Net ecosystem carbon balance is a comprehensive assessment of ecosystem function that can test restoration effectiveness. Coastal peatlands are globally important carbon sinks that are vulnerable to carbon loss with saltwater intrusion. It is uncertain how wetland carbon stocks and fluxes change during freshwater restoration following exposure to saltwater and elevated nutrients. We restored freshwater to sawgrass (
- Award ID(s):
- 1832178
- NSF-PAR ID:
- 10390588
- Date Published:
- Journal Name:
- Science
- Volume:
- 376
- Issue:
- 6593
- ISSN:
- 0036-8075
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Cladium jamaicense ) peat monoliths from freshwater marshes of the Everglades (Florida, U.S.A.) that had previously been exposed to elevated salinity (approximately9 ppt) and phosphorus (P) loading (1 g P m−2year−1) in wetland mesocosms. We quantified changes in water and soil physicochemistry, plant and soil carbon and nutrient standing stocks, and net ecosystem productivity during restoration. Added freshwater immediately reduced porewater salinity from >8 to approximately 2 ppt, but elevated porewater dissolved organic carbon persisted. Above‐ and belowground biomass, leaf P concentrations, and instantaneous rates of gross ecosystem productivity (GEP) and ecosystem respiration (ER) remained elevated from prior added P. Modeled monthly GEP and ER were higher in marshes with saltwater and P legacies, resulting in negative net ecosystem productivities that were up to 12× lower than controls. Leaf litter breakdown rates and litter P concentrations were 2× higher in marshes with legacies of added saltwater and P. Legacies of saltwater and P on carbon loss persisted despite freshwater restoration, but recovery was greatest for freshwater marshes exposed to saltwater alone. Our results suggest that restoration in nutrient‐limited freshwater wetlands exposed to saltwater intrusion and nutrient enrichment is a slow process. -
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Abstract Emissions of methane (CH4) and nitrous oxide (N2O) from soils to the atmosphere can offset the benefits of carbon sequestration for climate change mitigation. While past study has suggested that both CH4and N2O emissions from tidal freshwater forested wetlands (TFFW) are generally low, the impacts of coastal droughts and drought‐induced saltwater intrusion on CH4and N2O emissions remain unclear. In this study, a process‐driven biogeochemistry model, Tidal Freshwater Wetland DeNitrification‐DeComposition (TFW‐DNDC), was applied to examine the responses of CH4and N2O emissions to episodic drought‐induced saltwater intrusion in TFFW along the Waccamaw River and Savannah River, USA. These sites encompass landscape gradients of both surface and porewater salinity as influenced by Atlantic Ocean tides superimposed on periodic droughts. Surprisingly, CH4and N2O emission responsiveness to coastal droughts and drought‐induced saltwater intrusion varied greatly between river systems and among local geomorphologic settings. This reflected the complexity of wetland CH4and N2O emissions and suggests that simple linkages to salinity may not always be relevant, as non‐linear relationships dominated our simulations. Along the Savannah River, N2O emissions in the moderate‐oligohaline tidal forest site tended to increase dramatically under the drought condition, while CH4emission decreased. For the Waccamaw River, emissions of both CH4and N2O in the moderate‐oligohaline tidal forest site tended to decrease under the drought condition, but the capacity of the moderate‐oligohaline tidal forest to serve as a carbon sink was substantially reduced due to significant declines in net primary productivity and soil organic carbon sequestration rates as salinity killed the dominant freshwater vegetation. These changes in fluxes of CH4and N2O reflect crucial synergistic effects of soil salinity and water level on C and N dynamics in TFFW due to drought‐induced seawater intrusion.
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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 (
), 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), , 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. -
Coastal forested wetlands support many endemic species, sequester substantial carbon stocks, and have been reduced in extent due to historic drainage and agricultural expansion. Many of these unique coastal ecosystems have been drained, while those that remain are now threatened by saltwater intrusion and sea level rise in hydrologically modified coastal landscapes. Several recent studies have documented rapid and accelerating losses of coastal forested wetlands in small areas of the Atlantic and Gulf coasts of North America, but the full extent of loss across North America’s Coastal Plain (NACP) has not been quantified. We used classified satellite imagery to document a net loss of 13,682 km2 (8%) of forested coastal wetlands across the NACP between 1996 and 2016. Most forests transitioned to scrub-shrub (53%) and marsh habitats (24%). Even within protected areas, we measured substantial rates of wetland deforestation and significant fragmentation of forested wetland habitats. Variation in the rate of sea level rise, the number of tropical storm landings, and the average elevation of coastal watersheds explained about 78% of the variation in coastal wetland deforestation extent along the south Atlantic and Gulf Coasts. The rate of coastal forest loss within the NACP (684 km2/y) exceeds the recent estimate of global losses of coastal mangroves (210 km2/y). At the current rate of deforestation, in the absence of widespread protection or restoration efforts, coastal forested wetlands may not persist into the next century.more » « less
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Abstract Coastal marshes are globally important, carbon dense ecosystems simultaneously maintained and threatened by sea‐level rise. Warming temperatures may increase wetland plant productivity and organic matter accumulation, but temperature‐modulated feedbacks between productivity and decomposition make it difficult to assess how wetlands and their thick, organic‐rich soils will respond to climate warming. Here, we actively increased aboveground plant‐surface and belowground soil temperatures in two marsh plant communities, and found that a moderate amount of warming (1.7°C above ambient temperatures) consistently maximized root growth, marsh elevation gain, and belowground carbon accumulation. Marsh elevation loss observed at higher temperatures was associated with increased carbon mineralization and increased microtopographic heterogeneity, a potential early warning signal of marsh drowning. Maximized elevation and belowground carbon accumulation for moderate warming scenarios uniquely suggest linkages between metabolic theory of individuals and landscape‐scale ecosystem resilience and function, but our work indicates nonpermanent benefits as global temperatures continue to rise.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/science.abn1479
