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Freshwater wetlands of the temperate north are exposed to a range of pollutants that may alter their function, including nitrogen (N)-rich agricultural and urban runoff, seawater intrusion, and road salt contamination, though it is largely unknown how these drivers of change interact with the vegetation to affect wetland carbon (C) fluxes and microbial communities. We implemented a full factorial mesocosm (378.5 L tanks) experiment investigating C-related responses to three common wetland plants of eastern North America (Phragmites australis, Spartina pectinata, Typha latifolia), and four water quality treatments (fresh water control, N, road salt, sea salt). During the 2017 growing season, we quantified carbon dioxide (CO2) and methane (CH4) fluxes, above- and below-ground biomass, root porosity, light penetration, pore water chemistry (NH4+, NO3-, SO4-2, Cl-, DOC), soil C mineralization, as well as sediment microbial communities via 16S rRNA gene sequencing. Relative to freshwater controls, N enrichment stimulated plant biomass, which in turn increased CO2 uptake and reduced light penetration, especially in Spartina stands. Root porosity was not affected by water quality, but was positively correlated with CH4 emissions, suggesting that plants can be important conduits for CH4 from anoxic sediment to the atmosphere. Sediment microbial composition was largely unaffected by N addition, whereas salt amendments induced structural shifts, reduced sediment community diversity, and reduced C mineralization rates, presumably due to osmotic stress. Methane emissions were suppressed by sea salt, but not road salt, providing evidence for the additional chemical control (SO4-2 availability) on this microbial-mediated process. Thus, N may have stimulated plant activity while salting treatments preferentially enriched specific microbial populations. Together our findings underpin the utility of combining plant and microbial responses, and highlight the need for more integrative studies to predict the consequences of a changing environment on freshwater 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 CO 2 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 CO 2 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 CO 2 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.

Typha is an iconic wetland plant found worldwide. Hybridization and anthropogenic disturbances have resulted in large increases in Typha abundance in wetland ecosystems throughout North America at a cost to native floral and faunal biodiversity. As demonstrated by three regional case studies, Typha is capable of rapidly colonizing habitats and forming monodominant vegetation stands due to traits such as robust size, rapid growth rate, and rhizomatic expansion. Increased nutrient inputs into wetlands and altered hydrologic regimes are among the principal anthropogenic drivers of Typha invasion. Typha is associated with a wide range of negative ecological impacts to wetland and agricultural systems, but also is linked with a variety of ecosystem services such as bioremediation and provisioning of biomass, as well as an assortment of traditional cultural uses. Numerous physical, chemical, and hydrologic control methods are used to manage invasive Typha , but results are inconsistent and multiple methods and repeated treatments often are required. While this review focuses on invasive Typha in North America, the literature cited comes from research on Typha and other invasive species from around the world. As such, many of the underlying concepts in this review are relevant to invasive species in other wetland ecosystems worldwide.

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-form Spartina alterniflora zones (mean, 95% CI: 4, 3–6 mg Nh⁻¹ m⁻²) than in S. patens (25, 15–36 mg N h⁻¹ m⁻²) and Phragmites 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 by S. patens to short-form S. 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 of S. patens by short-form S. alterniflora were 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.

The resource budget model for mast seeding hypothesizes that soil nutrients proximately influence reproduction. Plants in high soil nutrient (particularly N) areas are predicted to have lower reproductive variability over time and higher mean reproduction. While often examined theoretically, there are relatively few empirical tests of this hypothesis. We quantified cone production of 110 individual white spruce (Picea glauca) trees over seven years and quantified plant-available soil macronutrients (N, Ca, K, Mg, P, S) in natural forest conditions across three years with different cone crop conditions. Each of these plant-available soil nutrients were correlated across years (r s = 0.55–0.89; all > 0.81 for total-N); spatially, total-N availability varied 366-fold across trees. Plant-available soil nutrients did not influence variability or mean annual reproduction, contrary to nutrient perturbation experiments. We examined within-year nutrient and cone-production relationships, and observed significant positive relationships between reproduction and plant-available soil nutrients only in a low-reproduction year preceding a mast event. Both during a mast event and the following year, when overall cone production was very high or very low, there were no relationships. Both external drivers (e.g., weather) and internal resource budgets likely influence soil nutrient-reproduction relationships. These results suggest that plant-available soil nutrients may not be a large factor influencing mast-seeding patterns among individuals in this species.

In Laurentian Great Lakes coastal wetlands (GLCWs), dominant emergent invasive plants are expanding their ranges and compromising the unique habitat and ecosystem service values that these ecosystems provide. Herbiciding and burning to control invasive plants have not been effective in part because neither strategy addresses the most common root cause of invasion, nutrient enrichment. Mechanical harvesting is an alternative approach that removes tissue‐bound phosphorus and nitrogen and can increase wetland plant diversity and aquatic connectivity between wetland and lacustrine systems. In this study, we used data from three years of Great Lakes‐wide wetland plant surveys, published literature, and bioenergy analyses to quantify the overall areal extent of GLCWs, the extent and biomass of the three most dominant invasive plants, the pools of nitrogen and phosphorus contained within their biomass, and the potential for harvesting this biomass to remediate nutrient runoff and produce renewable energy. Of the approximately 212,000 ha of GLCWs, three invasive plants (invasive cattail, common reed, and reed canary grass) dominated 76,825 ha (36%). The coastal wetlands of Lake Ontario exhibited the highest proportion of invasive dominance (57%) of any of the Great Lakes, primarily from cattail. A single growing season's biomass of these invasive plants across all GLCWs was estimated at 659,545 metric tons: 163,228 metric tons of reed canary grass, 270,474 metric tons of common reed, and 225,843 metric tons of invasive cattail, and estimated to contain 10,805 and 1144 metric tons of nitrogen and phosphorus, respectively. A one‐time harvest and utilization for energy of this biomass would provide the gross equivalent of 1.8 million barrels of oil if combusted, or 0.9 million barrels of oil if converted to biogas in an anaerobic digester. We discuss the potential for mitigating non‐point source nutrient pollution with invasive wetland plant removal, and other potential uses for the harvested biomass, including compost and direct application to agricultural soils. Finally, we describe the research and adaptive management program we have built around this concept, and point to current limitations to the implementation of large‐scale invasive plant harvesting.

