the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Dec 2023
Spatial patterns of Organic Matter content in the surface soil of the salt marshes of the Venice Lagoon (Italy)
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.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-2784', Anonymous Referee #1, 29 Jan 2024
The authors set out to understand spatial patterns on soil organic matter content in salt marshes around Venice Lagoon. The topic itself is interesting and timely and of high relevance for the readers of biogeosciences. Although the authors put a lot of effort in collecting and analyzing samples, in my opinion the data analysis and interpretation does not go far enough in understanding the uncovered spatial patterns.
Main comments:
- It seems increasing distance from the channel edge does not always mean increase in SOC, why, what could be the underlying causes? This is missing in the current version.
- I suggest to characterize the selected sites in being driven by different processes, e.g. driven by sedimentation through the tide, driven by sedimentation through waves, and then to analyze specific subgroups together, to gain an in-depth understanding of the encountered patterns
- Vegetation biomass data: either the authors show that the used literature-data-approach is valid, e.g. with some field sites comparisons, or I suggest removing all the vegetation biomass sections
- What is the variability in OM over the depth of the core, is it possible to constrain the OM variability at a give site?
Detailed comments:
Abstract
Line15: is organic matter only deposited or can it also originate from autochthonous production?
Line 19: what are the authors referring to when, stating in surface salt marsh soils, are they only considering surface samples or are did they also analysis samples throughout different depths?
Line 20-25,.. how was sedimentary OM distinguished from autochthonous OM?
Line 25: what do the authors mean with “carbon sink environments”?
Line 33: do the authors mean in micro to macro tidal regimes, ?
Line 50: Please rephrase “vertical accretion is driven by the deposition of OM”, yes OM can be deposited from the water column (e.g. POC) but OM in the sediment bed can also orginiate from plant production (above and below ground)
Line 54 : “Halophytic plants, spatially organized in characteristic patches”, I suggest to replace patches by zones, since the cited literature suggests the authors are referring to mid and high-marshes which in general are characterized by closed vegetation cover.
Line 57, please rephrase, what do the authors mean by tidal marsh volume ?
Line 59, I suggest to add autochthonously produced aboveground plant material which after dying of gets decomposed at the salt marsh surface.
Line 62: carbon stored in the soil, as SOC is not necessarily originating from C captured from the atmosphere, please refer to the studies of van den Broek et al.
Line 82: are the authors referring to soil organic content
Line 89; Please rephrase the sentence added here, although I agree with the core message, it is nevertheless difficult to understand: “Considerable variability in sediment organic content has also been observed at different scales across vegetation types (Ewers Lewis et al., 2020; Saintilan et al., 2013), which determine above and belowground biomass production both quantitatively and qualitatively, in terms of decay resistance (Scarton et al., 2002; Stagg et al., 2018). ”
Line 90: why did the authors not also analyze soil organic carbon stocks? why did the authors only analyze the top 20 cm?
Line 105ff: Since it might be relevant for understanding the SOC data, could that authors give some background information/estimates on the age of the sampled salt marshes?
Line 135ff: Could the authors provide information of the salinity of the water, i.e. is the entire lagoon brackish or are their freshwater to saltwater gradients at some sampled marshes?
Line 152: how was the aboveground biomass estimated? Was the stem-density per species assess, ? Are the literature references from the same are, hydroperiod, how comparable are the literature values to the site-specific conditions?
Line 154: what do the authors mean by sample community?
Line 155: was only 1 core taken per location? I know it is always easy to ask for more samples, but knowing the local variability would help in interpreting the significance of spatial patterns? Maybe there are previous studies which constrained that already ?
Line 165: here we see SOC stocks were calculated, please also indicate this at the end of the introduction.
Line 170: the reason for focusing on the top 20 cm, should also be communicated earlier on in the introduction.
