Salt marshes may be defined as halophytic grasslands or dwarf brushwoods encroaching sediment platforms bordering saline tidal water bodies (Beeftink, 1977; Silvestri and Marani, 2004). Salt marshes lie at the upper margin of the intertidal environment, and they are regularly flooded by the tides and occasionally by storm surges (Allen, 2000; Friedrichs and Perry, 2001; D'Alpaos and D'Alpaos, 2021; D'Alpaos et al., 2007; D'Alpaos, 2011). They occur worldwide, particularly in middle and high latitudes on low-energy coasts, from microtidal to macrotidal regimes (Allen and Pye, 1992). Coastal wetlands, situated at the interface between terrestrial and marine ecosystems, often lie in some of the world's most densely populated areas and deliver a wide range of ecosystem services that contribute to human wellbeing, as they are simultaneously some of the most economically relevant and vulnerable ecosystems on Earth (D'Alpaos and D'Alpaos, 2021; Costanza et al., 1997; de Groot et al., 2012). Salt marshes provide a critical habitat for numerous species, protect coastal areas by buffering floods and wave action, improve water quality, support commercial fisheries, provide recreational opportunities, and contribute to climate regulation by acting as carbon (C) sinks (e.g. Barbier et al., 2011; Boesch and Turner, 1984; Costanza et al., 1997; Lefeuvre et al., 2003; Temmerman et al., 2013; Fairchild et al., 2021; Möller et al., 2014).

However, salt marsh ecosystems are exceptionally vulnerable to the effects of climate changes and increasing human pressure such as accelerating relative sea level rise, declining sediment supply, and coastal land reclamation (D'Alpaos et al., 2011; Gedan et al., 2009; Kirwan et al., 2010; Marani et al., 2007; Morris et al., 2002; Mudd et al., 2004; Mudd, 2011; Temmerman et al., 2005; Silvestri et al., 2018; Breda et al., 2022), having lost between 25 % and 50 % of their global historical coverage (Mcowen et al., 2017).

Salt marshes are intrinsically dynamic environments, and their evolution importantly depends on complex feedbacks between physical and biological components. The interplay between hydrodynamics, geomorphology, and vegetation controls marsh vertical accretion, which has allowed marshes to keep pace with relative sea level rise over thousands of years (D'Alpaos et al., 2007; Marani et al., 2007; Fagherazzi et al., 2012; Mudd et al., 2009, 2004; Allen, 2000; Brückner et al., 2019). Vertical accretion in tidal marshes is driven by the contribution of both organic matter (OM) and inorganic sediments (Mudd et al., 2009; Fagherazzi et al., 2012; Nyman et al., 2006; Neubauer, 2008). Tides, wind waves, and storms are the main mechanisms for sediment transport, deposition, or mobilisation on the marsh platform, shaping marsh morphology, which in turn influences water depth and flow velocity. Halophytic plants, spatially organised in characteristic patches related to soil elevation and hydroperiod (Bertness and Ellison, 1987; Pennings and Callaway, 1992; Silvestri et al., 2005; Silvestri and Marani, 2004), contribute considerably to marsh accretion, influencing the water flow, enhancing sediment settling, damping sediment resuspension, and producing biomass, which results in OM accumulation (D'Alpaos et al., 2007; Marani et al., 2006; Mudd et al., 2009). In some cases, OM contribution to marsh surface vertical accretion can be much greater than that of sediment deposition (Ewers Lewis et al., 2020; Morris et al., 2016; Allen et al., 2021). The organic material that helps to build marsh elevation is likely a combination of in situ production of belowground root tissue inserted into the sediments (Craft et al., 1993; Day et al., 1999) and autochthonous or allochthonous organic materials that are deposited over the surface (Nyman et al., 2006; Mudd et al., 2009; Ewers Lewis et al., 2019; Mueller et al., 2019). Surface litter produced during the annual cycles of plant growth and decay settles on the ground and is trapped within the inorganic sediments deposited. A variable proportion of the salt marsh soil organic matter (SOM) (Drexler et al., 2020; Middelburg et al., 1997; Mueller et al., 2019) has an allochthonous source, deriving from suspended particulate organic matter, and is often adsorbed on mineral matter and on estuarine and marine phytoplankton, microphytobenthos, and non-local macrophyte litter carried to the marsh surface by waves and tides. Furthermore, tidal flooding inhibits microbial aerobic activity and slows down decomposition, fostering C accumulation in marsh soils (Keuskamp et al., 2013; Mueller et al., 2018; Kirwan et al., 2014; Morris et al., 2016). As a result of these dynamics, the C captured through plant photosynthesis is buried and preserved as soil organic carbon (SOC) and may be locked away from the atmosphere over centennial to millennial timescales (Perillo et al., 2009; Duarte et al., 2005). This process allows salt marsh environments to act as carbon sinks, serving as natural or artificial reservoirs that accumulate and store carbon-containing compounds, thereby helping to offset the effects of greenhouse gas emissions on the Earth's climate (Watson et al., 2000). The C sink function of vegetated coastal ecosystems, including salt marshes, mangrove forests, and seagrass meadows, has been increasingly recognised in recent years, and the term “blue carbon” was coined to indicate the C sequestered in these ecosystems, with a potential role in climate change mitigation (Chmura et al., 2003; Duarte et al., 2005; McLeod et al., 2011; Macreadie et al., 2019; Nellemann et al., 2009).

