the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Feb 2024
Silicon isotopes in juvenile and mature Cyperus papyrus from the Okavango Delta, Botswana
Abstract. The three most abundant stable isotopes of Silicon (Si), 28Si, 29Si, and 30Si, all occur in plants. Isotope studies are a potential tool to explore uptake and function of plant Si, and it is a developing field. However, there is a lack of studies from natural environments, and species from the African continent, and all plant parts including reproductive structures. In this study, naturally grown papyrus plants were sampled from the Okavango Delta and divided into five organs, i.e. umbel, culm, scales, rhizome, and roots. Samples were analysed for TN, TOC, BSi, TP concentrations, and for Si isotopes. Each organ of papyrus is represented by two samples, one from juvenile tissue and one mature (apart from the roots where age is difficult to determine). The study confirms that papyrus is a high Si-accumulating species, with BSi ranging from 0.88 % in rhizomes to 6.61 % in roots. High Si precipitation in the roots leads to an enrichment in heavy Si isotopes in the residual mobile Si pool, as light Si isotopes precipitate in phytoliths in the roots, even though in this study phytoliths were identified for all organs except for roots. In papyrus, shoot organs gradually become enriched in heavy Si isotopes along the transpiration stream, with an increase in heavy isotopes from rhizomes to scales, culm, and umbel, same pattern that has been observed for other plants in literature.
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RC1: 'Comment on egusphere-2024-225', Damien Cardinal, 24 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-225/egusphere-2024-225-RC1-supplement.pdf
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AC1: 'Reply on RC1', Giulia Lodi, 12 Jul 2024
This paper reports the Si isotopic compositions together with TOC, TN, TP and BSi concentrations in papyrus from the Okavango Delta. The data were measured from different plant parts (umbel, scales, culm, rhizome, roots) and for both mature and juvenile plants.
The Okavongo Delta is a unique environment where the silicon cycle has already been studied, including with Si isotopes, but not on this type of samples. The topic is therefore original and suitable for publication in Biogeosciences.
The number of data reported is relatively limited (10 in total: 5 plant parts for juvenile and mature), which partly justifies this short article. However, there are a significant number of missing information, approximations and limitations to general statements that make the paper not ready for publication yet and would require another round of review. The main points - detailed below – are: 1) explanations of the number of samples collected and how they were processed; 2) some issues related to the methods: cleaning for diatoms as well as dealing with the DOC matrix effect; 3) lack of reporting and discussion of the standard deviation / range of variation throughout the paper (tables, figures, results and discussion sections); 4) some vague statements and inappropriate references that makes the study mostly descriptive limiting its interest despite the originality of data.
Response: Thank you very much for your positive and constructive comments about the manuscript. We are glad you found the topic original and suitable for publication in Biogeosciences. The main criticisms were associated with the methods. We have clarified the section about the sampling, and also made terminology more consistent throughout the text. We have also elaborated on standard deviations and the supplementary table.
Detailed comments
L51-53, please refer at least to Opfergelt et al. 2008 biogeochemistry which precisely reports BSi isotopic compositions of different plants parts in Africa.
Response: We have referred to Opfergelt et al. 2008 and cited this reference.
The relevant section now reads (Line 50-52): There is a lack of isotopic studies on plants (Opfergelt et al., 2008) from natural environments and from species on the African continent. In addition, apart from a study on banana plants (Opfergelt et al., 2008), no other studies have measured different parts of plants including reproductive structures.
L52-54 The last sentence of this paragraph is vague, and the specific questions the authors wish to address could be more appropriately listed here. There is already a body of field work on Si isotopes in plants that has proven to be a useful tool for Si cycle (papers by Ding et al., Opfergelt et al., Riotte et al. ….).
Response: We have re-written according to your suggestions.
The relevant section now reads (Line 52-57): Si isotopes have been identified as a promising tool to resolve outstanding questions around plant Si use and biogeochemical studies (Ding et al., 2005, 2008; Opfergelt et al., 2006, 2008; Riotte et al., 2018; Sun et al., 2008; Y. Sun, Wu, and Li, 2016; Y. Sun, Wu, Li, et al., 2016), but more studies on natural plants are needed to understand their natural Si uptake and fractionation, and their role in biogeochemical cycles. In this study we analysed the different plant parts of natural, juvenile and mature Cyperus papyrus, from the roots up to the inflorescence, from the Okavango Delta, Botswana.
