Marsh Migration Into Forests and Farms: Effects on Soil Biogeochemistry Along the Salinity Gradients

Abstract Sea level rise (SLR) and increased storm intensity are causing landward expansion of intertidal zones in the low‐lying Delmarva Peninsula, allowing marsh migration into forests and agricultural fields. Transitional zones along the marsh‐upland transects are visible aboveground as ghost forests and crop die‐off, respectively. While the aboveground impacts of marsh migration are clear, the effects on belowground biogeochemistry are understudied. To characterize the impacts of marsh migration on soil biogeochemistry, we collected soil cores from marsh‐upland transects at 3 agricultural and 3 forested sites along the Delmarva Peninsula. Soil cores were analyzed for both porewater chemistry and solid‐phase characterization. Marsh end members support sulfate reduction; transitional zones support iron reduction; and upland end members support aerobic metabolisms at the surface, with iron reduction occurring at depth. In addition, the quality and quantity of dissolved organic matter changed across the transects, indicating differences in carbon source and cycling dynamics. Furthermore, our results show that soil carbon concentration varies drastically from lowland marsh to uplands, with marshes having 4–50 times more soil carbon than their upland endmembers. We also observed site‐specific differences, where at the site with the lowest slope, the migrating marsh layer was relatively thin and was underlain by low‐carbon aerobic soil that was coarser‐textured. These findings have important implications for better understanding the incremental and belowground effects of SLR on coastal forests and agricultural lands.