Salt Marsh Ecosystem Responses to Restored Tidal Connectivity Across a 14y Chronosequence

Salt marshes provide a variety of ecosystem services including carbon storage, nitrogen filtration, and nursery habitat for juvenile fish. However, salt marshes have been stressed over the years, resulting in changes in salt marsh biogeochemistry and the reduction of ecosystem services. Recently, efforts have been made to restore marshes back to their pre-disturbance states. This study focused on Cape Cod marshes that were blocked by tidal restrictions due to urban development and that have been reintroduced to tidal flow over the past 14 years. However, as scientists are not certain if degraded salt marshes ever truly regain their functionality, I looked at marsh restoration from a biogeochemical stance, assessing plant diversity, bulk density, and pore water geochemistry in marshes of varying restoration ages. The sites have been restored, via the installation of a culvert, at different points in time, thus allowing us to develop a chronosequence, defined as a space for time substitution, of marsh restoration. I found that restored marshes were recovering more quickly than anticipated. Salt marsh plant species were present within 3 years. By eleven years, the percent cover values were similar to those of natural marshes, and the plant species richness in the restored marshes was similar to that of a natural marsh. Pore water pH values in restored marshes were similar to natural marshes, suggesting that pH recovered within 3 years. Sulfide values were comparable to natural values within 7 years. Bulk density values in the upper horizon were converging towards reference values, suggesting that organic matter was being deposited. However, trends varied downcore presumably due to site histories. Dissolved inorganic carbon (DIC) data from restored sites showed similar rates of respiration as the natural sites by 14 years. Younger restored sites had slower respiration rates. In light of the metrics assessed, the restored marshes sampled on Cape Cod seem to be moving towards their initial pre-disturbance states and the restoration process is working

<b>Dataset: High methane concentrations in tidal salt marsh soils: where does the methane go?</b>

Tidal salt marshes produce and emit CH4. Therefore, it is critical to understand the biogeochemical controls that regulate CH4 spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH4 production, and higher salinity concentrations inhibit CH4 production in salt marshes. Recent evidence shows that CH4 is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil-atmosphere CH4 and CO2 fluxes coupled with depth profiles of soil CH4 and CO2 pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH4 production and fate within a temperate tidal salt marsh. We found unexpectedly high CH4 concentrations up to 145,000 μmol mol-1 positively correlated with S2- (salinity range: 6.6 to 14.5 ppt). Despite large CH4 production within the soil, soil-atmosphere CH4 fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m-2 s-1). CH4 and CO2 within the soil pore water were produced from young carbon, with most Δ14C-CH4 and Δ14C-CO2 values at or above modern. We found evidence that CH4 within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH4 is produced, including diffusion into the atmosphere, CH4 oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH4 production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co-occur and vary in importance over the year. This study highlights the potential for high CH4 production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH4 budgets and blue carbon in salt marshes.</p