L93-94 The wording is unclear. It appears as if high temperature combustion has been used for Si isotopes as well as for TOC and TN. However, in section 2.3.3. on Si isotope digestion, there is no mention of combustion to remove organic matter. This sentence should be removed as it is confusing and should not appear in the sampling section since the methods are detailed elsewhere.
Response: We have deleted that sentence.
Sections 2.2, 3 and supp mat.
The actual number of samples/replicates is unclear. In the sampling section, the authors refer to the collection of 10 samples at 60 m distance, but when looking at the Supp. mat. there is only mention of 2 ID samples per category. Does this mean that leaves from 5 plants at the same site were split into 5 different parts (umbel, culm, scale, rhizomes and roots) and then combined to measure a composite geochemical and isotopic characterisations for each plant part at one site? And then the same method for the second site which is 60m from the first? If this is the case, please clarify and detail how were the composites prepared? Or is it that only two plants were collected at each site, so the 10 samples would mean only 5 different parts of 2 plants? In this case the number of samples is minimal and may not meet the standard for a journal such as Biogeosciences.
Response: We agree this was confusing. The sampling was performed at the sampling location Sepopa. The papyrus plants were collected at a 60-m distance from the river channel (referred to as backswamp in the text). During the sampling of papyrus, 3 papyrus plants were harvested from a 1-metre radius, and then divided per organ. To make 1 sample, the 3 same organs (3 umbels, 3 culms, 3 scales, 3 rhizomes, 3 roots) of the 3 plants were combined together. Finally, the composite was dried and ground.
The relevant section now reads (Line 94-100): Naturally-grown papyrus plants were sampled in 2017 from Sepopa, a site located in the Panhandle region of the Okavango Delta, Botswana (Fig. 1). Ten samples were collected at a 60-m distance from the river channel (called the backswamp, hereafter). Every sample is a combination of three papyrus plants, collected from a 1-metre radius. Papyrus plants were harvested and separated into the different organs with respect to the age of the plants in the field. The organs of papyrus are umbels, culms, scales, roots, and rhizomes, and all of them were collected both as juvenile and mature tissues except for roots (Fig. 2). Harvesting of plants was executed during flood recession or low flood (August-November 2017). The samples were rinsed, dried, and ground.
This confusion extends to every method used. In sections 2.2, 2.3.1, 2.3.2, 2.3.2, 2.3.3: the term subsample is used: what is a subsample of 2-3 mg e.g. for TOC? Is it an aliquot of a composite of leaves (or other parts) from 5 plants? Or is a subsample a 2-3 mg aliquot of leaves from 1 plant at 1 location? How the representativeness of a 2-3 mg (composite or not?) subsample has been ensured?
Response: With the term “subsample” we mean, e.g. for TOC, the 2-3 mg of sample have been weighed and analysed for TOC and TN. The amount of sample used for the analysis is a standard amount. As we clarified what we mean by sample (See Line 96), we hope you now consider the term subsample clearer in the manuscript.
Similarly, the Supp Mat table is unclear.
- There is no need to have three columns that are the same for every row (sampling date, location and site),
- In contrast, there is no explanation in the sample ID, with _2 and 'dup'? What do the 2 duplicates in the SuppMat stand for? Also, there is no duplicate for Si isotope, is there?
- The data are given as % SiO2 or uM SiO2. See also comment on Fig. 4: what does the unit uM represent in a plant part? Moreover, it's written BSi in the text and figure and % SiO2 in the table. If the unit of mass is BSi, the concentration is about half that of BSiO2, so is it SiO2 or Si? This needs to be homogenised and clarified. If the concentration is given for silica mass unit then use BSiO2, if it's silicon then use BSi.
Response: we have deleted the 3 columns (sampling date, location and site) and added this information in the supplementary table caption. The 2 duplicates were made by the machine itself while analysing the samples. Every sample, except for 1 root, was analysed twice for BSi. There is no replicate for Si isotope. uM was the BSi concentration measured by the machine, and that is afterwards converted to %wt SiO2. BSi for us was representing biogenic silica, so it has been changed into BSiO2 and homogenised throughout the text.
The relevant table caption now reads: Supplementary Table 1. Table with raw data from samples collected in May-June 2017, in the backswamp environment at the sampling site Sepopa. Every sample is a combination of three papyrus plants, collected from a 1-metre radius.
2.3.3
If high-temperature combustion was not performed prior to alkaline digestion, did the authors check for residual DOC that could induce a matrix effect (Hughes et al. 2011)? If this was done, it needs to be written more clearly.