Line 188: please do not start a sentence with “the figure shows”, i.e. making the figure the focus of attention, rather describe what the figures shows(the results) and refer to the figure at the end of the sentence, please rephrase,
Figure2: I am not sure that showing DBD and LOI adds value, I suggest to rescale the LOI axes especially for h,k and n to see whether there are spatial gradients visible? Why did the authors chose to present the mean values, how do the different layers look like ? if the mean values are chosen I suggest to add a standard deviation over the 20cm to be able to set the different layer in context with each other. could some general description of how LOI and DBD changes over depth be added in the appendix?
Line 205: here also a description of OM/LOI over depth should be added, additionally the mean values reported should be supplemented by a standard deviation.
Line 210: I suggest rephrasing in a more or less clear increase, in my opinion some do not show an increase at all, I think adding standard deviations might help with the interpretation.
Line 225: I am not convinced this is a fair comparison, i.e. simply lumping all the data. As visible in fig.2 a positive trend could be interpreted in maybe PA,SF,SE,SA and in fig.3 CV, CO,VB, which is the majority of the sites. However there are also clear sites showing that there is no increase in LOI with distance from the channel. I suggest to focus how the difference between sites can be explained. See comments above, to interpret this patterns it is important to know how SOC was changing over depth and whether the spatial variability can be estimated.
Line 240: See previous comment, it is unclear to me whether the estimated above-ground biomass is related to reality. Since previous studies, e.g. Kirwan et al, has shown the biomass production is highly dependent on local conditions and external drivers such as inundation period. It is moreover unclear how this estimate was achieved ? Fig.5a if the estimate of aboveground biomass cannot be better constrained I suggest to remove it from the manuscript !
Fig.5c, is it difficult to see but it seems that for some species there are only 1-2 data points, which in my opinion would make the shown box-plots highly uncertain and potentially misleading. I would prefer to show the points (data) and potentially a horizontal line per plant signifying the average.
Fig.6 what do we learn from this plot? what data is shown here? Are the authors comparing LOI, Carbon stock, SCB and D50 of all location irrespective of distance to channel? Assuming that the relationship in Fig.4 is correct and LOI is increasing with distance from channel (which I am not convinced of to be true for all sites) is it a good idea to compare averages of different parameters between sites? The authors could for instance compare samples close to the channel to samples in the interior, which could give more insights on whether OM import or local production differ between sites?
Line 260-270, same comment as for Fig.6
Line 290: I am sorry but I cannot see the coarser sediment on top of levee in fig.2 and fig.3
Line 295: Fig.4b “median sediment grain size (D50) on the marsh surface was found to be significantly correlated to the distance from the marsh edge and to surface elevations ” I cannot support this statement, looking at fig.4b there seems to be not a consistent relationship.
I recommend to not try to establish a general relationship between LOI and distance from channel but rather discuss where this relationship is present and where it is not and why?
For instance Line 297, is it discussed that CA and CO are more exposed to winds and waves and therefore might have a different morphology and different spatial patterns in SOC. ,
What are assumptions are linked to the expectation that SOC increases with distance from the channel, more organic deposition? more autochthons production?, in my opinion these factors should be further investigated using the existing dataset?
Line 300ff: to stress how SOC and morphology changes on the lagoonal scale an additional figure would be necessary, e.g. classifying all sampling locations as distance from the lagoon-mainland boundary, now this paragraph is difficult to very using the data presente.
Line 312-317: the described two scales describing variations in OM content are in the current form of the manuscript difficult to find? Since unfortunately no stable isotope markers or have been used or data on autochthonous production at different sites has been compared, it stays unclear why SOC sometimes increase with distance from the channel and why sometimes not. Whether this is linked to incoming OM and transport phenomena remains unclear?
Maybe the authors could characterized locations in tide or wave dominated areas and use this distinction to understand spatial patterns?
Line 335: I would argue that no clear results were obtained regarding plant biomass since it was not measured but extracted from literature which is unclear how it relates to the field situation, see criticism above. Also belowground biomass was not estimated !
Line 340ff: See comment above, to link OM to vegetation patterns seems very simplistic and does not do justice to what to authors have set out to do. I suggest to compare different channel distances for different vegetation patterns, to try to understand whether vegetation production causes increase of SOC or OM import from SPM.
