Sea level rise resilience in coastal wetlands: plant-soil carbon responses to a Thin Layer Placement experiment

Background/Question/Methods

Thin Layer Placement (TLP) of sediment is an increasingly used restoration method in coastal marshes to stimulate plant productivity, subsequently promoting soil accretion and resilience to accelerated sea level rise. Additionally, there is increasing interest in restoring salt marshes to improve carbon sequestration. However, few experimental field studies have investigated using dredge material for TLP in northeastern Atlantic coast estuaries, and none holistically examine how sediment application has implications for carbon dynamics, including CO2 emissions. Our goal was to investigate the biological and biogeochemical responses of applying dredge material for restoration of a microtidal (1.7m) coastal salt marsh in Connecticut, USA. Our objectives were to determine how application of varying depths of sediment application affect: (1) above and belowground biomass growth of Spartina alterniflora, and (2) soil carbon cycling processes including decomposition, CO2 and CH4 fluxes. We used an in-situ experiment to manipulate soil surface elevation by applying a set of sediments depths (low: +5 cm, medium: +10 cm, and high: +15 cm) in 1 x 1-m plots. We monitored plant responses (above and belowground biomass, stem height, stem density, leaf area) and soil parameters (EC, pH, redox, NH4+, sulfides, C:N, decomposition, bulk density) over two growing seasons, collecting gas flux data in the peak of the second season.

Results/Conclusions

We found adding sediment at depths of 5-7 cm to a drowning marsh surface promoted rapid vegetation growth, alleviated phytotoxic sulfides, enhanced CO2 uptake and reduced CH4 emissions. Low treatments had reached similar cover to the controls after the first growing season, though medium and high treatments >10 cm deep showed a delay in growth. Low levels of sediment addition stimulated the highest root biomass in the underlying soil, promoting CO2 efflux, but this was offset by increased production in aboveground biomass, promoting the net CO2 uptake. Phytotoxic sulfides were reduced in all sediment treatments, but our data indicate that coastal managers aiming to promote rapid vegetative recovery should target conservative depths when using fine-grained dredged material. Collectively, our work will help guide wetland managers as they develop restoration specifications for perpetuating coastal marshes in the face of rising seas.