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Coastal wetlands, among the most productive ecosystems, are important global reservoirs of carbon (C). Accelerated sea level rise (SLR) and saltwater intrusion in coastal wetlands increase salinity and inundation depth, causing uncertain effects on plant and soil processes that drive storage. We exposed peat-soil monoliths with sawgrass (Cladium jamaicense) plants from a brackish marsh to continuous treatments of salinity (elevated (~ 20 ppt) vs. ambient (~ 10 ppt)) and inundation levels (submerged (water above soil surface) vs. exposed (water level 4 cm below soil surface)) for 18 months. We quantified changes in soil biogeochemistry, plant productivity, and whole-ecosystem flux (gross ecosystem productivity, GEP; ecosystem respiration, ER). Elevated salinity had no effect on soil CO₂ and CH₄ efflux, but it reduced ER and GEP by 42 and 72%, respectively. Control monoliths exposed to ambient salinity had greater net ecosystem productivity (NEP), storing up to nine times more C than plants and soils exposed to elevated salinity. Submersion suppressed soil CO₂ efflux but had no effect on NEP. Decreased plant productivity and soil organic C inputs with saltwater intrusion are likely mechanisms of net declines in soil C storage, which may affect the ability of coastal peat marshes to adapt to rising seas.

Nitrogen and phosphorus concentrations were quantified in interstitial water, soil, and the roots of Cladium jamaicense Crantz from four herbaceous wetlands in southeast Mexico, locally known as sabanas, which are established in the karstic valley of the Holbox fracture system (northern Quintana Roo). We used the physicochemical and hydrogeochemical properties of the water to identify the existence of any relationships between nutrient (nitrogen and phosphorus) concentration and stock, and the hydrogeochemistry of each wetland. The wetlands have different classifications: H1 and H2 are palustrine, H3 is lacustrine, and H4 is estuarine. We found greater total phosphorus mass (mg kg −1 ) in the roots compared to the soil, which was particularly large in the wetland located at the south end of the western fracture. In general, phosphorus and nitrogen had a trend in the interstitial water and soil in which concentration and mass were higher H1 > H3 > H4, different from H2; these trends were not observed in the soil or roots. The N and P concentrations in the soil and roots were different among the wetlands, with the lowest measured at the site with brackish influence. The results presented in this research allow us to compare the nitrogen and phosphorus that can be stored in tropical karst wetlands and relate them to hydrogeochemistry.

Coastal wetlands are globally important stores of carbon (C). However, accelerated sea-level rise (SLR), increased saltwater intrusion, and modified freshwater discharge can contribute to the collapse of peat marshes, converting coastal peatlands into open water. Applying results from multiple experiments from sawgrass (Cladium jamaicense)-dominated freshwater and brackish water marshes in the Florida Coastal Everglades, we developed a system-level mechanistic peat elevation model (EvPEM). We applied the model to simulate net ecosystem C balance (NECB) and peat elevation in response to elevated salinity under inundation and drought exposure.Using amass C balance approach, we estimated net gain in C and corresponding export of aquatic fluxes (F AQ) in the freshwater marsh under ambient conditions (NECB = 1119 ± 229 gC m−2 year−1; F AQ = 317 ± 186 gC m−2 year−1). In contrast, the brackish water marsh exhibited substantial peat loss and aquatic C export with ambient (NECB = −366 ± 15 gC m−2 year−1; F AQ = 311 ± 30 gC m−2 year−1) and elevated salinity (NECB = 594 ± 94 gC m−2 year−1; F AQ = 729 ± 142 gC m−2 year−1) under extended exposed conditions. Further, mass balance suggests a considerable decline in soil C and corresponding elevation loss with elevated salinity and seasonal dry-down. Applying EvPEM, we developed critical marsh net primary productivity (NPP) thresholds as a function of salinity to simulate accumulating, steady-state, and collapsing peat elevations. The optimization showed that ∼150–1070 gC m−2 year−1 NPP could support a stable peat elevation (elevation change ≈ SLR), with the corresponding salinity ranging from 1 to 20 ppt under increasing inundation levels. The C budgeting and modeling illustrate the impacts of saltwater intrusion, inundation, and seasonal dry-down and reduce uncertainties in understanding the fate of coastal peat wetlands with SLR and freshwater restoration. The modeling results provide management targets for hydrologic restoration based on the ecological conditions needed to reduce the vulnerability of the Everglades’ peat marshes to collapse. The approach can beextended to other coastal peatlands to quantify C loss and improve understanding of the influence of the biological controls on wetland C storage changes for coastal management.