Aims We examined how mechanical management of invasive macrophyte, Typha × glauca alters plant-soil interactions underlying carbon processes and nutrient cycling, which are important to wetland function but under-represented in restoration research. Methods In the northern Great Lakes, we compared plant biomass, light transmittance, soil nutrient availability and carbon mineralization rates of Typha -dominated controls with Typha stands harvested above the waterline (harvest) and at the soil surface (submerged harvest). Results Relative to controls, harvested stands had 50% less litter and twice as much light transmittance to the water surface after one year. However, Typha stems re-grew, and soil nutrient availability rates were similar to controls. Submerged harvest eliminated Typha litter and stems, and increased light transmittance through the water column. P and K soil availability rates were 70% greater with submerged harvest than in controls. Soil C mineralization rates were not affected by treatment (mean ± 1 SE; 40.11 ± 2.48 μg C-CO 2 and 2.44 ± 0.85 μg C-CH 4 g −1 soil C hr. −1 ), but were positively correlated with soil Fe availability. Conclusions While submerged harvest effectively decreased invasive Typha biomass after one year, it increased soil nutrient availability, warranting further examination of macronutrient cycling and export during invasive plant management.

Plant invasions result in biodiversity losses and altered ecological functions, though quantifying loss of multiple ecosystem functions presents a research challenge. Plant phylogenetic diversity correlates with a range of ecosystem functions and can be used as a proxy for ecosystem multifunctionality. Laurentian Great Lakes coastal wetlands are ideal systems for testing invasive species management effects because they support diverse biological communities, provide numerous ecosystem services, and are increasingly dominated by invasive macrophytes. Invasive cattails are among the most widespread and abundant of these taxa. We conducted a three‐year study in two Great Lakes wetlands, testing the effects of a gradient of cattail removal intensities (mowing, harvest, complete biomass removal) within two vegetation zones (emergent marsh and wet meadow) on plant taxonomic and phylogenetic diversity. To evaluate native plant recovery potential, we paired this with a seed bank emergence study that quantified diversity metrics in each zone under experimentally manipulated hydroperiods. Pretreatment, we found that wetland zones had distinct plant community composition. Wet meadow seed banks had greater taxonomic and phylogenetic diversity than emergent marsh seed banks, and high‐water treatments tended to inhibit diversity by reducing germination. Aboveground harvesting of cattails and their litter increased phylogenetic diversity and species richness in both zones, more than doubling richness compared to unmanipulated controls. In the wet meadow, harvesting shifted the community toward an early successional state, favoring seed bank germination from early seral species, whereas emergent marsh complete removal treatments shifted the community toward an aquatic condition, favoring floating‐leaved plants. Removing cattails and their litter increased taxonomic and phylogenetic diversity across water levels, a key environmental gradient, thereby potentially increasing the multifunctionality of these ecosystems. Killing invasive wetland macrophytes but leaving their biomass in situ does not address their underlying mechanism of dominance and is less effective than more intensive treatments that also remove their litter. We conducted a 3‐year study in two Laurentian Great Lakes wetlands, testing the effects of dominant invasive plant removal techniques within two vegetation zones on plant taxonomic and phylogenetic diversity. Removal of invasive cattails and their litter increased phylogenetic diversity and species richness across water levels, more than doubling richness compared to unmanipulated controls, thereby potentially increasing the multifunctionality of these ecosystems.

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.

Denitrification removes reactive nitrogen (N) from ecosystems by transforming nitrate (NO 3 − ) to dinitrogen (N 2 ) gas. Incomplete denitrification produces nitrous oxide (N 2 O), a potent greenhouse gas. In salt marshes, denitrification, N 2 O production, and N 2 O yield (the fraction of denitrification that produces N 2 O) have implications for N load reduction and greenhouse gas emissions. We collected soil cores from three salt marsh zones (low marsh, high marsh, and invasive Phragmites australis ) on five sampling dates that spanned the growing season (May through October) to quantify seasonal patterns of potential denitrification, N 2 O production, and N 2 O yield. Potential denitrification peaked at the beginning and end of the growing season, with rates several times higher in May and October than in June, but we found no significant differences among salt marsh zones. In contrast, seasonal patterns of potential N 2 O production depended on marsh zone; N 2 O production was aseasonal in high marsh and P. australis zones, but in the low marsh, N 2 O production was lower in July compared to all other months sampled. In this salt marsh, seasonal variation of potential denitrification and N 2 O production was greater than spatial variation over the marsh zones, highlighting the importance of understanding temporal patterns of salt marsh N cycling.