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Coastal tidal wetlands and estuaries play important roles in the global carbon budget by contributing to the net withdrawal of CO₂from the atmosphere. We quantified the linkages between terrestrial and oceanic systems, marsh-to-bay carbon exchange, and the uptake of CO₂from the atmosphere in the wetland-dominated Plum Island Sound (MA, USA) and Duplin River (GA, USA) estuaries. The C budgets revealed that autotrophic marshes [primary production:ecosystem respiration (P:R) ~1.3:1] are tightly coupled to heterotrophic aquatic systems (P:R ~0.6:1). Levels of marsh gross primary production are similar in these systems (865 ± 39 and 768 ± 74 gC m⁻²year⁻¹in Plum Island and the Duplin, respectively) even though they are in different biogeographic provinces. In contrast to inputs from rivers and coastal oceans, tidal marshes are the dominant source of allochthonous matter that supports heterotrophy in aquatic systems. Dissolved inorganic carbon (DIC) exported from marshes to the coastal ocean was a major flux pathway in the Duplin River; however, there was no evidence of DIC export from Plum Island marshes and only minor export to the ocean. Burial was a sink for 53% of marsh net ecosystem production (NEP) on Plum Island, but only 19% of marsh NEP in the Duplin. Burial was the dominant blue carbon sequestration pathway at Plum Island, whereas in the Duplin, DIC and organic carbon export to the ocean were equally important. Regional- and continental-scale C budgets should better reflect wetland-dominated systems to more accurately characterize their contribution to global CO₂sequestration. 

Abstract This study examines the seasonal and spatial variabilities of total alkalinity (TA) and dissolved inorganic carbon (DIC) concentrations and fluxes in the Parker River, part of the Plum Island estuary, the largest macro tidal saltmarsh system in northeastern US. Our investigation reveals that the Parker River estuary is a net source of TA and DIC, with outwelling fluxes ranging 8.9–13.2 mmol m⁻² d⁻¹for TA and 10.4–18.1 mmol m⁻² d⁻¹for DIC. Porewater fluxes from marshes contributed to 71% (55%–96%) and 59% (51%–78%) of the TA and DIC outwelling flux, respectively. Spatial heterogeneity is pronounced, with high‐salinity section porewater fluxes nearly three times higher than in low‐salinity section. TA fluxes peak in summer and are lowest in the spring, with the seasonal changes controlled by both hydrology and marsh phenology. The interplay of primary production and aerobic and anaerobic respiration governs carbon transformations in the estuary, leading to net DIC export and contributing to the acidification of adjacent coastal waters. Overall, approximately 20% saltmarsh net primary production is laterally exported to the coastal ocean as TA during 2023, providing potential long‐term carbon sinks comparable to about 40% of sediment organic carbon burial. Our research underscores the spatial and temporal variabilities in TA and DIC fluxes driven by biotic processes and the importance of considering both TA and DIC fluxes to fully understand the carbon sequestration potential of saltmarshes. 

Abstract. Atmospheric concentrations of nitrous oxide (N2O), a potent greenhouse gas that is also responsible for significant stratospheric ozone depletion, have increased in response to the intensified use of agricultural fertilizers and other human activities that have accelerated nitrogen cycling processes. Microbial denitrification in soils and sediments is a major source of N2O, produced as an intermediate during the reduction of oxidized forms of nitrogen to dinitrogen gas (N2). Substrate availability (nitrate and organic matter) and environmental factors such as oxygen levels, temperature, moisture, and pH influence rates of denitrification and N2O production. Here we describe the role of physicochemical perturbation (defined here as a change from the ambient environmental conditions) in influencing rates of denitrification and N2O production. Changes in salinity, temperature, moisture, pH, and zinc in agricultural soils induced a short-term perturbation response characterized by lower rates of total denitrification and higher rates of net N2O production. The ratio of N2O to total denitrification (N2O : DNF) increased strongly with physicochemical perturbation. A salinity press experiment on tidal freshwater marsh soils revealed that increased N2O production was likely driven by transcriptional inhibition of the nitrous oxide reductase (nos) gene and that the microbial community adapted to altered salinity over a relatively short time frame (within 1 month). Perturbation appeared to confer resilience to subsequent disturbance, and denitrifiers from an environment without salinity fluctuations (tidal freshwater estuarine sediments) demonstrated a stronger N2O perturbation response than denitrifiers from environments with more variable salinity (oligohaline and mesohaline estuarine sediments), suggesting that the denitrifying community from physicochemically stable environments may have a stronger perturbation response. These findings provide a framework for improving our understanding of the dynamic nature of N2O production in soils and sediments, in which changes in physical and/or chemical conditions initiate a short-term perturbation response that promotes N2O production that moderates over time and with subsequent physicochemical perturbation. 