Recent models of intertidal system dynamics predict marsh evolution incorporating both physical and ecological processes (e.g. D'Alpaos et al., 2007; Kirwan and Murray, 2007; Marani et al., 2007, 2013; Morris et al., 2002; Mudd et al., 2004, 2009). However, to refine current representations of OM accumulation, a deeper understanding is needed of crucial biogeomorphological factors driving OM dynamics in tidal environments. Drivers of SOM variability in salt marshes are innately difficult to identify, as SOM content is the result of many complex processes acting simultaneously. The study of the interplay between these processes is complicated by the fact that they occur over a wide range of spatial scales (Marani et al., 2006; Ewers Lewis et al., 2020).

While, at the global scale, climatic parameters appear to be important drivers for C sequestration and storage (Chmura et al., 2003), at the system scale, the position within the gradient generated by marine and fluvial influence was observed to be a key predictor of organic content, with the environmental conditions resulting from the interplay between physical and biological factors (Kelleway et al., 2016; Van de Broek et al., 2016). Geomorphological factors and vegetation characteristics appear to be important drivers of soil organic content variability when considering finer spatial scales (e.g. Chen et al., 2016; Leonard et al., 2002; Roner et al., 2016). For instance, elevation and hydroperiod importantly affect vegetation characteristics, organic matter supply, and sediment deposition (Chmura et al., 2003) other than microbial community and organic matter preservation conditions (Kirwan et al., 2013; Marani et al., 2006; Mudd et al., 2009; Yousefi Lalimi et al., 2018). The latter are also influenced by sediment type and grain size, which are additionally associated with different concentrations of suspended particulate organic matter (Kelleway et al., 2016; Van de Broek et al., 2016; Saintilan et al., 2013). Therefore, sediment grain size was pointed out as a key predictor of organic content by Ford et al. (2019) and Kelleway et al. (2016), whereas Van de Broek et al. (2016) reported variations in suspended sediment concentration and stable particulate OC as the main drivers of observed differences in organic storage. The vegetation type was also observed to significantly affect soil organic content at different scales (Ewers Lewis et al., 2020; Saintilan et al., 2013). Above- and belowground production greatly varies across and within plant species, both in terms of quantity and quality, the latter determining different degrees of decay resistance (Scarton et al., 2002; Stagg et al., 2018). In addition, Van de Broek et al. (2018) show that SOC from allochthonous sources (e.g. particulate organic matter, phytoplankton, microphytobenthos, and non-local macrophyte litter) may be a main component of SOC stock preserved in marsh sediments, and this finding could significantly impact organic carbon sequestration assessments (Van de Broek et al., 2018).

Here we aim to investigate spatial patterns of OM in salt marsh soils, providing further insights into the physical and biological factors driving OM dynamics, which affect salt marsh survival and C sink potential. Toward these goals, we analysed soil organic content and SOC stock for the surface soil layer (0–20 cm) in 10 salt marshes of the Venice Lagoon from 60 sediment cores together with different variables including soil and morphological and vegetation characteristics. The choice to analyse surface marsh soil for assessing spatial patterns of soil organic content is driven by the need to capture the layer most directly influenced by current environmental variables. By contributing to a better understanding of OM distribution and its controls in tidal marshes, our study has the potential to improve spatially explicit biomorphological models of salt marsh evolution, allowing the evaluation of marsh resilience and associated C storage as an ecosystem service. Improved representation of SOM dynamics in biomorphodynamic models will provide critical information for management and conservation strategies, favouring the identification of optimal locations and actions to direct protection and restoration efforts.

