Spatial patterns of Organic Matter content in the surface soil of the salt marshes of the Venice Lagoon (Italy)

Abstract

Abstract. Salt marshes are crucial eco-geomorphic features of tidal environments as they provide numerous important ecological functions and deliver a wide range of ecosystem services that contribute to human well-being. Being controlled by the interplay between hydrodynamics, geomorphology and vegetation, the deposition of both organic matter (OM) and inorganic sediments drives salt-marsh vertical accretion. This allows marshes to keep pace with relative sea-level rise, and likewise capture and store carbon, making them valuable allies in climate mitigation strategies. Thus, Soil Organic Matter (SOM), i.e. the organic component of the soil, plays a key role within salt-marsh environments, directly contributing to soil formation and supporting carbon storage. This study aims at inspecting spatial patterns of OM in surface salt-marsh soils, providing further insights into the physical and biological factors driving OM dynamics, affecting salt-marsh survival and carbon sink potential. Our results reveal two scales of variations in sedimentary OM content in salt-marsh soils. At the marsh scale OM variability is influenced by the interplay between surface elevation and changes in sediment supply linked with the distance from tidal channels. At the system scale, OM content distribution is dominated by the gradient generated by marine and fluvial influence. Variations in inorganic and organic inputs, both autochthonous and allochthonous, sediment grain size, and preservation conditions may explain the observed variations in SOM. Our results highlight marsh importance as carbon sink environments, furthermore emphasizing that environmental conditions within a tidal system may generate strongly variable and site-specific carbon accumulation patterns, enhancing blue carbon assessment complexity.