Abstract Methane (CH₄) is a potent greenhouse gas (GHG) with atmospheric concentrations that have nearly tripled since pre‐industrial times. Wetlands account for a large share of global CH₄emissions, yet the magnitude and factors controlling CH₄fluxes in tidal wetlands remain uncertain. We synthesized CH₄flux data from 100 chamber and 9 eddy covariance (EC) sites across tidal marshes in the conterminous United States to assess controlling factors and improve predictions of CH₄emissions. This effort included creating an open‐source database of chamber‐based GHG fluxes (https://doi.org/10.25573/serc.14227085). Annual fluxes across chamber and EC sites averaged 26 ± 53 g CH₄m⁻² year⁻¹, with a median of 3.9 g CH₄m⁻² year⁻¹, and only 25% of sites exceeding 18 g CH₄m⁻² year⁻¹. The highest fluxes were observed at fresh‐oligohaline sites with daily maximum temperature normals (MATmax) above 25.6°C. These were followed by frequently inundated low and mid‐fresh‐oligohaline marshes with MATmax ≤25.6°C, and mesohaline sites with MATmax >19°C. Quantile regressions of paired chamber CH₄flux and porewater biogeochemistry revealed that the 90th percentile of fluxes fell below 5 ± 3 nmol m⁻² s⁻¹at sulfate concentrations >4.7 ± 0.6 mM, porewater salinity >21 ± 2 psu, or surface water salinity >15 ± 3 psu. Across sites, salinity was the dominant predictor of annual CH₄fluxes, while within sites, temperature, gross primary productivity (GPP), and tidal height controlled variability at diel and seasonal scales. At the diel scale, GPP preceded temperature in importance for predicting CH₄flux changes, while the opposite was observed at the seasonal scale. Water levels influenced the timing and pathway of diel CH₄fluxes, with pulsed releases of stored CH₄at low to rising tide. This study provides data and methods to improve tidal marsh CH₄emission estimates, support blue carbon assessments, and refine national and global GHG inventories. 

Abstract Tidal wetlands sequester carbon (C) at much higher rates per area than other ecosystems, helping to offset C emissions. The burial of organic C in tidal wetland soils, “blue C”, is tightly linked to the cycling of nitrogen (N), which is a key pollutant and limiting nutrient for many ecosystems. The large fluxes of N in and out of tidal wetlands can have strong impacts on surrounding water quality. However, the global burial rate of “blue nitrogen” and its potential response to future sea‐level rise (SLR) and wetland extent changes have not been quantified. Here, we assembled a global database of 8012 soil N measurements from 255 tidal wetland sites and found that the relationship between soil C and N concentrations was consistent but differed between the two dominant types of emergent tidal wetlands—marshes and mangroves. Leveraging extensive knowledge of blue C accumulation, we estimated that tidal wetlands, which cover less than 0.1% of the Earth's surface, bury 3.2 Tg N yr⁻¹(2.5 Tg N yr⁻¹in mangroves, and 0.7 Tg N yr⁻¹in marshes), representing 13%–15% of marine N burial. This rate could more than double globally by 2,100 if wetland elevation increases with accelerating SLR, enhancing wetland burial of coastal N prior to release into estuarine bays and oceans. Alternatively, if wetlands submerge and soil erodes, N sinks could reverse and become sources, exacerbating coastal pollution. 

Abstract Saltwater intrusion is the leading edge of sea-level rise, preceding tidal inundation, but leaving its salty signature far inland. With climate change, saltwater is shifting landward into regions that previously have not experienced or adapted to salinity, leading to novel transitions in biogeochemistry, ecology, and human land uses. We explore these changes and their implications for climate adaptation in coastal ecosystems. Biogeochemical changes, including increases in ionic strength, sulfidation, and alkalinization, have cascading ecological consequences such as upland forest retreat, conversion of freshwater wetlands, nutrient mobilization, and declines in agricultural productivity. We explore the trade-offs among land management decisions in response to these changes and how public policy should shape socioecological transitions in the coastal zone. Understanding transitions resulting from saltwater intrusion—and how to manage them—is vital for promoting coastal resilience.