Response: As described in the section 2.3.3, phytoliths materials were treated with HCl, HNO3 and H2O2 before being digested for Si isotopic analysis to remove carbonates and organic matter. Also before the digestion step, images of the phytoliths were made to check the purity of the sample. The matrix effect due to DOC mentioned by Hughes et al., 2011 concerns the river waters and unfortunately we have not measured river waters.
L135-140. The authors here honestly refer to some diatom contamination in the samples and describe their method to remove it, which consists of 2 hot alkaline leachings (0.1M Na2CO3 then 0.2M NaOH). Such leaching should dissolve not only diatoms but also some plant BSi, but no mention is made of whether this was considered and tested. How many samples were affected by the presence of diatoms? Are they mainly from root/rhizome samples? Is it possible to estimate the potential loss of plant BSi by this method and its effect on the Si isotope, e.g. if the BSi pool of the plant organ is not isotopically homogeneous? Can we also neglect the diatom contribution to plant BSi estimated by another digestion (§2.3.1)?
Response: All samples were visually inspected and we found diatoms only in root samples. We cleaned them thoroughly as described and re-inspected finding no diatoms presented. We limited the digestion to 20 min which is known to only dissolve the diatoms. Phytoliths are much more resistant (Fraysse et al., 2006, 2009) than diatoms and need hours before being attacked by NaOH like for spicules and radiolarians. The visual inspection was not only made to look at the presence of diatoms but also the surface of the phytoliths.
The relevant section now reads (Line 146-148): We limited the digestion to 20 min which is known to only dissolve the diatoms. Phytoliths are much more resistant than diatoms, with dissolution rates between those of quartz and vitreous silica (Fraysse et al., 2009) and are less reactive than diatom frustules (Fraysse et al., 2009).
Results sections 3.1, 3.2 and Figures 3 and 4 give average concentrations per tissue type and d30Si, but do not give the st dev and number of samples for each category. Please provide these - and keep the figures to significant digits only. St. dev. is only given for TN and TOC in roots in Fig. 3. For the other items, is the st. dev. within the symbol size? This must be mentioned in the caption. Significant differences in concentrations between juvenile and mature plants and between tissue types must be appropriately reported.
Response: We have added st.dev. for BSiO2 and 𝛿30Si. We do not have st.dev. for TN, TOC, and TP. In Figure 3, we provide the st.dev. for the roots, as the point in the graph represents the average of the 2 measurements of the 2 roots samples. For TP, the st.dev. is within the symbol size and therefore not visible. The same is true for Figure 4 where the st.dev. is not visible.
The legend for Figure 4 is incomplete, e.g. panel (c) is not listed. It is unclear what BSi refers to in the abscissa of panel c: units are uM, does it refer to BSi concentration in water? Why would this be relevant? This unit is different from panel (a) where the BSi concentration is given as % of dry weight. The reference to Frings et al. 2014 data is also unclear, particularly the shaded rectangle in (c). It would be more appropriate to present the Frings et al. data as a single point representing the mean +/- st dev in panel (c) and add the mean as a horizontal line in panel (b). Finally, the choice of different symbols (size, color, type…) could be improved to better differentiate the series.
Response: We have corrected and updated the Figure 4 caption. We have changed Figure 4. In panel (c), the x-axis is now Biogenic Silica (1/wt% SiO2), and not [BSi] (uM). We have added the water 𝛿30Si as a horizontal line in panels (b) and (c). We have changed the symbols: they are now all dots in a colour-blind-friendly combination of colours representing the different age categories. We have also added 2 lines to better explain the relationship between points which is explored in the discussion. Lastly, the data on [DSi] in water in the Panhandle region are now added as a caption only and and not in panel (c).
The relevant figure caption now reads (Line 218-220): Figure 4: Scatter plots showing (a) BSiO2 divided by plant part and age, (b) 𝛿30Si divided by plant part and age, and (c) 1/BSiO2 vs 𝛿30Si divided by plant part and age. 𝛿30Si and [DSi] water data are taken from Frings et al. (2014) as a reference. The st.dev. is within the symbol size where it is not visible.
L195 this section should be 3.2, not 3.1
Response: we have corrected it.
Section 4 – Discussion
Differences should only be discussed if they are significant, so either mention / add st dev and limit the numbers to the significant digits.
