Beth A. Lawrence

Salt marshes are important blue carbon sinks and dynamic, connective ecosystems that experience change across a wide range of temporal scales, from daily tidal inundation to decadal and century‐scale human modifications to sea level rise over millennia. However, whether short‐term (∼100‐year) ecosystem shifts can be detected in soil carbon records remains unclear. Here, we use carbon isotopes ( δ ¹³ C) to reconstruct the recent vegetation and land‐use changes in a well‐studied salt marsh in Connecticut, USA. Leveraging the distinct isotopic signatures of dominant species (C ₃ Phragmites australis and C ₄ Spartina alterniflora ), we analyzed bulk δ ¹³ C, n ‐alkane δ ¹³ C, and soil CO ₂ , CH ₄ δ ¹³ C to (a) reconstruct plant community shifts associated with historical farming, ditching, tidal restrictions, and subsequent restoration, (b) evaluate the sensitivity of bulk versus leaf wax δ ¹³ C to ecological change, and (c) compare soil carbon legacies with soil CO ₂ and CH ₄ emissions. Both bulk sediment and leaf wax δ ¹³ C records capture the imprint of historical ditching and draining practices but do not clearly record vegetation changes associated with tidal restrictions and restoration over the past 60 years. n ‐Alkane sediment δ ¹³ C preserves evidence of C ₄ vegetation from 90 years ago that are not resolved in bulk sediment δ ¹³ C values. Soil CO ₂ δ ¹³ C reflects modern vegetation inputs combined with isotopic enrichment associated with microbial processing. These results demonstrate that carbon isotopes can effectively reconstruct ecosystem history where C ₃ and C ₄ vegetation are distinct, and leaf wax δ ¹³ C provides a more sensitive archive of plant‐derived inputs to marsh soil organic matter than bulk δ ¹³ C.

Coastal marshes and their carbon sequestration capacity are threatened by development and accelerated sea level rise, yet we lack an understanding of how carbon dynamics vary among dominant zones due to confounding feedbacks between environmental gradients and vegetation. We implemented a three‐year mesocosm experiment (48 379‐L tanks) manipulating plant species ( Spartina alterniflora , Phragmites australis , unvegetated tidal flat) under controlled hydroperiod and brackish salinity conditions to limit confounding physiochemical gradients. We quantified all biomass pools, net ecosystem exchange (NEE), and soil respiration (CO ₂ ) during the 2021 and 2022 growing seasons, and soil microbial communities in 2021. We measured field‐based soil respiration (CO ₂ ) rates in P. australis and S. alterniflora zones at one site in Stonington, Connecticut, USA, to compare with experimental fluxes. We observed greater biomass in vegetated than in tidal flat treatments, but no differences between vegetated P. australis and S. alterniflora treatments. Spartina alterniflora had the highest rates of NEE yet also had the highest soil respiration rates; both patterns were likely due to differential physiology between S. alterniflora and P. australis . Field‐based fluxes were comparable in magnitude to our experimental fluxes and similar across vegetation zones. Soil microbial community diversity was higher in vegetated treatments compared to unvegetated, whereas community composition moderately differed between vegetated treatments. Overall, growing two dominant coastal marsh grasses under similar physiochemical conditions neutralized many species differences typically observed in situ, suggesting strong environmental control on carbon cycling in coastal marshes.

