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Salinization of coastal freshwater wetlands is an increasingly common and widespread phenomenon resulting from climate change. The ecosystem consequences of added salinity are poorly constrained and highly variable across prior observational and experimental studies. We added 1.8 metric tons of marine salts to replicated 200 m2 plots within a restored forested wetland in Eastern North Carolina over the course of four years. Based on prior small-scale experiments at this site, we predicted that salinization would lead to slower tree growth and suppressed soil carbon cycling. Results from this large-scale field experiment were subtle and inconsistent over space and time. By the fourth year of the experiment, we observed the predicted suppression of soil respiration and a reduction of water extractable carbon from soils receiving salt treatments. However, we found no cumulative effects of four years of salinization on total soil carbon stocks, tree growth, or root biomass. We observed substantial variation in soil solution chemistry (notably, pH and base saturation) across replicated treatment blocks; the effective salt levels, ionic composition, and pH varied following treatment depending upon pre-existing differences in edaphic factors. Our multi-year monitoring also revealed an underlying trend of wetland acidification across the entire site, a suspected effect of ecosystem recovery following wetland restoration on former agricultural land. The overwhelming resistance to our salt treatments could be attributed to the vigor of a relatively young, healthy wetland ecosystem. The heterogeneous responses to salt that we observed over space and time merits further investigation into the environmental factors that control carbon cycling in wetlands. This work highlights the importance of multi-year, large-scale field experiments for investigating ecosystem responses to global environmental change. [ABSTRACT FROM AUTHOR]

Salinization of freshwater ecosystems impacts carbon cycling, a particular concern for coastal wetlands, which are important agents of carbon sequestration. Previous experimental work using salt additions as a proxy for sea level rise, reveals widely divergent effects of salt on soil carbon processes. We performed a laboratory salt addition experiment on two different types of wetland soils (Ponzer muck and Hyde loam, both poorly drained organic soils) from the Coastal Plain of North Carolina. We used a commercial aquarium salt mix to make treatment solutions of 0, 2.5 and 10 ppt salinity and independently manipulated solution pH (5.5, 7.2, 8.8) for a full factorial experimental design. Our goal was to identify the effects of increasing ionic strength and increasing soil solution pH on soil carbon solubility and turnover. Microbial respiration and dissolved organic carbon solubility were depressed by marine salts, while pH manipulation alone had minimal effect. The addition of marine salts substantially reduced rates of carbon mineralization, reduced carbon solubility, and preferentially reduced the abundance of phenolic and aromatic organic molecules in solution. In the more acidic Ponzer muck, where salt additions dropped the pH from > 5 to < 4, we measured more substantial declines in DOC concentrations than in the base saturated Hyde loam. In contrast, in the base saturated Hyde loam, more marine salts remained in solution and the treatment effects on rates of carbon mineralization were more pronounced. Our results provide a clear demonstration of how ion exchange mechanisms result in indirect effects of salinization on the pH of soil solution and the solubility of organic matter. These indirect effects may explain much of the existing variation in reports of salt effects on soil carbon dynamics. [ABSTRACT FROM AUTHOR]

Climate change is driving ecological shifts in coastal regions of the world, where low topographic relief makes ecosystems particularly vulnerable to sea‐level rise, salinization, storm surge, and other effects of global climate change. The consequences of rising water tables and salinity can penetrate well inland, and lead to particularly dramatic changes in freshwater forested wetlands dominated by tree species with low salt tolerance. The resulting loss of coastal forests could have significant implications to the coastal carbon cycle. We quantified the rates of vegetation change including land loss, forest loss, and shrubland expansion in North Carolina's largest coastal wildlife refuge over 35 yr. Despite its protected status, and in the absence of any active forest management, 32% (31,600 hectares) of the refuge area has changed landcover classification during the study period. A total of 1,151 hectares of land was lost to the sea and ~19,300 hectares of coastal forest habitat was converted to shrubland or marsh habitat. As much as 11% of all forested cover in the refuge transitioned to a unique land cover type—"ghost forest"—characterized by standing dead trees and fallen tree trunks. The formation of this ghost forest transition state peaked prominently between 2011 and 2012, following Hurricane Irene and a 5‐yr drought, with 4,500 ± 990 hectares of ghost forest forming during that year alone. This is the first attempt to map and quantify coastal ghost forests using remote sensing. Forest losses were greatest in the eastern portion of the refuge closest to the Croatan and Pamlico Sounds, but also occurred much further inland in low‐elevation areas and alongside major canals. These unprecedented rates of deforestation and land cover change due to climate change may become the status quo for coastal regions worldwide, with implications for wetland function, wildlife habitat, and global carbon cycling. [ABSTRACT FROM AUTHOR]

Coastal wetlands have the capacity to retain and denitrify large quantities of reactive nitrogen (N), making them important in attenuating increased anthropogenic N flux to coastal ecosystems. The ability of coastal wetlands to retain and transform N is being reduced by wetland losses resulting from land development. Nitrogen retention in coastal wetlands is further threatened by the increasing frequency and spatial extent of saltwater inundation in historically freshwater ecosystems, due to the combined effects of dredging, declining river discharge to coastal areas due to human water use, increased drought frequency, and accelerating sea-level rise. Because saltwater incursion may affect N cycling through multiple mechanisms, the impacts of salinization on coastal freshwater wetland N retention and transformation are not well understood. Here, we show that repeated annual saltwater incursion during late summer droughts in the coastal plain of North Carolina changed N export from organic to inorganic forms and led to a doubling of annual NH(4)(+) export from a 440 hectare former agricultural field undergoing wetland restoration. Soil solution NH(4)(+) concentrations in two mature wetlands also increased with salinization, but the magnitude of increase was smaller than that in the former agricultural field. Long-term saltwater exposure experiments with intact soil columns demonstrated that much of the increase in reactive N released could be explained by exchange of salt cations with sediment NH(4)(+). Using these findings together with the predicted flooding of 1661 km(2) of wetlands along the NC coast by 2100, we estimate that saltwater incursion into these coastal areas could release up to 18 077 Mg N, or approximately half the annual NH(4)(+) flux of the Mississippi River. Our results suggest that saltwater incursion into coastal freshwater wetlands globally could lead to increased N loading to sensitive coastal waters., (© 2013 John Wiley & Sons Ltd.)