indicating an intermediate position in carbon content response along the salinity gradient. However, the mean carbon densities among these marsh types were similar, ~ 0.2 g C cm^-3. This study helps to characterize historic and current carbon content and storage in four marsh types and provides insight into how it may change as marsh habitats transition with global climate change and coastal restoration.