Questions: Formerly cultivated lands are common targets for wetland restoration, but human interventions needed to achieve diverse plant communities in sites with agricultural legacies are not well understood. We studied pre‐restoration seed banks and pre‐ and post‐restoration plant communities of former cranberry farms, whose native peat was covered with anthropogenically placed sand during cultivation and that underwent hydrologic restoration. Location: Three retired cranberry ( Vaccinium macrocarpon Aiton) farm complexes in southeast Massachusetts, USA, that underwent hydrologic restoration between 2017 and 2020. Methods: We quantified seed germination from sand and peat collected from former farms in a greenhouse under three hydrologic treatments (inundated, saturated, and well‐drained) to assess the pre‐restoration seed bank composition. In the field, we surveyed plant species presence and cover both before and after restoration at permanent 9 m ² plots. We assessed species composition, richness, and diversity in seed bank treatments and in situ . We also measured changes in in situ vegetation over time and compared seed bank and in situ vegetation. Results: The 30 species that germinated from retired cranberry farm seed banks were primarily native, perennial, wetland‐adapted, and occurred in the sand but not peat. Highest germination rates and species richness occurred in saturated substrate, with very little germination in the well‐drained treatment. Seed banks were not predictive of post‐restoration in situ species richness, but post‐restoration vegetation was similarly native, perennial, and wetland‐adapted. Restoration increased in situ total plant and wetland‐adapted plant species richness compared with pre‐restoration, and this richness persisted for multiple years post‐restoration. Wetland plant cover was more variable and likely affected by year‐to‐year changes in growing season precipitation. Conclusions: Vegetation responses to wetland restoration at formerly cultivated sites indicate that diverse, native plant communities can regenerate following hydrologic restoration in sites with sandy, nutrient‐poor soils.

Restorations of former cranberry farms (“bogs”) aim to re-establish native wetland vegetation, promote cold water habitat, and attenuate nitrogen (N) delivery to coastal waters. It is unclear, though, how elements of restoration design such as microtopography, groundwater interception, and plant communities affect N removal via denitrification. In a recently restored riparian cranberry bog with created microtopography, we compared denitrification potential, nitrous oxide (N₂O) yield of denitrification (ratio of N₂O:N₂O + N₂ gases), in situ N₂O fluxes, soil chemistry, and plant communities at the highest and lowest elevations within 20 plots and at four side-channel groundwater seeps. Denitrification potential was > 2 × greater at low elevations, which had plant communities distinct from high elevations, and was positively correlated with plant species richness (Spearman’s rho = 0.43). Despite detecting high N₂O yield (0.86 ± 0.16) from low elevation soils, we observed small N₂O emissions in situ, suggesting minimal incomplete denitrification even in saturated depressions. Groundwater seeps had an order of magnitude higher denitrification potentials and 100–300 × greater soil NO₃− concentrations than the typically saturated low elevation soils. Groundwater seeps also had high N₂O yield (1.05 ± 0.15) and higher, but spatially variable, in situ N₂O emissions. Our results indicate that N removal is concentrated where soils interact with NO₃–rich groundwater, but other factors such as low soil carbon (C) also limit denitrification. Designing restoration features to increase groundwater residence time, particularly in low lying, species rich areas, may promote more N attenuation in restored cranberry bogs and other herbaceous riparian wetlands.

Tidal wetlands are important blue carbon reservoirs, but it is unclear how sea-level rise (SLR) may affect carbon cycling and soil microbial communities either by increased inundation frequency or via shifting plant species dominance. We used an in-situ marsh organ experiment to test how SLR-scenarios (0, +7.5, +15 cm) and vegetation treatments (Spartina alterniflora, Spartina patens, Phragmites australis, unvegetated controls) altered CO2 fluxes (net ecosystem exchange, ecosystem respiration), soil carbon mineralization rates, potential denitrification rates, and microbial community composition. Increasing inundation frequency with SLR treatments decreased the carbon sink strength and promoted carbon emissions with +15-cm SLR. However, SLR treatments did not alter soil chemistry, microbial process rates, or bacterial community structure. In contrast, our vegetation treatments affected all carbon flux measurements; S. alterniflora and S. patens had greater CO2 uptake and ecosystem respiration compared to P. australis. Soils associated with Spartina spp. had higher carbon mineralization rates than P. australis or unvegetated controls. Soil bacterial assemblages differed among vegetation treatments but shifted more dramatically over the three-month experiment. As marshes flood more frequently with projected SLR, marsh vegetation composition is predicted to shift towards more flood-tolerant S. alterniflora, which may lead to increased CO2 uptake, though tidal marsh carbon sink strength will likely be offset by increased abundance of unvegetated tidal flats and open water. Our findings suggest that plant species play a central role in ecosystem carbon dynamics in vegetated tidal marshes undergoing rapid SLR.