Increasing rates of sea-level rise (SLR) threaten to submerge coastal wetlands unless they increase soil elevation at similar pace, often by storing soil organic carbon (OC). Coastal wetlands face increasing salinity, marine-derived nutrients, and inundation depths from increasing rates of SLR. To quantify the effects of SLR on soil OC stocks and fluxes and elevation change, we conducted two mesocosm experiments using the foundation species sawgrass (Cladium jamaicense) and organic soils from freshwater and brackish Florida Everglades marshes for 1 year. In freshwater mesocosms, we compared ambient and elevated salinity (fresh, 9 ppt) and phosphorus (ambient, + 1 g P m⁻² year⁻¹) treatments with a 2 × 2 factorial design. Salinity addition reduced root biomass (48%), driving 2.8 ± 0.3 cm year⁻¹ of elevation loss, while soil elevation was maintained in freshwater conditions. Added P increased root productivity (134 %) but also increased breakdown rates (k) of roots (31%) and leaves (42%) with no effect on root biomass or soil elevation. In brackish mesocosms, we compared ambient and elevated salinity (10, 19 ppt) and inundated and exposed conditions (water level 5-cm below and 4-cm above soil). Elevated salinity decreased root productivity (70%) and root biomass (37%) and increased k in litter (33%) and surface roots (11%), whereas inundation decreased subsurface root k (10%). All brackish marshes lost elevation at similar rates (0.6 ± 0.2 cm year⁻¹). In conclusion, saltwater intrusion in freshwater and brackish wetlands may reduce net OC storage and increase vulnerability to SLR despite inundation or marine P supplies.

Saltwater intrusion and salinization of coastal wetlands around the world are becoming a pressing issue due to sea level rise. Here, we assessed how a freshwater coastal wetland ecosystem responds to saltwater intrusion. In wetland mesocosms, we continuously exposed Cladium jamaicense Crantz (sawgrass) plants and their peat soil collected from a freshwater marsh to two factors associated with saltwater intrusion in karstic ecosystems: elevated loading of salinity and phosphorus (P) inputs. We took repeated measures using a 2 × 2 factorial experimental design (n = 6) with treatments composed of elevated salinity (∼9 ppt), P loading (14.66 μmol P/d), or a combination of both. We measured changes in water physicochemistry, ecosystem productivity, and plant biomass change over two years to assess monthly and two-year responses to saltwater intrusion. In the short-term, plants exhibited positive growth responses with simulated saltwater intrusion (salinity + P), driven by increased P availability. Despite relatively high salinity levels for a freshwater marsh (∼9 ppt), gross ecosystem productivity (GEP), net ecosystem productivity (NEP), and aboveground biomass were significantly higher in the elevated salinity + P treated monoliths compared to the freshwater controls. Salinity stress became evident after extended exposure. Although still higher than freshwater controls, GEP and NEP were significantly lower in the elevated salinity + P treatment than the +P treatment after two years. However, elevated salinity decreased live root biomass regardless of whether P was added. Our results suggest that saltwater intrusion into karstic freshwater wetlands may initially act as a subsidy by stimulating aboveground primary productivity of marsh plants. However, chronic exposure to elevated salinity results in plant stress, negatively impacting belowground peat soil structure and stability through a reduction in plant roots.

Compound specific carbon and deuterium stable isotope values ( delta super(13)C and delta D) and the relative abundance of mid-chain n-alkanes (Paq) were determined for a series of dominant wetland plants, a surface slough-to-ridge soil transect, and slough and ridge soil cores, to assess historical vegetation successions induced by hydrological modification in an anthropogenically impacted, subtropical wetland, the Florida Everglades, USA. A difference of as much as 3.6 and 130 ppt in their delta super(13)C and delta D values was observed between the two most abundant emergent macrophyte species (Cladium and Eleocharis), respectively. A clear n-alkane delta D value depletion (-130 to -167 ppt) and decreasing Paq was observed along the slough-to-ridge soil transect, likely the result of an eco-hydrological transition from slough-to-ridge dominated vegetation (Eleocharis to Cladium). In agreement with the relatively constant Paq values, the lack of significant changes in the delta D depth profile for the slough core, suggest a consistent slough type of vegetation composition over time at that location. In contrast, changes of both n-alkane delta super(13)C and delta D values for the ridge core, especially after ~1960 AD, coincide with the expected plant successions from historically long hydroperiod (>8 months), slough type plants (Eleocharis, Utricularia, Nymphaea) to present day, shorter hydroperiod (<8 months), ridge type plants (Cladium). These delta super(13)C and delta D changes seem to be driven by vegetation shifts associated with hydrological change. The application of the compound-specific stable isotope determinations may strongly complement the biomarker approach for paleo-hydrological assessments in wetland ecosystems.

Despite the incontrovertible importance of wetland ecosystems, our understanding of the causes underlying the spatial heterogeneity of their attributes is still insufficient. Here, we assess the spatial variation of the floristics (species composition and their relative abundances) and quantitative structure (vegetation cover) of the San Pedro Mártir River riparian herbaceous wetlands (Tabasco, Mexico), and explore potential factors responsible for within- and among community variation. Vegetation was sampled at 10 sites along a 40-km stretch of the river. Per site cover of the recorded morphospecies was assessed with digital photography, and community diversity was analyzed using the Hill numbers framework. Total richness was 76 morphospecies (67 identified to some taxonomic category); the drastic decrease in true diversity with increasing q values indicated that few species dominate the community matrix, while numerous remaining species occupy the small interstices among them. Three Cyperaceae species had the largest importance in the community, with Cladium jamaicense being the most frequent and having the largest cover along the river. Ward’s site classification and NMDS ordination indicated the existence of four floristically and structurally different vegetation groups. Within sites, GLMMs showed a weak inverse relationship between species richness and distance from the river, but no relationship for plant cover. Fluvial geomorphology, and possibly regional geological heterogeneity, are the main factors determining the spatial variation of these herbaceous wetlands. Deep understanding of the relationship of herbaceous riparian wetlands with their environment will improve the prediction of the effects of modifications of fluvial dynamics and support efficient conservation strategies.

