Estuarine wetlands provide invaluable ecosystem services such as storm abatement and water filtration. Soil in estuarine ecosystems plays an integral role in carbon sequestration, biogeochemical cycling, and physical stability. Estuaries affected by physical barriers, such as culverts, experience reduced hydrological inputs and reduced connectivity resulting in the loss of ecological processes above and below the site of impact. To address reduction in ecological connectedness, culverts at three sites were removed and replaced with bridges. An observational field study was conducted in Kitsap County, Washington of three sites where culverts were replaced with bridges three, eight, and 13 years ago. Data was collected at a reference site where the estuary contains a culvert. It is hypothesized that estuary restoration will recover factors such as homogeneity above and below the restoration site with regard to plant species, carbon sequestration, and nutrient cycling; however, the length of time for recovery is not well understood. Plant community composition was described using 50 m transects placed parallel to the estuary. Soil, plant height, and aboveground biomass were collected from random quadrats. Soil carbon and nitrogen were analyzed using an elemental analyzer and soil macro and micro-nutrients were analyzed using the Mehlich 3 Extraction method.
Plant species richness was significantly higher (P<0.01) at the intermediate and oldest post-restoration sites, and species diversity was significantly higher (P<0.01) at the oldest post-restoration site, which was comprised of a mix of native, naturalized, and invasive species. Hierarchical clustering produced two distinct clusters of plant communities associated by site location (above and below the bridge), and one cluster from the site aged eight years (χ2=40.82, df=6, P<0.001). Principal component analysis showed that Salicornia virginica and Distichlis spicata determined the separation in vegetation communities. Percent soil carbon below the site of impact was significantly lower at the youngest post-restoration site (P=0.02). Steady increases in carbon sequestration was noted through time with carbon levels recovering at year 12. Calcium, magnesium, sulfur, copper, boron, zinc, and phosphorus (all P<0.05) were negatively correlated with plant height. This indicates younger, unweathered soils (calcium and magnesium), fossil fuel emissions (sulfur), and storm water pollution (copper, boron and zinc) may have an influence on plant growth. The results of this study will aid resource managers by providing a timeline for the return of native plant communities and biogeochemical dynamics within a recovering estuary system.