Salt marsh vegetation zones shift in response to large‐scale environmental changes such as sea‐level rise (SLR) and restoration activities, but it is unclear if they are good indicators of soil nitrogen removal. Our goal was to characterize the relationship between denitrification potential and salt marsh vegetation zones in tidally restored and tidally unrestricted coastal marshes, and to use vegetation zones to extrapolate how SLR may influence high marsh denitrification at the landscape scale. We conducted denitrification enzyme activity assays on sediment collected from three vegetation zones expected to shift in distribution due to SLR and tidal flow restoration across 20 salt marshes in Connecticut, USA (n = 60 sampling plots) during the summer of 2017. We found lower denitrification potential in short‐formSpartina alterniflorazones (mean, 95% CI: 4, 3–6 mg N h⁻¹ m⁻²) than inS. patens(25, 15–36 mg N h⁻¹ m⁻²) andPhragmites australis(56, 16–96 mg N h⁻¹ m⁻²) zones. Vegetation zone was the single best predictor and explained 52% of the variation in denitrification potential; incorporating restoration status and soil characteristics (soil salinity, moisture, and ammonium) did not improve model fit. Because denitrification potential did not differ between tidally restored and unrestricted marshes, we suggest landscape‐scale changes in denitrification after tidal restoration are likely to be associated with shifts in vegetation, rather than differences driven by restoration status. Sea‐level‐rise‐induced hydrologic changes are widely observed to shift high marsh dominated byS. patensto short‐formS. alterniflora. To explore the implications of this shift in dominant high marsh vegetation, we paired our measured mean denitrification potential rates with projections of high marsh loss from SLR. We found that, under low and medium SLR scenarios, predicted losses of denitrification potential due to replacement ofS. patensby short‐formS. alterniflorawere substantially larger than losses due to reduced high marsh land area alone. Our results suggest that changes in vegetation zones can serve as landscape‐scale predictors of the response of denitrification rates to rapid changes occurring in salt marshes.

Forested wetlands of the temperate north are increasingly exposed to deicing salts, but it is unclear how this may alter wetland biogeochemistry and plant community composition. To investigate potential effects of deicing salts on exurban forested wetlands in southern New England, we employed a multi‐site field study to describe spatiotemporal patterns of soil physiochemical, water quality, and vegetation characteristics with distance from road deicing salt source. We surveyed nine road‐adjacent, red maple‐dominated wetlands to quantify a suite of soil parameters (Na⁺, K⁺, Mg²⁺, Ca²⁺, pH, electrical conductivity (EC), heavy metals, N, P, and soil moisture), as well as surface and groundwater salinity, and vegetation communities. With increasing distance from roads along 165‐m transects penetrating into each wetland, soil salinity (EC, Na⁺) decreased, while soil base cation concentrations (Mg²⁺, Ca²⁺) increased, likely due to cation exchange (Na⁺displacing other base cations). We also measured foliar chemistry and observed elevated Na⁺and reduced Mg²⁺of dominant species leaf tissue near roads, suggesting plant nutrient uptake responds to road salt‐related changes in soil physicochemical variables. Despite this, we did not detect differences in plant community composition (ground, shrub layers) along road salt‐induced soil chemistry gradients in the field, likely because surface and groundwater salinities were relatively low (maximum: 0.64 ppt). To determine at which field salinities we could potentially expect changes in wetland plant communities, we conducted a full‐factorial, manipulative seed bank experiment to examine how NaCl concentrations (0, 0.5, 1, 2, 4, and 8 parts per thousand (ppt)), frequency of salt exposure (pulse, constant), and water level (surface, 2 cm below surface) affected soil seed bank responses. Seedling richness was reduced at salinities exceeding 1 ppt, and seedling density was reduced above 2 ppt, but pulsing tended to alleviate salt‐induced reductions in seed bank responses. As salinization of freshwater ecosystems continues to increase, our results suggest that field salinity levels of exurban New England forested wetlands are nearing yet still typically below the threshold for which we expect to see strong plant community responses.