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Applying dredged sediments to build coastal resilience in salt marshes: soil biogeochemical responses
Citation:
Loffredo, J., J. Bishop, D. Perry, C. Tremper, C. Chaffee, W. Ferguson, AND C. Wigand. Applying dredged sediments to build coastal resilience in salt marshes: soil biogeochemical responses. Soil Science Society of America, Virtual, Arizona, November 09 - 13, 2020.
Impact/Purpose:
Some coastal marshes in the Northeast USA are drowning in place because of accelerated sea level rise. The RI Coastal Resources Management Council, Save the Bay, the US EPA and other state and academic partners have been collaborating on coastal marsh restoration using sandy dredge material to build elevation of the drowning marshes. In this study, the marsh and dredge soils are examined in the restored sites and in salt marsh controls. Initially, marsh soils buried under the added dredge material had lower oxygen levels and higher porewater sulfide concentrations. High sulfides in the soils may hinder seed germination and vegetation growth. However, over one growing season the marsh soil conditions became more similar to the controls. The study is ongoing to examine how these results in the short term are reflected during longer time periods.
Description:
Coastal marshes in the Northeast US are vulnerable to increased flooding due to sea level rise and an increase in the frequency and severity of storms. Some New England salt marshes are drowning in place, and observations of plant die-off and shoreline erosion have been reported. Increasing marsh surface elevation by applying dredged sediments is one approach to build coastal resilience. EPA has partnered with RI Coastal Resources Management Council and Save the Bay to examine the effects of varying depths of clean dredge material (+5cm, +10cm) on soil biogeochemistry and vegetation recruitment in a greenhouse mesocosm experiment. This experiment was coupled with field monitoring at the Quonochontaug Marsh, RI restoration site, which underwent sediment placement in 2019. The mesocosm soil materials originated from Quonochontaug. Here we focus and report on various biogeochemical responses. Initially, buried marsh soils had lower redox potentials and higher porewater sulfide concentrations, jointly indicating more reducing conditions in underlying marsh soils. However, over time subsurface redox potential tended to increase. This redox response may be attributable to vegetation recruitment and indicative of increasing soil aeration and biogeochemical complexity over time. Additionally, porewater salinity was negatively correlated with dredge application depth. We detected increased methane efflux, a potent greenhouse gas, with dredge treatment in the fall in the low marsh. Restoration managers should be aware of and monitor these initial biogeochemical responses. Previous research found that labile ammonium can accumulate in buried marsh soils, which could facilitate vegetation recruitment. However, the simultaneous accumulation of phytotoxic sulfides may inhibit revegetation success and plant-mediated nutrient uptake. Future work aims to validate the initial soil responses found in the greenhouse with field measurements and understand how these responses change with long-term ecosystem development. We will also assess nitrogen attenuation services in restored marshes through denitrification assays, genomic analyses, and isotopic methods.
