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Abstract Excess anthropogenic nitrogen is a key stressor in coastal and marine ecosystems, where it can alter primary production, create eutrophic and anoxic conditions, and affect biodiversity, structure, and function. The salt marsh cordgrass,Spartina alterniflora, is a foundation plant species in salt marsh ecosystems along the North American Atlantic and Gulf coasts that plays a key role in the fate of this exogenous nitrogen.S. alternifloraexhibits intraspecific variation in plant morphology, plant-microbe interactions, and consumer dynamics, which can drive ecosystem-level responses to stressors. However, little is known about whether or how intraspecific variation may affect nitrogen cycling or plant response to different forms of nitrogen input, even though such variation could have important implications for the marsh response to eutrophication and for restoration efforts. We conducted a greenhouse chamber experiment to test whether plant intraspecific variation or nitrogen form—nitrate (NO₃⁻) vs. ammonium (NH₄⁺)—affect nutrient uptake among sixS. alterniflorapopulations across two geographically distinct salt marsh regions (Plum Island Estuary, MA and North Inlet, SC). All populations demonstrated approximately 0.9 times greater uptake, on average, of NH₄⁺relative to NO₃⁻. In addition, we documented significant regional variation in absolute NO₃⁻uptake, resulting in an approximately 1.8-fold stronger preference for NH₄⁺relative to NO₃⁻among SC populations relative to MA populations. Our results indicate that regional intraspecific variation inS. alternifloraand nitrogen form may interact to influence the degree of exogenous nitrogen uptake in salt marshes, potentially influencing our ability to predict marsh nutrient cycling and its impact on marsh persistence. 

Abstract Saltmarshes provide vital ecosystem services, including coastal protection and habitat for fisheries. While feedbacks influencing vertical sediment accretion in marshes are well‐documented, including those between relative sea level and primary production, relationships among nutrient cycles, flooding, and primary production remain less explored. This study presents a unique 30‐year data set from the North Inlet estuary, South Carolina, tracking monthly growth of the dominant macrophyte,Spartina alterniflora, and porewater concentrations of sulfide, ammonium, and phosphate. Our findings reveal correlated seasonal and decadal patterns of sulfides, nutrients, and plant growth, with periodicities linked to seasonal climate and decadal tidal cycles.S.alterniflorathrives in anaerobic sediments where sulfides are toxic byproducts of sulfate‐reducing bacteria (SRB) produced through the oxidation of organic acids. Dissolved organic compounds fluctuated seasonally in tandem with plant growth. Organic acids utilized by SRB are released as root exudates and produced via cellulosic biomass fermentation. SRB fix nitrogen and produce sulfides that solubilize phosphate, compensating for nutrients lost in drainage and rafting of aboveground biomass. By directly and indirectly producing the substrates,Spartinaeffectively regulates SRB activity, thereby orchestrating biogeochemical cycles and accumulating sulfides that inhibit competing species. Our results challenge the traditional view of sulfides as mere phytotoxins and saltmarshes as passive nutrient transformers. Instead, we propose a model whereinS.alterniflorasustains its growth and supports estuarine productivity by regulating its nitrogen and phosphorus supplies through a mutualistic relationship with SRB. 

Wetlands are major carbon sinks because waterlogged sediments suppress decomposition, but the depth at which organic matter becomes effectively preserved remains poorly constrained. We quantified depth-dependent decomposition of labile and refractory organic matter using birch dowels deployed for 7 years in fertilized and control plots across salt and freshwater tidal marshes in South Carolina and Massachusetts. A surface-limited decay model revealed a consistent three-zone profile across contrasting climates, salinities, and nutrient regimes: rapid decomposition at the sediment surface, declining decomposition through an active rooting zone (~0–20 cm), and slow, nearly depth-invariant decomposition below ~20 cm. Labile material continued to decay after burial, whereas lignin degradation was largely restricted to the surface and strongly suppressed in deeper, reducing sediments. Fertilization accelerated surface loss of labile material but had little effect at depth, indicating that nutrient availability did not overcome the constraints imposed by burial depth. Estimated persistence increased from decades near the surface to a century or more at depth. These results identify the active rooting zone as a decomposition filter through which plant production must pass before entering deeper sediments where decay is greatly reduced. Consequently, long-term carbon burial depends not only on the amount of organic matter produced, but also on the balance between decomposition and the rate at which accretion transfers that material through the active zone. Depth-dependent decomposition thus provides a mechanistic link among plant production, carbon preservation, vertical accretion, and marsh resilience to relative sea-level rise. 

We review the functioning and sustainability of coastal marshes and mangroves. Urbanized humans have a 7,000-year-old enduring relationship to coastal wetlands. Wetlands include marshes, salt flats, and saline and freshwater forests. Coastal wetlands occur in all climate zones but are most abundant in deltas. Mangroves are tropical, whereas marshes occur from tropical to boreal areas. Quantification of coastal wetland areas has advanced in recent years but is still insufficiently accurate. Climate change and sea-level rise are predicted to lead to significant wetland losses and other impacts on coastal wetlands and the humans associated with them. Landward migration and coastal retreat are not expected to significantly reduce coastal wetland losses. Nitrogen watershed inputs are unlikely to alter coastal marsh stability because watershed loadings are mostly significantly lower than those in fertilization studies that show decreased belowground biomass and increased decomposition of soil organic matter. Blue carbon is not expected to significantly reduce climate impacts. The high values of ecosystem goods and services of wetlands are expected to be reduced by area losses. Humans have had strong impacts on coastal wetlands in the Holocene, and these impacts are expected to increase in the Anthropocene.