BIOPHYSICAL CONTROLS OF MARSH SOIL SHEAR STRENGTH ALONG AN ESTUARINE SALINITY GRADIENT

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 is thought to have higher soil shear strength than freshwater marshes with shallow root systems. However, 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 to assess the direct and indirect impacts of salinity on soil shear strength. Five sites were examined in the York River estuary in Virginia, ranging from high salinity, monospecific salt marshes to freshwater marshes with more than four vegetation species. We measured shear strength using a Humboldt shear vane at a variety of soil depths at the marsh edge and the marsh interior, and 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. Results indicate that soil shear strength is higher in salt marshes (5 - 40 kPa) than freshwater marshes (4 -10 kPa). Marsh edge sites had lower shear strength values than interior sites regardless of salinity. Belowground biomass was determined to be the primary driver of differences in soil shear strength at interior sites. Soil properties were the most closely related to variations in shear strength at the marsh edge. Despite enhanced biodiversity and less favorable environments for organic matter decomposition, our 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 essential to help predict the future of marshes.