Differences in long-term vs short-term carbon and nitrogen sequestration in a coastal river delta wetland: Implications for global budgets

Coastal wetlands can serve as a considerable sink for carbon (C) gases. However, the capacity for wetlands to serve as more permanent C and N sinks over the long term is less clear given the time dependence of sediment deposition, organic matter decomposition, and anthropogenic land use change. In this study, we compare the short-term (decadal scale) and long-term (millennial scale) C and N accumulation rates estimated using ¹³⁷Cs and radiocarbon dating of vibracores collected from a freshwater coastal wetland in the Louisiana Mississippi River deltaic plain (Atchafalaya River delta). The mean short-term (60 yrs) sediment accumulation rate was 1.4 cm/yr while the mean rate of long-term (100–1000 yrs) sediment accumulation was an order of magnitude lower at 0.12 cm/yr. Annual rates of C and N accumulated over the past several thousand years were approximately 10% of that over the past 60 years after correcting for bulk density. These results are similar to other coastal wetlands and suggest that time scale must be considered in determining the relative permanence of C and N storage in coastal wetland soils. This difference is especially important for assessing the role of C cycling in relation to global change models and N cycling related to water quality in accurately quantifying the role of coastal deltaic fresh water wetlands in regulating these biogeochemical cycles.