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

Abstract

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 (ERCO2), 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), ERCO2, 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.

Seasonal sediment dynamics in a constructed and natural tidal marsh in the northern Gulf of Mexico

“…Sections of pine savannah were excavated in parallel strips to an elevation of +0.3 m mean sea level (MSL) and connected to the West Fowl River through a nearby existing tidal creek by dredging artificial tidal creeks through the center of each marsh (Figure 1c; Vittor et al 1987). NAT is slightly higher in elevation with respect to sea level than CON-1 (0.36 ± 0.01 in NAT, 0.32 ± 0.04 in CON-1; Ledford et al 2021). The marsh platform in CON-1 was planted with J. roemerianus and S. alterniflora in 1988 but is now dominated nearly entirely by J. roemerianus.…”

Seasonal and Zonal Succession of Bacterial Communities in North Sea Salt Marsh Sediments

“…Nitrification at the terrestrial sites and denitrification in the marine zone of our salt marshes exhibit synergistic effects for nitrogen removal. Together with the ability to reduce oxidized nitrogen species along the whole transect, these processes are underpinning the functioning of wetlands in mitigating reactive nitrogen species of the downstream water bodies [ 67 , 68 ]. While denitrification removes nitrogen from the salt marshes by N 2 formation, assimilatory nitrate and sulfate reduction increased towards the terrestrial zones.…”

“…Furthermore, it could irreversibly damage the balance of natural oceanic communities, including fish populations, thereby having strong implications for human food availability and nature preservation. Thus, it is not only of great interest to intensify the ongoing efforts to reduce the anthropogenic nitrogen inputs to the environments [ 73 ], but also to increase the attempts to protect and restore wetlands [ 68 ], including salt marshes. Detailed investigations of the N-cycle, combining N-measurements and conversion rate measurements with the microbial diversity and their metabolic potential, would deepen our understanding of salt marsh functioning.…”

Stratified vertical sediment profiles increase burrowing crab effects on salt marsh edaphic conditions

“…All three sites are located within a subtropical estuary with an average annual temperature of 18.8°C, an average rainfall of 1.63 m, and diurnal microtides with an amplitude of approximately 0.26 m (Smyth, 2020). All sites ranged in elevation from 0.26 to 0.36 NAVD88 (in meters; Ledford et al, 2021).…”

“…The plant community at each marsh is dominated by Juncus roemerianus Scheele (hereafter, Juncus ), with smaller patches of Spartina alterniflora Loisel and Distichlis spicata Greene along creek banks and in the high marsh, respectively. The aboveground biomass at all three sites is comparable; however, CON‐1 and CON‐2 have lower belowground biomass, pore water and extractable nutrient concentrations, and organic matter content than NAT (Ledford et al, 2021; Tatariw et al, 2021). These differences likely result from the mitigation methods employed to create CON‐1 and CON‐2, which were converted to tidal marshes by harvesting pine savanna habitat and excavating topsoil down to a clay layer that was parallel with the water table (Vittor et al, 1987).…”