The Role of the Upper Tidal Estuary in Wetland Blue Carbon Storage and Flux

Carbon (C) standing stocks, C mass balance, and soil C burial in tidal freshwater forested wetlands (TFFW) and TFFW transitioning to low-salinity marshes along the upper estuary are not typically included in blue carbon accounting, but may represent a significant C sink. Results from two salinity transects along the tidal Waccamaw and Savannah rivers of the U.S. Atlantic Coast show that total C standing stocks were 322-1,264Mg C/ha among all sites, generally shifting to greater soil storage as salinity increased. Carbon mass balance inputs (litterfall, woody growth, herbaceous growth, root growth, and surface accumulation) minus C outputs (surface litter and root decomposition, gaseous C) over a period of up to 11years were 340-900g Cm(-2)year(-1). Soil C burial was variable (7-337g Cm(-2)year(-1)), and lateral C export was estimated as C mass balance minus soil C burial as 267-849g Cm(-2)year(-1). This represents a large amount of C export to support aquatic biogeochemical transformations. Despite reduced C persistence within emergent vegetation, decomposition of organic matter, and higher lateral C export, total C storage increased as forests converted to marsh with salinization. These tidal river wetlands exhibited high N mineralization in salinity-stressed forested sites and considerable P mineralization in low-salinity marshes. Large C standing stocks and rates of C sequestration suggest that TFFW and oligohaline marshes are considerably important globally to coastal C dynamics and in facilitating energy transformations in areas of the world in which they occur. Plain Language Summary Tidal wetlands that occur along the upper reaches of river-associated estuaries are not normally considered when determining the amount of carbon stored, exchanged, or exported from terrestrial ecosystems globally. Examples of these wetland types include tidal freshwater forested wetlands and low-salinity marshes, as well as wetlands at different stages of transition between forest and marsh as salinity intrudes and sea levels rise. However, these tidal wetlands store more carbon than many coastal wetland types documented throughout the world, including classically defined blue carbon wetlands, and they support high rates of annual carbon sequestration (uptake of CO2 from the atmosphere) and lateral carbon export into aquatic environments that can influence critical near-shore and marine energy transformations.