We assessed the impact of nutrient additions on greenhouse gas fluxes using dark static chambers in a microtidal and a macrotidal marsh along the coast of New Brunswick, Canada approximately monthly over a year. Both were experimentally fertilized for six years with varying levels of N and P. For unfertilized, N and NPK treatments, average yearly CO₂ emissions (which represent only respiration) at the microtidal marsh (13, 19, and 28 mmoles CO₂ m^-2 hr^-1, respectively) were higher than at the macrotidal marsh (12, 15, and 19 mmoles m^-2 hr^-1, respectively, with a flux under the additional high N/low P treatment of 21 mmoles m^-2 hr^-1). Response of CH₄ to fertilization was more variable. At the macrotidal marsh average yearly fluxes were 1.29, 1.26, and 0.77 μmol CH₄ m^-2 hr^-1 with control, N, and NPK treatments, respectively and 1.21 μmol m^-2 hr^-1 under high N/low P treatment. At the microtidal marsh CH₄ fluxes were 0.23, 0.16, and -0.24 μmol CH₄ m^-2 hr^-1 in control, N, and NPK and treatments, respectively. Fertilization changed soils from sinks to sources of N₂O. Average yearly N₂O fluxes at the macrotidal marsh were -0.07, 0.08, and 1.70, μmol N₂O m^-2 hr^-1 in control, N, NPK and treatments, respectively and 0.35 μmol m^-2 hr^-1 under high N/low P treatment. For the control, N, and NPK treatments at the microtidal marsh N₂O fluxes were -0.05, 0.30, and 0.52 μmol N₂O m^-2 hr^-1, respectively. Our results indicate that N₂O fluxes are likely to vary with the source of pollutant nutrients but emissions will be lower if N is not accompanied by an adequate supply of P (e.g., atmospheric deposition vs sewage or agricultural runoff). With chronic fertilization the global warming potential of the increased N₂O emissions may be enough to offset the global cooling potential of the C sequestered by salt marshes.

Our study of a St. Lawrence Estuary marsh reveals that, compared to native Spartina patens‐dominated vegetation, invasive Phragmites australis makes a greater contribution to soil volume and carbon stock (referred to as blue carbon). Phragmites' contributions to soil volume enhance marsh sustainability in face of sea level rise, and its greater contribution to soil carbon helps to reduce the atmospheric concentration of CO₂. Phragmites australis (common reed) is a cosmopolitan species growing in fresh to brackish wetlands. An invasive genetic strain, introduced from Europe or Asia, has expanded extensively along the St. Lawrence River in the last few decades but has been little studied on the estuarine portion. We collected soil cores from three sites within an invasive Phragmites stand and one site within S. patens‐dominated stand in a St. Lawrence Estuary salt marsh near la Pocatiere, Quebec. We measured the bulk density, carbon content, volume, and mass of belowground organic matter in 2‐cm‐thick soil layers of three cores at each Phragmites site. Bulk density and carbon content were measured in 5‐cm‐thick soil layers of three cores at S. patens site. Results showed that soil in the Phragmites stands held 37–77% more blue carbon than in the S. patens‐dominated marsh. Based upon their diameter size, Phragmites rhizomes could be contributing 7.4–10.2 cm to the thickness in the upper 20 cm of soil. We suggest that any management of invasive Phragmites include consideration of its role in increasing blue carbon stocks and marsh resilience along with other ecosystem services.

The data presented here includes a table of soils measurements taken at high resolution depth intervals (5 cm) for three salt marshes, two along the New Brunswick coast of Canada and one on the southern coast of Maine, USA. The data includes a table which includes the bulk density, percent organic matter, percent organic carbon, carbon stock, and rhizome dominance (if identifiable) at 5 cm depth intervals for each soil core. Shapefiles are also included which indicates the GPS position of acquired cores and sites where marsh depth was measured but no material was recovered. These shapefiles also include marsh peat depth and estimates of carbon stock for each point. For further information and interpretation of the included data please see the companion research article titled "The Importance of Geomorphic Context for Estimating the Carbon Stock of Salt Marshes" [1].

We examined the sediment profiles, surface accretion rates, and soil carbon (C) accumulation rates of two brackish tidal marshes on Kilbella and Wannock Rivers of the central coast of British Columbia, Canada, a region where such tidal marsh studies are rare. Sediment grain size is variable within these deposits and is a significant predictor of soil C density. The average soil C density of the Kilbella (0.027 ± 0.01 g C cm⁻³) and the Wannock marsh (0.029 ± 0.007 g C cm⁻³) are within the range of reported global averages. Compared to global estimates, the depth weighted mean rates of soil C accumulation are low, between 29 and 50 g C m⁻² year⁻¹, although high sectional rates (up to 490 ± 230 g C m⁻³ year⁻¹) occur in deeper segments of the marsh profile at Kilbella. Modern accretion rates are below moderate to high sea level rise scenarios. The Kilbella marsh contains sand layers from periods of mass sedimentation events (landslides and tsunamis), which provide the marsh with elevation capital that can be critical to its sustainability. The Kilbella profile demonstrates that variation in carbon density with depth cannot be predicted in marshes on geomorphologically dynamic coasts.

The determination of rates and stocks of carbon storage in salt marshes, as well as their protection, require that we know where they and their boundaries are. Marsh boundaries are conventionally mapped through recognition of plant communities using aerial photography or satellite imagery. We examined the possibility of substituting the use of 1 m resolution LiDAR-derived digital elevation models (DEMs) and tidal elevations to establish salt marsh upper boundaries on the New Brunswick coasts of the Gulf of St. Lawrence and the Bay of Fundy, testing this method at tidal ranges from ≤2 to ≥4 m. LiDAR-mapped marsh boundaries were verified with high spatial resolution satellite imagery and a subset through field mapping of the upland marsh edge based upon vegetation and soil characteristics, recording the edge location and elevation with a Differential Geographic Positioning System. The results show that the use of high-resolution LiDAR and tidal elevation data can successfully map the upper boundary of salt marshes without the need to first map plant species. The marsh map area resulting from our mapping was ~30% lower than that in the province’s aerial-photograph-based maps. However, the difference was not primarily due to the location of the upper marsh boundaries but more so because of the exclusion of mudflats and large creeks (features that are not valued as carbon sinks) using the LiDAR method that are often mapped as marsh areas in the provincial maps. Despite some minor limitations, the development of DEMs derived from LiDAR can be applied to update and correct existing salt marsh maps along extensive sections of coastlines in less time than required to manually trace from imagery. This is vital information for governments and NGOs seeking to conserve these environments, as accurate mapping of the location and area of these ecosystems is a necessary basis for conservation prioritization indices.