Variability in nutrient and carbon cycling of constructed marshes

Salt marshes provide valuable ecosystem services, including filtering nutrients and trapping sediments from the water column, but are increasingly threatened by sea-level rise and coastal development. Constructed marshes offer a potential pathway to offsetting marsh loss, but the extent to which they can replace the ecosystem services provided by mature natural marshes is not well understood. Using a chronosequence of constructed marshes (1, 21, and 44 years old) and a natural reference marsh (>100y old) in Beaufort, NC, we assessed the development of plant communities, sediment characteristics, and nutrient and carbon cycling over time. We compared aboveground biomass, surface sediment texture and bulk density, soil carbon (C) and nitrogen (N) stocks, sediment respiration, and porewater nutrient and sulfide concentrations to assess trajectories toward functional equivalence. Vegetation structure, sediment texture, bulk density, porewater nutrients, and soil respiration converged with the natural reference marsh within approximately two decades of construction. In contrast, surface sediment C and N stocks remained lower than reference levels even after ~20 years, and patterns appeared to be influenced by geomorphic setting and episodic sediment deposition rather than marsh age alone. Methane fluxes were negligible across all sites, and carbon dioxide respiration exhibited seasonal patterns but limited age-related differences. Our results demonstrate that while constructed marshes in sandy back barrier environments can achieve vegetation and physical equivalence with natural marshes within a few decades, long-term C and N storage develop more slowly. Further, the trajectory of development can be particularly complicated in shoreline marshes as a result of episodic deposition events. These findings highlight the importance of sediment texture, geomorphic context, and long-term monitoring when evaluating restoration success and estimating carbon benefits of estuarine living shoreline projects.