L234: There is another discrepancy between the text, figure and table. Here the authors refer to a BSi concentration of 6.61% in roots corresponding to the Supp mat, but in the figure it is less than 5%? This again causes a lot of confusion.
Response: we have deleted the reference to Fig. 4. In the figure, an average of the 2 roots samples with its relative st.dev. represented, while in the text we referred to the individual measurements and to the table in the supplementary material.
Fig. 5: It's a very nice illustration to identify the location of the phytolith, but it's a pity there's no close-up, as we miss a focus on a single phytolith scale to see the "conical morphotypes with satellites".
Response: Yes, a close up of a single phytolith would be good, but we do not have it. We have added the best close up we have.
L241-244. Unclear, please rephrase.
Response: we have rephrased it.
The relevant section now reads (Line 253-256): The Cyperaceae are prolific producers of phytoliths. Phytoliths have been reported from the leaves and culms of multiple species of Cyperus (Murungi and Bamford, 2020) and the culms and inflorescences of C. papyrus (Albert et al., 2006). To the best of our knowledge, phytoliths have not been isolated from Cyperus spp. roots (Murungi and Bamford, 2020).
L267-260 Since it's unclear what the x-axis of Fig. 4c represents, this sentence is also unclear. How could we expect a linear relationship between d30Si and 'plant part', which is not a numerical value, and/or why would we expect a linear relationship between d30Si and BSi in a plant?
Response: we have re-written this section to better explain what we meant with linear relationship.
The relevant section now reads (Line 281-291): Si isotopes can be used to investigate how Si is transported from the roots up to the umbel. If we consider the roots and the umbel as end members, the DSi enters papyrus plants through the roots with the umbel as the most distant organ and the last organ forming. If DSi travels passively from the roots to the umbel then we would expect fairly consistent changes in isotopic composition from organ to organ. In juvenile parts, it appears that Si is transported from the roots to all the other organs. In mature parts, one hypothesis is that after precipitation in the roots, heavy Si follows the transpiration stream moving from rhizome to scales to culm and to umbel. A second hypothesis is that there are two different transport pathways (Fig. 4c): one going from the roots up to culm and umbel, and one going from the rhizome to the scales. These two different pathways could be explained by both roots and rhizomes being underwater and therefore directly in contact with water. A third hypothesis is that the roots represent a constant supply to both pathways. Si enters the papyrus plant from the roots, and the lightest isotopes are deposited in the rhizome. Then, in one pathway Si moves from the rhizome to the scales and the culm, while in the other one the roots supply Si to the culm and umbel.
L290-292. The statement that ‘heavy Si isotopes were found to be more mobile than light isotopes in plants, contradicting the belief that the transport of light isotopes is favoured in plant biological processes (Dawson et al. 2002)’ is seriously flawed.
- First, contrary to what is written, there are no Si isotope data in Dawson et al. 2002, which focuses on C, N, O, H isotopes.
- Secondly, after a quick look at the paper, it doesn’t seem there is reference to mobility and associated isotopic fractionation within the plant. There is a discussion of C transport and bidirectional exchange between root and fungi, but nothing related to preferential transport of heavy isotopes within the plant. Note that comparison of C and O with Si isotopic systematics is difficult and should be justified because of the multiple processes and sources at stake in a plant for C and O. Comparison with N isotope could perhaps be more straightforward and useful if done properly since the N source is acquired via the roots as Si.
- Third, it's not a "belief" that biological processes favour the transport of Si light isotopes. There is plenty of data on Si isotopes (as well as on other isotopic systems) and the rationale is based on physical theory and isotopic fractionation data which clearly show that light isotopes move and react faster. Enrichments of heavy isotopes do exist, but they are related to bidirectional exchange with preferential incorporation of the heavier isotopes into the product due to the formation of more stable chemical bonds.
So either delete this sentence or provide appropriate references and discussion.
Response: We have deleted that sentence.
Table 1 and associated discussion should provide ranges or st. dev. to be useful
Response: We have checked the original sources, and added any measures of variance where it was available.
L302-303. It is true that this study provides evidence for a high concentration of BSi in the roots, which could lead to this pool being overlooked in previous studies. The authors could strengthen this statement by calculating the BSi fraction in each plant part relative to the total BSi content of the plant, if the biomass of the plant parts is known. Root biomass may be difficult to obtain accurately, but at least a range could be known. Do we expect the root biomass of papyrus to be significant compared to other parts?
