Mon, Aug 02, 2021:On Demand
Background/Question/Methods
Marsh restoration and creation can mitigate losses of global salt marsh surface area and the subsequent reductions in the ecosystem functions they provide. While these constructed or created marshes can rapidly recover ecosystem structure, recovery of carbon sequestration, nitrogen removal, and other biogeochemical processes occurs over longer timescales. If the pace of recovery for carbon sequestration and nitrogen removal, two highly valued ecosystem services, is too slow, creating or restoring marshes will need to proceed at a faster rate than current efforts to compensate for marsh loss. We compared seasonal rates of denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and ecosystem CO2 exchange, along with above- and below-ground plant biomass and porewater nutrients at a 33-year-old excavation-created salt marsh (CON) and a nearby natural reference (NAT) salt marsh in Alabama, USA to assess functional recovery. In addition to biogeochemical measurements, we used available data on land cover changes associated with urbanization, sea-level rise, or hurricane impacts during the past three decades to assess the pace at which marsh creation or restoration needs to proceed in the region to compensate for marsh loss.
Results/Conclusions We found that while standing plant aboveground and belowground biomass were similar between NAT and CON, soil organic matter (SOM) was 6× lower in CON. Gross ecosystem productivity, ecosystem respiration, and net ecosystem exchange were similar across marshes. In both systems, N-retention by DNRA accounted for upwards of 75% of nitrate reduction, and rates of DNRA and denitrification were similar across marshes. While porewater ammonium concentrations were similar across all marshes, porewater phosphate and nitrate concentrations were greater in the NAT than in CON. However, the lag in recovery of porewater nutrient stocks and SOM content and nitrogen cycling rates suggests that some biogeochemical functions take longer than a few decades to fully recover in constructed marshes. Trends of land cover change in the past three decades indicate an annual marsh loss equivalent to 1% (–1106 m2 y-1) in the region. While current marsh construction practices could be a suitable tool for recovering plant structure and some ecosystem functions, the long-term rates of marsh loss suggest that a much greater pace of marsh construction in the region would be needed to compensate for losses of ecosystem functions.
Results/Conclusions We found that while standing plant aboveground and belowground biomass were similar between NAT and CON, soil organic matter (SOM) was 6× lower in CON. Gross ecosystem productivity, ecosystem respiration, and net ecosystem exchange were similar across marshes. In both systems, N-retention by DNRA accounted for upwards of 75% of nitrate reduction, and rates of DNRA and denitrification were similar across marshes. While porewater ammonium concentrations were similar across all marshes, porewater phosphate and nitrate concentrations were greater in the NAT than in CON. However, the lag in recovery of porewater nutrient stocks and SOM content and nitrogen cycling rates suggests that some biogeochemical functions take longer than a few decades to fully recover in constructed marshes. Trends of land cover change in the past three decades indicate an annual marsh loss equivalent to 1% (–1106 m2 y-1) in the region. While current marsh construction practices could be a suitable tool for recovering plant structure and some ecosystem functions, the long-term rates of marsh loss suggest that a much greater pace of marsh construction in the region would be needed to compensate for losses of ecosystem functions.