Carbon accumulation in salt marsh soils
Authors:   , Unger Viktoria R. , Kilham Susan Soltau 
Published: July 2021

Carbon accumulation in salt marsh soils is a beneficial ecosystem service that occurs as a result of in-situ macrophyte production, slow decomposition, and deposition of allochthonous sediments. Variation in rates of carbon accumulation within and among salt marshes may be influenced by factors including tidal flooding dynamics and sediment availability (Chmura et al. 2003, Mudd et al. 2009, DeLaune and White 2011). In this study, carbon accumulation and biomass dynamics were compared in two distinct estuary types with large differences in sediment availability and tide range. Nine soil cores approximately 60 cm long were collected in Spartina alterniflora-dominated marshes in a coastal plain estuary, Delaware Bay, and a coastal lagoon, Barnegat Bay. Accretion and mass-based accumulation of inorganic matter (sediment), and organic carbon were determined using 137Cs dating. Sediment accumulation rates were seven times greater whereas accretion, organic matter and organic carbon accumulation was two times greater in Delaware Bay than in Barnegat Bay. Accretion and accumulation processes were significantly different between the two estuaries. Inorganic sediment and organic matter in both estuaries contributed to vertical accretion and carbon accumulation. Our data suggest that the high sediment accumulation rates in Delaware Bay increase carbon accumulation rates to some degree, but that the degree to which it enhances carbon accumulation is influenced by some other factor(s). Coarse and fine root morphology in the soil cores was determined and related to sediment, organic matter and organic carbon content at analogous depth sections, as well as accretion and accumulation rates since 1963. Aboveground live and dead biomass was also measured at each core location. In Delaware Bay, the live:dead ratio of aboveground biomass was greater than that of Barnegat Bay. Live aboveground biomass was significantly greater in Delaware Bay, while dead aboveground biomass was slightly greater in Barnegat Bay. In Barnegat Bay there was little to no aboveground biomass at Island Beach and Reedy Creek, and little belowground biomass at Reedy Creek. Aboveground biomass dynamics such as stem density, stem height, and root:shoot ratios did not correlate with carbon accumulation processes at any sites. There were no significant differences in coarse, fine or total root material between the estuaries, however total belowground biomass contributed significantly to the soil carbon inventory in Barnegat Bay but not in Delaware Bay. Comparison of the biomass dynamics with soil characteristics and accretion and accumulation rates further emphasizes that factors other than sediment accumulation may be responsible for some of the differences in carbon accumulation within and among marshes in the two estuaries. The data suggest that belowground biomass may provide structural support to the soil column that is crucial for maintaining accretion and carbon accumulation processes, and more research should be conducted on the role of belowground biomass with respect to carbon accumulation.