Response: We do not have any data on biomass, but it will be interesting in the future to expand on this. However, we have found that Mnaya et al. (2007) reported some biomass percentages for this species, in a study from Tanzania. Hence, we can’t calculate the BSi fraction of each plant part but we have incorporated this new reference into the text to strengthen our arguments. Line 248-250
The relevant section now reads (Line 318-322): Other data for C. papyrus BSiO2 concentrations are limited, enabling only some comparisons. As a consequence, Si cycling calculations made with existing literature would lead to an underestimation because, as found in this study, the most BSiO2-rich organs (i.e., roots, culms and umbels) have not previously been measured for BSiO2. Mnaya et al. (2007) reported roots account for 14% of the total plant biomass in a case study from Tanzania. We can’t calculate the BSiO2 fraction of each plant part but roots’ biomass is significant compared to the other plant parts and therefore likely to be an overlooked BSiO2 pool.
Citation: https://doi.org/10.5194/egusphere-2024-225-AC1
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AC1: 'Reply on RC1', Giulia Lodi, 12 Jul 2024
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RC2: 'Comment on egusphere-2024-225', Anonymous Referee #2, 22 Jun 2024
I would first like to apologize to the authors for the delay in handing my review in. I also would like to clarify that at this current stage of submitting my own review, I haven’t read yet the other reviewer’s comment, which I saw was already posted.
To summarize, this manuscript by Lodi et al. reports the C, N, P and Si content and Si isotope composition of plants (Cyperus papyrus) collected in the Okavango Delta in Botswana. The study focuses on characterising those chemical signatures in various plant parts of both juvenile and mature individuals, with the aim to contribute to a better understanding of the Si cycling in wetland environments in general, and in particular in the Okavango Delta system, where this cycling is thought to be very dynamic in particular through the role of Si accumulating species.
The manuscript is properly organized, with acceptable phrasing (which could be improved at places still), and the analytical methods seem robust. Generally speaking, any study providing a refined understanding of Si biological cycling at the Earth surface, and/or of how Si isotopes fractionate during absorption by (and transport through) plants is timely and of interest to a wide audience, and potentially appropriate for publication in Biogeosciences.
However, I fail to identify the scientific question(s) this particular manuscript is asking. As an example, the end of the introduction states that the authors “sought to (1) Quantify the amounts of
macronutrients, including Si, in papyrus plant parts, and (2) Explore Si isotope distribution/fractionation in papyrus plants.” I don’t think either of the two makes for a sound scientific question, as (1) is merely about measuring concentrations of elements; and (2) is unclear (what does it mean “to explore” exactly?).I have more specific comments below, but I do think that as it stands this study is more of a data report. Note that this is not to dismiss the time and effort that went into this work, which I think features a very nice dataset that can be useful to the community. I think that if the authors still want to go ahead with a publication in something like Biogeosciences, this would require clarification of the scientific questions, and a more thorough investigation of e.g. a) the mechanisms at play in Si absorption and translocation in the targeted plants, and/or b) the ecosystem-scale Si fluxes involved in the context of the Okavango Delta.
***** Specific comments *****- l. 36: I don’t think “Silicon” needs to be capitalised.
- l. 39: “had been” - > “has been”.
- l. 67-69: The word “important” is repeated 3 times.
- l. 75-76: Two occurrences of “big”, for which the authors might prefer “large” or some other word.
- l. 77: “uptakes” -> “takes up”.
- Figure 1 could be made more informative, with DEM as background for example.
- l. 88: It says here that 10 plant samples were collected, but it remains unclear to me whether each of these samples was split for the different plant parts and analysed separately, or whether a selection only was analysed (and if yes, all from the same plant or across individuals?), or whether a “composite” was made…? This is important to understand what the numbers reported in the Results section (and in e.g. Fig. 3) mean (are they average? If yes this should be mention together with S.D. and n). Maybe this is something I missed in the text.
- l. 111: Use subscripts in “Na2CO3” (as l. 107).
- l. 177: Shouldn’t “C. papyrus” be italicised?
- l. 182-186: Is it reasonable to have two digits for the TOC numbers here?
- Figure 4: The way the Frings et al. (2014) data is represented in panel c is slightly misleading: I think it should just be a rectangle extending ~55 -> ~110 on the X-axis and ~-0.5 -> 1.1 on the Y-axis.
- l. 266: I don’t understand this statement: how could there be a “linear relationship between d30Si and plant part”? The latter is not a quantitative parameters… Also, why would such a linear relationship tell us that the transport os Si in papyrus is “conservative”, and what is meant by “conservative” in this context?
