Towards an improved understanding of the magnitude and drivers of carbon and nitrogen storage and accumulation in urban saltmarshes: a case study from New England (USA)

Urban saltmarshes, although often heavily modified yet critical for nature-based climate and water quality management strategies, remain insufficiently integrated into coastal sustainability policy and practice. This study quantified soil organic carbon (SOC) and nitrogen (SN) stocks and accumulation in Belle Isle Marsh, Boston, USA, using 26 soil cores collected to 1 m depth across upland transitional marsh, high marsh, low marsh, and unvegetated marsh habitats. Mean (± SD) SOC and SN stocks in the upper meter were 332 ± 144 Mg C ha− 1 and 17 ± 4 Mg N ha− 1, respectively, values comparable to global temperate saltmarsh estimates. High marshes consistently held the highest stocks, while unvegetated and upland transitional marsh habitats exhibited lower and more variable stocks. Stocks and accumulation were strongly influenced by dry bulk density and sediment-size distribution, reflecting how sediment composition affects carbon and nitrogen storage. Using ¹³⁷Cs radionuclide dating, sediment accretion rates ranged from 0.29 to 0.79 cm− 2 yr− 1, and corresponding to carbon and nitrogen accumulation rates from 80 to 183 g C m− 2 yr− 1 and 2 to 15 g N m− 2 yr− 1, respectively. These findings demonstrate that urban saltmarshes can sustain carbon and nitrogen stocks and burial rates comparable to global ranges, even under sustained anthropogenic pressure. However, the same forces that shape these systems also threaten their long-term storage capacity, underscoring the urgency of conserving and restoring urban wetlands and fully integrating them into regional and global carbon and nutrient management frameworks.