Tidal salt marshes are widespread along the World's coasts, and are ecologically and economically important as they provide several valuable ecosystem services. In particular, their significant primary production, coupled with sustained vertical accretion rates, enables marshes to sequester and store large amounts of organic carbon and makes them one of the most carbon-rich ecosystems on Earth. Organic carbon accumulation results from the balance between inputs, i.e. organic matter produced by local plants or imported, and outputs through decomposition and erosion. Additionally, organic matter deposition actively contributes to marsh vertical accretion, thus critically affecting the resilience of marsh ecosystems to rising relative sea levels. A better understanding of organic-matter dynamics in salt marshes is key to address salt-marsh conservation issues and to elucidate marsh importance within the global carbon cycle. Toward this goal, we empirically derived rates of organic matter decomposition by burying 712 commercially available tea bags at different marshes in the microtidal Venice Lagoon (Italy), and by analyzing them following the Tea Bag Index protocol. We find values of the decomposition rate (k) and stabilization factor (S) equal to 0.012±0.003 day -1 and 0.15±0.063, respectively. Water temperature critically affects organic matter decomposition, enhancing decomposition rates by 8% per °C on average. We argue that, at least in the short term, the amount of undecomposed organic matter that actively contributes to carbon sequestration and marsh vertical accretion strongly depends on the initial organic matter quality, which is a function of marsh and vegetation characteristics.

<p>Salt-marsh evolution importantly depends on complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial ecogeomorphic structures support a diverse range of ecosystem services, including coastal protection and biodiversity increase. In addition, they are among the most carbon‐rich ecosystems on Earth, as their high primary production coupled with rapid surface accretion results into the ability to sequester atmospheric carbon at high rates. However, salt-marsh future is at risk today, due to the effects of climate changes and local anthropogenic disturbances, in particular sea-level rise and reduced fluvial sediment delivery to the coasts. The organic matter captured and stored by salt marshes results from the balance between inputs and outputs and may contribute to marsh surface accretion, which determines their ability to keep pace with sea-level rise. Therefore, a better understanding of the processes regulating organic matter dynamics on salt marshes is a critical step to elucidate their carbon sink potential and to address salt-marsh management and conservation issues. Toward this goal, we analysed organic matter decomposition processes within salt-marsh ecosystems by burying 712 commercially available tea bags within different marshes in the Venice Lagoon (Italy), following the Tea Bag Index protocol. The process provides the values of two key parameters: the decomposition rate (k) and litter stabilisation factor (S). Based on standardized litter bag experiments, the Tea Bag Index focuses on the effects of abiotic conditions, neglecting litter-quality influences. The mean values of the decomposition metrics from our analyses are in general consistent with previous results and indicate a quite fast decomposition of the organic matter with a remaining mass of about 34% of the initial labile mass after 90 days. We next explore the possible dependence of k and S on environmental drivers. Temperature showed the most significant relationship with decomposition processes, suggesting an organic-matter decay acceleration with warming temperature, in line with previous literature. Moreover, the statistical analysis indicated some significant trends of the decomposition rate also with surface elevation and distance from the marsh edge. This suggests that, at the marsh scale, higher and probably less frequently flooded sites are exposed to faster decomposition, likely due to greater oxygen availability enhancing microbial respiration. In conclusion, the organic matter decay we observed is rapid enough to consume all the labile material before it can be buried and stabilized, hence increased global temperatures may not have a significant effect in increasing organic matter decomposition in coastal marshes. Therefore, we argue that, at least in the short term, the remaining mass of the organic matter contributing to carbon sequestration and marsh accretion, strongly depends on the initial litter quality, recalcitrant or labile, which may differ considerably between different species and plant parts and may be affected by climate change effects.</p>

