Tidal wetlands contain large reservoirs of carbon in their soils and can sequester carbon dioxide (CO ) at a greater rate per unit area than nearly any other ecosystem. The spatial distribution of this carbon influences climate and wetland policy. To assist with international accords such as the Paris Climate Agreement, national-level assessments such as the United States (U.S.) National Greenhouse Gas Inventory, and regional, state, local, and project-level evaluation of CO sequestration credits, we developed a geodatabase (CoBluCarb) and high-resolution maps of soil organic carbon (SOC) distribution by linking National Wetlands Inventory data with the U.S. Soil Survey Geographic Database. For over 600,000 wetlands, the total carbon stock and organic carbon density was calculated at 5-cm vertical resolution from 0 to 300 cm of depth. Across the continental United States, there are 1,153-1,359 Tg of SOC in the upper 0-100 cm of soils across a total of 24 945.9 km of tidal wetland area, twice as much carbon as the most recent national estimate. Approximately 75% of this carbon was found in estuarine emergent wetlands with freshwater tidal wetlands holding about 19%. The greatest pool of SOC was found within the Atchafalaya/Vermilion Bay complex in Louisiana, containing about 10% of the U.S. total. The average density across all tidal wetlands was 0.071 g cm across 0-15 cm, 0.055 g cm across 0-100 cm, and 0.040 g cm at the 100 cm depth. There is inherent variability between and within individual wetlands; however, we conclude that it is possible to use standardized values at a range of 0-100 cm of the soil profile, to provide first-order quantification and to evaluate future changes in carbon stocks in response to environmental perturbations. This Tier 2-oriented carbon stock assessment provides a scientific method that can be copied by other nations in support of international requirements.

Coastal wetlands store carbon dioxide (CO 2 ) and emit CO 2 and methane (CH 4 ) making them an important part of greenhouse gas (GHG) inventorying. In the contiguous United States (CONUS), a coastal wetland inventory was recently calculated by combining maps of wetland type and change with soil, biomass, and CH 4 flux data from a literature review. We assess uncertainty in this developing carbon monitoring system to quantify confidence in the inventory process itself and to prioritize future research. We provide a value-added analysis by defining types and scales of uncertainty for assumptions, burial and emissions datasets, and wetland maps, simulating 10 000 iterations of a simplified version of the inventory, and performing a sensitivity analysis. Coastal wetlands were likely a source of net-CO 2 -equivalent (CO 2 e) emissions from 2006-2011. Although stable estuarine wetlands were likely a CO 2 e sink, this effect was counteracted by catastrophic soil losses in the Gulf Coast, and CH 4 emissions from tidal freshwater wetlands. The direction and magnitude of total CONUS CO 2 e flux were most sensitive to uncertainty in emissions and burial data, and assumptions about how to calculate the inventory. Critical data uncertainties included CH 4 emissions for stable freshwater wetlands and carbon burial rates for all coastal wetlands. Critical assumptions included the average depth of soil affected by erosion events, the method used to convert CH 4 fluxes to CO 2 e, and the fraction of carbon lost to the atmosphere following an erosion event. The inventory was relatively insensitive to mapping uncertainties. Future versions could be improved by collecting additional data, especially the depth affected by loss events, and by better mapping salinity and inundation gradients relevant to key GHG fluxes. Social Media Abstract: US coastal wetlands were a recent and uncertain source of greenhouse gasses because of CH 4 and erosion.

With current losses of saltmarsh running at > 100 ha per year in the UK, creation of new intertidal habitats through managed realignment is likely to be increasingly used. Potentially, this has biodiversity as well as engineering benefits. However, assessing the conservation value of many of the current UK schemes is difficult as the biological monitoring has been generally poor, with a few notable exceptions. At the Tollesbury and Orplands realignment sites, Essex, bird communities were dominated by terrestrial species during the first year of inundation and waterbird communities rapidly developed during the second and third years. Five years after the initial breach in the sea wall, communities were similar to surrounding mudflats but with some notable exceptions. Dunlin Calidris alpina and Common Redshank Tringa totanus that prey on the early colonizing Nereis and Hydrobia used the sites in the first 2 years. Eurasian Oystercatcher Haematopus ostralegus did not occur on the realignment site as there were no large bivalves, whereas Red Knot Calidris canutus used the site after 4–5 years coincidentally with the appearance of Macoma balthica. The differences in the bird communities occurred because UK sites are often small, enclosed and poorly drained. If at a suitable height in the tidal frame, UK managed realignment sites are successful in that they have developed saltmarsh and biologically active mudflats but they may lack the full range of biodiversity found in surrounding natural intertidal habitats, even decades after inundation.

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