The sources of OM content in salt marsh soils are influenced both by local and non-local processes. Firstly, OM is the result of in situ production of belowground (root, rhizome, and tuber tissue) (Craft et al., 1993; Rybczyk and Cahoon, 2002) and aboveground biomass, thus directly depending on the local primary production. OM content is also affected by the accumulation of organic material produced at other sites (Nyman et al., 2006; Mudd et al., 2009; Ewer Lewis, 2019; Mueller et al., 2019), which is transported eventually deposited on the marsh surface by hydrodynamic processes (i.e. tides and waves) acting at larger spatial scales . Both autochthonous and allochthonous organic materials, once part of marsh soil, are also affected by decomposition resulting from local topographic, sedimentological, and environmental conditions (Chen et al., 2016). Due to the intricate interaction of these local and non-local dynamics, two main spatial scales in OM variations can be identified: the marsh scale (metres to tens of metres) and the system scale (ranging from kilometres, encompassing the entire lagoon or estuary). In the following discussion, we will first examine evidence of OM variations in our results at the marsh scale by considering trends along the transects. Subsequently, we will shift our focus to the system scale by comparing results among study sites.

At the marsh scale, our results show that OM content in surface soils displays a significant trend with the distance from the marsh edge, with OM content generally increasing towards the inner marsh (Fig. a), consistently with previous findings (e.g. Chen et al., 2016; Leonard et al., 2002; Roner et al., 2016). This overall trend in OM with distance from the edge is influenced by various processes acting at the marsh scale, among which the interaction between sediment delivery and local topography plays a pre-eminent role. Suspended material is primarily delivered onto the marsh platform through inundation by overbank flow along tidal channels (Bayliss-Smith et al., 1979) and by apical flow at creek heads (Torres and Styles, 2007). As soon as the flow reaches the vegetated marsh platform, current velocities and turbulent energy rapidly decrease (D'Alpaos et al., 2007; Mudd et al., 2010), thus promoting the deposition of more abundant sediments during the flooding initial phase in close proximity of the marsh edge (Christiansen et al., 2000; Roner et al., 2016). As a result of the larger deposition close to the edge, inner marshes generally present slightly lower elevation than that of the marsh margin (Figs.  and ). Lower elevations promote the persistence of an anaerobic environment, which slows down OM decomposition by reducing microbial respiration (Halupa and Howes, 1995; Kirwan et al., 2013; Puppin et al., 2023a; Roner et al., 2016).

Progressive energy dissipation over the vegetated marsh platform also promotes selective material settling. Coarser, denser inorganic sediment is mainly deposited near the marsh edge, while the inner marsh receives a higher proportion of finer sediments, as supported by the observed grain size distribution along the transect (Fig. c), and less dense organic material (Leonard et al., 2002; Miller et al., 2022). In addition to the already larger proportion of organic material settling in the inner marsh, the supply of finer inorganic sediment (Fig. c) may also promote conditions favourable for OM preservation. This can occur due to the reduced oxygen exchange resulting from the lower porosity and drainage capacity of finer sediments, as well as their greater potential for protecting C from decay through organic mineral interactions and the formation of micro- or macro-aggregates (Kelleway et al., 2016).

Considering all available data along transects allows us to capture global trends and average out variabilities related to local conditions. However, analysing site-specific trends can offer an interesting perspective on drivers locally affecting OM dynamics. Overall, 7 out of 10 analysed transects show an increasing trend in OM with the distance from the marsh edge (CA, PA, SF, SE, CV, CO, and VB). At the SA, MI, and FO study sites the trend in OM content deviates from the average behaviour, and this can be attributed to the effects of local variability. At the SA site, observed OM content is very low (Fig. n), and its distribution pattern may be masked by the intrinsic variability in the measurements. At the MI site, Phragmites australis grows on the marsh edge, providing a contribution of OM that outcompetes that provided by the halophytic vegetation of the inner marsh (Fig. S1). At the FO site, we observed abundant beach-cast seagrass wracks on the marsh edge, which are likely transported from the extensive seagrass meadows located on the tidal flats adjacent to the FO area (Fig. f). These wracks may serve as an additional source of OM, locally influencing the OM trend with the distance from the marsh margin.