Line 355ff: see comment above, catchment scale conclusion are not be related to a figure comparing different degrees of fluvial or marine influence
Citation: https://doi.org/10.5194/egusphere-2023-2784-RC1 - AC1: 'Reply on RC1', Alice Puppin, 21 Feb 2024
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RC2: 'Comment on egusphere-2023-2784', Anonymous Referee #2, 01 Feb 2024
The paper “Spatial patterns of Organic Matter content in the surface soil of the salt marshes of the Venice Lagoon (Italy)” presents an impressive field campaign, measuring key soil variables at several points along a transect perpendicular to the marsh edge, in ten different marshes of the Venice Lagoon. These results will be useful for the broader salt marsh community.
A few important comments to be addressed:
- There is a description of the outliers in the statistical analysis section missing, as they are only thus far mentioned in the figure legends. How did the authors determine outliers? Were these excluded from all statistical analyses?
- In the results presented in Figure 5, it is unclear why the authors binned values in values and b, and how this then influenced their statistical tests. I would think that some sort of linear regression would be more appropriate given that both x and y are continuous data. Thus, I am not convinced by the discussion in L348-352
- The main conclusion from the authors is about the two scales of variation in sedimentary OM content in salt marsh soils. The current version of the discussion lacks the structure for the reader to understand where this conclusion is coming from. One option would be to separate the discussion into two sections based on the marsh scale variation and the basin scale variation, instead of splitting it by variable measured as it is currently. Acknowledging that this would be a lot of work, another option could be to more clearly discuss the two scales of variation in the current discussion to make it evident to the reader the main important results and conclusions of this work.
Comments applicable throughout the paper:
- Choose between the wording “salt marsh” or “salt-marsh” to be used throughout the manuscript (with a preference for the former)
- When reporting very small p-values, it is preferred to use the format p < 0.0001 than to give the exact number
Specific edits:
- L44 insert comma after references
- L58 helps build (remove “to”)
- L82-83 needs to be rewritten (not sure what is meant here)
- L160 please add the SOM to SOC conversion equation used
- Figure 4 legend remove text about outliers plotted individually since it seems none are shown in this figure
- L297-300 split up the sentence (too long)
- L 300 move parentheses of reference to (2019)
- L303 remove “to” from near to the Dese River
- L314 remove “one” before is associated with
- L319 remove "a” before considerable variability
- L326 change marsh to marshes
- L331 consider changing supply to inputs
- L337-339 I would refrain from concluding on belowground biomass since it wasn’t studied. Or, make it clearer that it’s a variable that likely has an influence but wasn’t measured in this study
- L349 add parentheses to (2019)
- L372 change “being this effect overcome” to “as this effect may have been overcome”
- L376 define RSLR
- L 375-376 change order to sentence to “Based on our estimates of mean SOC density in the top 20 cm, and considering an expected accretion rate of about 0.3 cm yr−1 for salt marshes in equilibrium with RSLR…”
- L379-381 What are the implications of your findings?
- Section 4.3 should be combined with the previous section because it is currently too short. Another option is to restructure the discussion
- L398-399 This conclusion doesn’t say much. Either further describe or remove
- L401 how does the data constrain model representations? It may be more appropriate to write “inform model representations of SOM accumulation, with the possibility to help improve the ability for biogeomorphological models to describe marsh responses to the effects of climate change and anthropogenic perturbations.”
- L402-403 how do the analyses further elucidate marsh importance within the global C cycle?
- L403-404 how do your findings inform conservation strategies and restoration interventions?
Finally, I strongly recommend adding the following to the published dataset:
- Change depth to two separate columns with an upper and lower depth of each sample
- Add a latitude and longitude measurement for each soil core
Citation: https://doi.org/10.5194/egusphere-2023-2784-RC2 - AC2: 'Reply on RC2', Alice Puppin, 21 Feb 2024
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