Aims Litter decomposition in wetlands is an important component of ecosystem function in these detrital systems. In oligotrophic wetlands, such as the Florida Everglades, litter decomposition processes are dependent on nutrient availability and litter quality. The aim of this study was to assess the differences and changes in chemical composition of above- and belowground plant tissues at different stages of decomposition, and to compare them to organic matter accumulating in wetland surface soils. Methods To understand the chemical changes occurring during the early stages of litter decomposition in wetlands, short-term subaqueous decomposition patterns of above- and belowground tissues from Cladium jamaicense and Eleocharis cellulosa were investigated at two freshwater marsh sites in the Florida Everglades. The composition of litter at different stages of decomposition was compared to that of the two end-members, namely fresh plant tissues and soil organic matter (SOM), in an effort to assess both the gradual transformation of this organic matter (OM) and the incorporation of above- vs. belowground biomass to wetland soils. The chemical composition of the litter and of surface soils was assessed using solid-state 13C nuclear magnetic resonance spectroscopy. Results Decomposition indices (alkyl/O-alkyl ratio, Aromaticity index) of Cladium and Eleocharis leaves varied during incubation likely reflecting physical leaching processes followed by a shift to microbial decomposition. Overall, Eleocharis leaves were more labile compared to Cladium leaves. Relative to aboveground litter, the belowground biomass of both species was more resistant to degradation, and roots were more resistant than rhizomes. Compared to the observed early diagenetic transformations of the plant litter, the SOM is at a more advanced stage of degradation, suggesting that the decomposition of litter and belowground biomass prior to its incorporation into wetland soils requires longer degradation times than those applied in this study. Conclusions Litter decomposition in Everglades' freshwater marshes is driven by a combination of tissue quality and site characteristics such as hydroperiod and nutrient availability, ultimately leading to the accumulation of peat.

Estimates of gaseous carbon (C) fluxes in wetlands are heavily based on temperature. However, isolating specific effects of temperature on anaerobic C processing from other controls (C quality and nutrients) has proven difficult. Here, we test the hypothesis that temperature sensitivity of soil organic matter (SOM) decomposition is more influenced by C quality than nutrient availability in subtropical freshwater, sawgrass (Cladium jamaicense)-based peats. Carbon age (characterized by depth: 0–10 and 10–20 cm) was used as a surrogate of C quality while two sites were selected with contrasting levels of nutrient (P) availability. In anaerobic laboratory incubations temperature was increased in 5 °C steps to assess the proportion of C available at a given temperature (i.e. thermo-labile C) as productions of gaseous (CO₂ and CH₄) and dissolved organic C (DOC) fractions. Thermo-labile C increased 3.1–3.6 times from 15 °C to 30 °C in all soils. Disproportionate increase in the production of gaseous forms versus DOC as well as CH₄:CO₂ was observed with warming. Observed Q₁₀ values followed the trend of CH₄ (~ 14) ≫ CO₂ (~ 2.5) > DOC (~ 1.7) and temperature sensitivity was more dependent on C quality than nutrient availability over the entire temperature range. Spectral analysis indicated more bio-available DOC production at higher temperature. Regression analysis also indicated that C quality primarily influenced SOM decomposition at lower temperature, while at higher temperature nutrient limitation dominantly controlled SOM decomposition. These findings confirm the role of C quality in temperature sensitivity of warm peat soils, but also indicate an increased importance of nutrient limitation at higher temperature.

Encroachment of woody shrubs into graminoid-dominated wetlands can impact ecosystem carbon and water cycling due to differences in species physiology. In subtropical Florida, shortened hydroperiods have led to the expansion of Carolina willow ( Salix caroliniana ) in sawgrass ( Cladium jamaicense ) marsh communities, potentially compromising ecosystem health. In this study, we assessed differences in leaf gas exchange between willow and sawgrass in Blue Cypress Marsh Conservation Area (BCMCA). Stomatal conductance ( g s ) and photosynthetic CO 2 exchange ( A net ) were measured across a range of photosynthetically active radiation (PAR; 0–2000 μmol m −2 s −1 ). Leaf area index (LAI; m 2 leaf m −2 ground) was determined for each species and used in conjunction with land cover estimates to extrapolate leaf measurements to the plant canopy and assess the consequences of shrub encroachment on landscape atmospheric carbon and water exchange. Willow had higher average rates of leaf g s and A net than sawgrass. However, willow had lower water use efficiency (WUE) and greater LAI, resulting in greater loss of water through transpiration by willow populations and diminishing projected landscape water availability despite marginally increased C assimilation. Climate drying or potential positive feedbacks of shrubs to autogenic drying may accelerate shrub encroachment and increase risk of wetland loss.