Megan Kelsall

M.S. Candidate, Louisiana State University

2019 Conference Travel Grant Type 2 (Society of Wetland Scientists)

Source and Chemical Stability of Soil Carbon Across a 32-Year Chronosequence of Created Coastal Marshes

“Tidal marshes have been recognized for providing a number of important ecological services including soil carbon (C) sequestration. However, the loss of tidal marsh habitat due to sea level rise, subsidence, erosion, and anthropogenic stressors exposes previously stored soil organic carbon (SOC) to oxidation and reduces the overall capacity to sequester C. Mitigation through marsh restoration and creation have been used to compensate for marsh loss; however, the timescale and development of these blue C sinks are not well understood. The vulnerability of organic C to oxidation depends on its chemical stability and environmental conditions that may limit decomposition. Labile organic carbon (LC), or the fraction of soil carbon with rapid turnover time, is predicted to decompose quickly unless abiotic conditions, or mineral interactions limit decomposition. Recalcitrant organic carbon (RC) is the fraction of carbon that have slow rates of turnover, therefore remain stable for long time periods. For tidal marshes, predicting the fate and long-term storage of SOC is limited by complications such as the potential for multiple C sources, differences in their chemical stability, and the influence of environmental conditions that may preserve chemically labile carbon for long-time periods. In order to increase our understanding of soil C dynamics in created tidal marshes, SOC sources were estimated using δ13C analyses and chemical stabilities measured by acid hydrolysis along a time series of created marshes ranging from 1 to 32 years old in Sabine National Wildlife Refuge, southwest Louisiana. We hypothesized that the transition from algal to vegetation sources as marshes age is predicted to be supported by stable carbon isotope data and will be coincident with a shift from predominantly LC to RC. Our preliminary results support this hypothesis as the LC pool increased from 52 to 347 g m-2 in marshes from 5 to 32 years old, respectively while the RC pool increased more so such that there was approximately two times more RC in older marshes (e.g., 678 g m-2 in 32 yr old marsh). Additionally, the rate of RC accumulation was nearly twice that of LC in older marshes (22 g m-2 yr-1 compared to 12 g m-2 yr-1) , however both LC and RC accumulation rates were approximately 70% lower in created marshes than natural marshes. We further predict that C source, marsh elevation, hydroperiod, mineral sediment, and vegetation characteristics including species, stem density and biomass may influence LC and RC accumulation. Overall, our findings are anticipated to improve understanding of carbon sequestration potential and SOC development in created wetlands.”