Ecosystem carbon exchange and nitrogen removal rates in two 33-year-old constructed salt marshes are similar to those in a nearby natural marsh

Human activities have led to 1–2% of coastal wetlands lost per year globally, with subsequent losses in ecosystem services such as nutrient filtering and carbon sequestration. Wetland construction is used to mitigate losses of marsh cover and services resulting from human impacts in coastal areas. Though marsh structure can recover relatively quickly (i.e., <10 years) after construction, there are often long-term lags in the recovery of ecosystem functions in constructed marshes. We conducted a year-long study comparing seasonal plant productivity, ecosystem respiration ((Formula presented.)), denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) between two 33-year-old constructed marshes (CON-1, CON-2) and a nearby natural reference marsh (NAT). We found that CON-1 and CON-2 were structurally similar to NAT (i.e., plant aboveground and belowground biomass did not differ). Likewise, gross ecosystem productivity (GEP), (Formula presented.), and net ecosystem exchange (NEE) were similar across all marshes. Further, DNRA and denitrification were similar across marshes, with the exception of greater denitrification rates at CON-2 than at the other two sites. While pore-water ammonium concentrations were similar across all marshes, organic matter (OM) content, pore-water phosphate, nitrate + nitrite, and hydrogen sulfide concentrations were greater in NAT than CON-1 and CON-2. Collectively, this work suggests that current marsh construction practices could be a suitable tool for recovering plant structure and some ecosystem functions. However, the lag in recovery of pore-water nutrient stocks and OM content also suggests that some biogeochemical functions may take longer than a few decades to fully recover in constructed marshes.