<p>Salt marshes are intertidal coastal ecosystems characterized by mostly herbaceous halophytic vegetation and shaped by complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial yet endangered environments provide a diverse range of ecosystem services but are severely exposed to climate change and human pressure. The importance of salt marshes as ‘blue carbon’ (C) sinks, deriving from their primary production coupled with rapid surface accretion, has been increasingly recognized within the framework of climate mitigation strategies. However, uncertainties remain in the estimation of salt-marsh C stock and sequestration at the basin scale and large knowledge gaps still linger in the response of marsh C pools under increasing anthropogenic interventions, such as storm-surge regulation. In order to provide further knowledge in salt-marsh C assessment and investigate marsh C pool response to management actions under different scenarios, we analysed organic matter content in salt-marsh soils in 720 samples from 60 sediment cores to the depth of 1 m, and we estimated C stocks and accumulation rates in different areas of the Venice Lagoon (Italy), which has recently become regulated by a storm-surge barrier system. OC stocks in the surface 1 m were highly variable in different marshes averaging 17,108 ± 5,757 ton OC km<sup>-2 </sup>(range 9,800 − 24,700 ton OC km<sup>-2</sup>). The estimated OC accumulation rate was 85 ± 25 ton OC km<sup>-2</sup> yr<sup>-1</sup>, confirming the CO<sub>2</sub> sequestration potential of tidal environments, which, however, resulted to be crucially affected by marsh accretion rates and their human-induced variations. By hindering sediment supply provided by storm surges which are largely responsible for marsh accretion, flood regulation can dramatically reduce the CO<sub>2</sub> sequestration potential of salt marshes. We estimate that storm-surge barrier operations in the Venice Lagoon may reduce the annual marsh CO<sub>2</sub> sequestration potential by about 33%, with high costs in terms of ecosystem service loss. Our results highlight the need for integrated coastal management policies to enhance the resilience of anthropic and natural environments and to preserve the ecosystem services delivered by coastal wetlands.</p>

Tight interplays between physical and biotic processes in tidal salt marshes lead to self‐organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long‐term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation‐vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea‐level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7–4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site‐specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species‐specifically to changes in environmental forcings by modifying species‐preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation‐species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution.

<p>Tidal channel networks control tide propagation and, therefore, fluxes of water, sediment, nutrients, and particulate matter in wetlands and low-lying coastal areas. Furthermore, tightly linked wetland-channel systems deliver multiple ecosystem services, among which blue carbon sequestration is critically important. While carbon fluxes associated with both vertical and lateral dynamics of salt marshes have been extensively studied, the role of tidal channel abandonment still needs to be further investigated. Reduced flow velocities promote rapid particle settling within abandoned channels thus rapidly storing large volumes of inorganic and organic sediment, both from autochthonous and allochthonous sources. Hence, a better understanding of the processes that lead to the abandonment of active tidal channels and the characterization of the related sedimentary deposits are critical steps to assess their potential sequestration capacity and storage of blue carbon.</p> <p>Towards this aim, here we investigate the sedimentary features and the related depositional processes in abandoned tidal channels by analyzing several undisturbed sediment cores retrieved in the microtidal Venice Lagoon, Italy. Cores were cut longitudinally and photographed for classical sedimentary facies analysis and description of the main sedimentary units. In each core, soil subsamples were taken every 5 cm and were prepared for different laboratory analyses. Organic matter content was estimated as the difference in weight before and after the Loss-On-Ignition (LOI), while organic carbon was directly measured using an elemental analyzer.</p> <p>The deposits accumulated during the abandonment phase and the related infill volumes were identified thanks to sedimentary facies analyses. The combination of the infill volume and organic carbon content allowed us to estimate the carbon stock potential of abandoned channels. Preliminary results show that, although the organic matter content in abandoned channel deposits is lower than that of the surrounding salt marshes, high infill rates make the carbon accumulation rate comparable between these different depositional systems. Moreover, the analysis shows that there is a very high spatial variability in sedimentary features of abandoned channel deposits, which needs to be taken into account to assess the potential of abandoned tidal channels as carbon sinks.</p>