Conversely, in some transects where the organic matter trend aligns with the average increasing trend with distance from the marsh margin, we observe particular cases in the behaviour of grain size and topographic variables, which occasionally deviate from the average trend. For example, at the inner end of the SE transect, we can observe an unexpected slight increase both in sediment grain size (Fig. l) and in topographic elevation (Fig. m). This may be related to the presence of a tidal flat at the inner border of the SE marsh (Fig. j), which can represent an additional source of sediment supply. Two notable exceptions in terms of marsh topography are also represented by the CA and CO marshes, which face a tidal flat (Fig. b and h) and are exposed to energetic wind waves. As a consequence of the influence of wind waves, the elevation profiles exhibit a subtly convex shape, with the disappearance of the raised margin and a slight inward shift (i.e. between 5 and 10 m from the edge) of the maximum elevation along with the locations of higher median grain size values. However, these variations do not appear to affect the overall increasing trend of OM content towards the inner marsh at the SE, CA, and CO sites.

We also observe variations and patterns of OM at the system scale. The position within the gradient generated by marine and fluvial influence was previously observed to be a key predictor of OM content (e.g. Van de Broek et al., 2016; Ewers Lewis et al., 2020; Kelleway et al., 2016; Macreadie et al., 2017). At the lagoon scale, our results show lower mean OM content in surface soil in areas which are directly affected by marine influence, being closer to the inlets or along the main channels branching from them (CO, SE, SF, and SA) (Figs.  and a). Conversely, higher OM contents are observed at sites closer to the mainland, e.g. CA, MI, and PA. In agreement with this result, a significant negative relationship is observed between OM content and water salinity. However, it is unlikely that salinity as such directly controls soil organic content, as previous observations suggest an inverse relationship between soil salinity and decomposition (Hemminga et al., 1991; Wang et al., 2019). The effect of salinity on decomposition may be overcome by the co-occurring effects of other factors acting at different positions within the lagoon, such as vegetation characteristics, hydrodynamic conditions, sediment supply, and freshwater inputs. OM increase at less-saline sites is likely minimally related to the supply of already-stabilised organic suspended particles from terrestrial sources as suggested for other study areas (e.g. Van de Broek et al., 2016, 2018; Gorham et al., 2021; Omengo et al., 2016) because, in the Venice Lagoon, after historical river diversions, the fluvial supply of organic and inorganic material dramatically decreased. However, residual freshwater inputs, especially in terms of groundwater, can still locally reduce salinity levels and consequently affect vegetation characteristics, with usually increasing macrophyte biomass at lower salinity values (Hansen et al., 2017; Van de Broek et al., 2016).

A relationship between OM and grain size can also be observed at the lagoon scale. Higher values of median grain size are observed at sites closer to the inlet (i.e. SF and SE) or adjacent to first-order channels connected to them (SA) (Fig. d), whereas higher fractions of fine sediments are observed at the lagoon–mainland boundary near the Dese River mouth (PA) (Fig. ). This pattern is consistent with the general grain size gradient observed in the Venice Lagoon, reflecting the typical pattern of decreasing hydrodynamic energy conditions from the inlets to the landward shore (Zonta et al., 2018). Considering the landward decreasing grain size gradient observed within the lagoon, enhanced C preservation capacity of fine sediments (Kelleway et al., 2016) may have a role in the observed organic content pattern. Furthermore, we may hypothesise that at sites where inorganic sediment inputs are greater, where hydrodynamic energy is higher, the soil organic fraction is proportionally lower.

Another potentially important factor controlling OM content is vegetation type, as different species exhibit varying biomass production and decomposition resistance (e.g. Van de Broek et al., 2016; Ewers Lewis et al., 2020; Ford et al., 2019; Saintilan et al., 2013; Yuan et al., 2020).

While our data do not allow for a full statistical analysis of the relationship between OM content and vegetation type, we can derive some interesting qualitative observations. We find no discernible trend in vegetation cover along the transects nor a significant relationship between OM content and vegetation cover.

Considering the relationship between SOM and dominant species, we observe that higher OM percentages in surface soil are not necessarily associated with dominant species having greater aboveground biomass (as reported in the literature; see Table S4). We speculate that the lack of a relationship between aboveground biomass and OM may be due to the continuous transport and mixing of locally produced litter by marsh flooding, weakening the effect of local aboveground biomass production. Moreover, previous studies indicate a major impact from belowground biomass, which, inserted into the sediments, directly contributes to OM content (Craft et al., 1993; Rybczyk and Cahoon, 2002). The highest mean organic contents are observed in the presence of Limonium narbonense and Phragmites australis as dominant species (Fig. c). Phragmites australis, characterised by high aboveground biomass and even higher belowground biomass, forms a dense and deep network of leathery stems, roots, and rhizomes  (Fig. f) (Moore et al., 2012; Scarton et al., 2002). Limonium narbonense, despite low aboveground biomass (Table S4), produces massive woody roots (Fig. b) (Brooks et al., 2021). The belowground biomass of these plants may importantly contribute to SOM content quantitatively and qualitatively, as belowground litter decomposition was observed to decline with increasing lignin content (Stagg et al., 2018; Puppin et al., 2023a). Interestingly, these species are more abundant in locations associated with higher values of SOM, such as the inner marsh for Limonium narbonense and brackish areas for Phragmites australis (Figs. a and S1). However, it was not feasible to directly measure or estimate the relationship between aboveground and belowground biomass and their effect on SOM content in this study.

