Experimental Sediment Addition in Salt-Marsh Management: Plant-soil Carbon Dynamics in Southern New England

Thin Layer Placement (TLP) is an increasingly used management technique to promote coastal resilience to accelerated sea level rise, but it is unclear how differential thickness of dredged sediment application alters plantsoil carbon dynamics in northeastern Atlantic coast salt marshes. We initiated a field experiment in a microtidal Spartina alterniflora (smooth cordgrass) dominated salt marsh in Connecticut (USA) to examine how a silt-loam dredge applied at different thicknesses (low: +5 cm, medium: +10 cm, and high: +15 cm) altered biological and biogeochemical responses. Over two growing seasons we monitored plant responses (above and belowground biomass, stem height, stem density, leaf area) and soil-biogeochemical parameters (EC, pH, redox, NH4+, sulfides, C:N, decomposition, bulk density, carbon fluxes). We found adding 5-7 cm of dredged sediment promoted rapid revegetation, alleviated phytotoxic sulfides, enhanced CO2 uptake, and reduced CH4 emissions. Low treatments reached similar vegetation cover to controls after one growing season, though sediment treatments greater than 10-cm thick showed delayed growth. Low levels of sediment addition stimulated the highest root biomass in the underlying soil, promoting CO2 efflux, but this was offset by aboveground biomass productivity, promoting net CO2 uptake. Phytotoxic sulfides were reduced in all sediment addition treatments, but our data indicate that coastal managers aiming to promote rapid vegetative recovery while increasing elevation in the short term should target conservative depths (< 10 cm) when using fine-grained dredged material. Collectively, our work will guide wetland managers as they develop restoration specifications for perpetuating coastal marshes in the face of rising seas.