Spatial variability of Soil Organic Matter and Carbon content in the salt marshes of the Venice Lagoon

Abstract

Salt marshes are crucial eco-geomorphic features of tidal environments, providing numerous important ecological functions and delivering a wide range of ecosystem services that contribute to human well-being. Salt-marsh evolution is controlled by the interplay between hydrodynamics, geomorphology, and vegetation, as marshes accrete vertically through the deposition of both organic matter (OM) and inorganic sediments. This allows marshes to keep pace with relative sea-level rise, and likewise capture and store carbon (C), making them valuable allies in climate mitigation strategies. Thus, Soil Organic Matter (SOM) plays a key role within salt-marsh environments, directly contributing to soil formation and supporting C storage. Distribution patterns of SOM in salt marshes may vary in space and time across and within tidal wetland types depending on different factors including vegetation, sediment, and morphodynamics.   To better understand variations in SOM distribution and further comprehend physical and biological factors driving OM and C dynamics in salt-marsh soils, we analyzed soil organic content in 10 salt marshes of the microtidal sediment-starved Venice Lagoon (Italy), from 60 sediment cores to the depth of 1 m. These analyses allowed us to relate SOM patterns to soil, vegetation, and morphological variables, as well as depositional patterns testified by recent sedimentary successions.   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.