Influence of Salinity and Vegetation on Tidal Marsh Soil Shear Strength

Sea level rise, salt water intrusion, and wave erosion threaten coastal marshes, but the direct influence of salinity on marsh erodibility remains poorly understood. Previous work identifies potentially competing influences. Salt marsh vegetation characterized by plant species with deeper roots are thought to have higher soil shear strength than freshwater marshes with shallow root systems. Yet, salt water intrusion increases the decomposition of soil organic matter thought to be important for binding soil and mitigating erosion. Here, we measure the strength of salt marsh soils along a salinity and biodiversity gradient at 5 sites along the York River, estuary in Virginia to assess the direct and indirect impacts of salinity on soil shear strength. Sites range from high salinity, monospecific salt marshes to freshwater marshes with more than 4 species. We measured shear strength using a Humboldt shear vane at a variety of soil depths at the marsh edge and the marsh interior. We compared shear strength profiles at each site and location to measurements of biomass, biodiversity, and core-derived soil properties such as bulk density and organic content. Additionally, we used the Tea Bag Index method to measure organic matter decomposition rates at each site. Preliminary results indicate that soil shear strength is higher in salt marshes (5 kPa to 40 kPa) than freshwater marshes (4 kPa to 10 kPa), with differences that vary with depth, with maximum values at intermediate depths. In salt marsh edge sites, maximum shear strength occurs between 50-60 cm below the soil surface, whereas salt marsh interior sites have maximum soil strength at 35-40cm below the surface. Shear strength of freshwater marshes at our sites increases to a depth of 60 cm below the surface, beyond which shear strength is constant. Despite enhanced biodiversity and less favorable environments for organic matter decomposition, our preliminary results suggest that freshwater marsh soils are consistently weaker than salt marsh soils. With global sea level rise and proposed wetland restoration, understanding interactions between salinity and erodibility are important to help predict the future of marshes.