Temporal variability in soil organic matter accretion rates in coastal deltaic wetlands under changing depositional environments

Abstract

The Mississippi River Deltaic Plain experiences high relative sea level rise, limited sediment supply, and high marsh edge erosion, leading to the substantial coastal wetland and stored soil organic matter (SOM) loss. The objective of this study was to understand the SOM accumulation rates over the past 1000 years related to the changes in the depositional environment in these highly eroding coastal wetlands. Soil cores (2 m) were collected from four sites in Barataria Basin, LA and analyzed for proportion of organic and mineral matter, total C, N, P, particle size, and stable isotopic composition (δ¹³C and δ¹⁵N), as well as ¹⁴C and ¹³⁷Cs dating. The soil carbon stock in the 2 m depth (62.4 ± 2 kg m⁻²) was approximately 88% greater than the carbon stock in just the 1 m depth (33.1 ± 0.6 kg m⁻²) indicating a need for considering deeper soil profiles (up to 2 m) to estimate blue carbon stock in deltaic environments. The average vertical accretion rate for Barataria Basin was 8.1 ± 0.6 mm year⁻¹ over 50 years. The long-term (1000-year time scale) C accumulation rate (39 g C m⁻² year⁻¹) was ∼14% of the short-term accumulation rate (254 ± 19 g C m⁻² year⁻¹). Wetlands in Barataria Basin started as fresh marsh and transitioned over time to intermediate to brackish. These marshes were able to maintain relative elevation through the accumulation of organic matter and mud despite high relative rates of sea-level rise. However, the high rates of edge erosion may limit these marshes to continue to sequester atmospheric carbon under accelerating sea level in the absence of restoration efforts.