Depth-Distribution Patterns of Soil Organic Matter in the Tidal Marshes of the Venice Lagoon (Italy): Signatures of Depositional and Environmental Conditions

Salt marshes are depositional landforms lying at the upper margin of intertidal environments. They provide a diverse range of valuable ecosystem services and yet are exceptionally vulnerable to climate change and human pressure. Salt marshes are intrinsically dynamic environments, shaped by complex feedback between hydrodynamic, morphological, and biological processes. Soil Organic Matter (SOM) has a crucial role within salt marsh environments, as on the one hand, its accumulation contributes to the build-up of marsh elevation which is necessary for marshes to keep pace with sea-level rise, and on the other it supports the high carbon sink potential of wetlands. To better understand variations in SOM depth distribution and further comprehend SOM drivers, we analyzed soil organic content in 10 salt marshes of the microtidal Venice Lagoon from 60 sediment cores to the depth of 1 m, relating SOM spatial and vertical patterns to the temporal and spatial variability of depositional sub-environments recorded in the study deposits. Our results suggest that changes in the depositional environment are of primary importance in determining organic matter depth distribution and caution is needed in SOM prediction at unsampled soil depths. We observed relationships between SOM vertical patterns and factors such as autochthonous and allochthonous organic inputs, sediment properties, relative sea level rise, fluvial inputs and wave action. Our findings emphasize the considerable carbon storage potential of marshes in intertidal environments and provide a conceptual framework for understanding the dynamics of SOM and their drivers, which can inform and enhance coastal management strategies.