In addition, soil organic content shows a significant positive correlation with vegetation species diversity (Fig. d and e) in agreement with Ford et al. (2019). Ford et al. (2016) found that plant species richness was one of the most important explanatory variables of root biomass, and Xu et al. (2020) suggested that species richness may increase biomass productivity due to multiple mechanisms including competition reduction, niche complementarity, selection effects, and biotic and abiotic facilitation.

Overall, the interplay of these dynamics at both marsh and system scales also impacts sediment bulk density. We observe higher soil densities along marsh edges and at sites such as CO, SE, SF, and SA, where organic content is generally lower and coarser sediments predominate (Figs. b and b). This observation aligns with previous research indicating that soil density is influenced by both organic matter content and grain size. Indeed, several studies have reported a significant and negative correlation between SOM and bulk density (e.g. Holmquist et al., 2018; Morris et al., 2016), while sand content has been shown to correlate positively with soil bulk density (Tanveera et al., 2016).

As a consequence of the variability in soil organic content and density both at the marsh and at the system scale, SOC density and C stock show significant variations across and within different salt marshes, enhancing the complexity of blue C assessment. Based on our estimates of mean SOC density in the top 20 cm, and considering an expected accretion rate of about 0.3 cmyr-1 for salt marshes in equilibrium with relative sea level rise (RSLR) (Day et al., 1998), the average C accumulation rate for the salt marshes in the Venice Lagoon is estimated to be approximately equal to 69 gCm-2yr-1. Our results are consistent with the mean C accumulation rate in the Australian tidal marshes of 54.52 gCm-2yr-1 calculated by Macreadie et al. (2017) from 323 soil cores to the depth of 30 cm all around Australia and using a mean accretion rate of 0.21 cmyr-1. However, we observed that, under the same accretion rate, estimated SOC from our study may result in C accumulation rates varying up to 50 % from one place to another because of the two-scale variability highlighted by our measurements. This underscores the need for careful consideration of local variability when assessing blue carbon sequestration and storage potential in wetland environments.

Overall, our data reveal two scales of variations in sedimentary OM content in salt marsh soils. One occurs at the marsh scale and is influenced by the interplay between surface elevation and sediment supply dynamics along the distance from the marsh edge. The other occurs at the system scale, dominated by the gradient generated by marine and fluvial influence.

The observations on morphological, sedimentological, and vegetational features emphasise the dynamic feedbacks between hydrological dynamics, sediment supply, surface elevation, and vegetation characteristics and allowed us to relate them to SOM spatial patterns.

Our results show that OM content generally follows an inverse relationship with the distance from the marsh edge and, broadly, is greatest in salt marshes subjected to freshwater inputs and characterised by the presence of fine-grained sediments. Although the interplay between physical and biological factors makes it difficult to unravel the direct cause–effect relationship, variations in allochthonous inputs, sediment grain size, and/or preservation condition may explain the observed variation in SOM. Our data on local vegetation do not allow us to detect plant community characteristics controlling overall variations in SOM. Hence, we highlight the need for further analyses of quantitative and qualitative characteristics of different tidal marsh vegetation types. However, species diversity was observed to be an important factor influencing SOM content.

Our data on the spatial distribution of SOM provide additional insights on organic matter dynamics in salt marshes and inform model representations of SOM accumulation, with the possibility to help improve the ability for biogeomorphological models to describe marsh response to the effects of climate change and anthropogenic perturbation. Moreover, our analyses emphasise the potential complications introduced by local variability in assessing the blue carbon sequestration and storage potential in wetland environments. This underscores the importance of carefully considering both the local variability and the factors influencing it. Finally, our findings may inform conservation strategies and restoration interventions, providing information on conditions promoting OM storage and preservation, such as the maintenance or recovery of freshwater inputs, the supply of finer sediments, and the enhancement of vegetation diversity.