- l. 275: Two occurrences of “numerous” in the same line.
- l. 274-279: Do the authors mean that if present, phytoliths were removed by the cleaning steps? It is not stated clearly.
- l. 316: What does “shift from decreasing to increasing available DSi” mean?Citation: https://doi.org/10.5194/egusphere-2024-225-RC2 -
AC2: 'Reply on RC2', Giulia Lodi, 12 Jul 2024
I would first like to apologize to the authors for the delay in handing my review in. I also would like to clarify that at this current stage of submitting my own review, I haven’t read yet the other reviewer’s comment, which I saw was already posted.
To summarize, this manuscript by Lodi et al. reports the C, N, P and Si content and Si isotope composition of plants (Cyperus papyrus) collected in the Okavango Delta in Botswana. The study focuses on characterising those chemical signatures in various plant parts of both juvenile and mature individuals, with the aim to contribute to a better understanding of the Si cycling in wetland environments in general, and in particular in the Okavango Delta system, where this cycling is thought to be very dynamic in particular through the role of Si accumulating species.
The manuscript is properly organized, with acceptable phrasing (which could be improved at places still), and the analytical methods seem robust. Generally speaking, any study providing a refined understanding of Si biological cycling at the Earth surface, and/or of how Si isotopes fractionate during absorption by (and transport through) plants is timely and of interest to a wide audience, and potentially appropriate for publication in Biogeosciences.
However, I fail to identify the scientific question(s) this particular manuscript is asking. As an example, the end of the introduction states that the authors “sought to (1) Quantify the amounts of macronutrients, including Si, in papyrus plant parts, and (2) Explore Si isotope distribution/fractionation in papyrus plants.” I don’t think either of the two makes for a sound scientific question, as (1) is merely about measuring concentrations of elements; and (2) is unclear (what does it mean “to explore” exactly?).
I have more specific comments below, but I do think that as it stands this study is more of a data report. Note that this is not to dismiss the time and effort that went into this work, which I think features a very nice dataset that can be useful to the community. I think that if the authors still want to go ahead with a publication in something like Biogeosciences, this would require clarification of the scientific questions, and a more thorough investigation of e.g. a) the mechanisms at play in Si absorption and translocation in the targeted plants, and/or b) the ecosystem-scale Si fluxes involved in the context of the Okavango Delta.
Response: Thank you very much for the acknowledgement of the importance of contributions to the understanding of Si biological cycling and Si isotope fractionation in plants for a wide audience, as well as the time and effort that went into this work. We also appreciate the constructive criticism and specific comments, which are addressed below.
Both reviewers have asked for clarification on what is the important scientific question(s) to be addressed. We have rewritten the concluding paragraph in the introduction to focus on the important scientific question.The relevant section now reads (Line 82-91): Current understanding of Si and other nutrient cycles in the Okavango Delta needs further study, including the key drivers of these cycles and an understanding of the uptake and fractionation of Si during the growth of plants (Mendelsohn, 2010). This study aims to provide new knowledge on the aquatic plant communities living in the area, focusing on C. papyrus, one of the most abundant sedges in the Delta. The few studies on nutrient concentrations are particularly focused on Nitrogen (N) in papyrus plants. In order to understand more about Si cycling in this system and potential uptake by C. papyrus we sought to (1) Quantify the ability of papyrus plants to accumulate Si in all plant parts in an aquatic ecosystem, (2) Determine the Si isotope composition in different plant parts, and (3) Test if Si isotope fractionation in papyrus plants behaves similar to land plants (i.e., increasing 𝛿30Si along the transpiration stream, with the preferential deposition of lighter isotopes). To our knowledge this is one of the few studies applying Si isotopes on non-agricultural plants to enhance our knowledge of the biogeochemical Si cycle.
The reviewer has suggested that we could a) examine the mechanisms of uptake and translocation and b) extend the results to the entire Okavango Delta. Both of these suggestions require extensive research and essentially require a new study with many additional measurements. We should note that in the discussion we have addressed a number of issues regarding Si cycling in the papyrus plants providing important new knowledge regarding the application of Si isotopes to natural plant communities.
***** Specific comments *****- l. 36: I don’t think “Silicon” needs to be capitalised.
Response: we have removed the capital S.
- l. 39: “had been” - > “has been”.Response: we have corrected it.
- l. 67-69: The word “important” is repeated 3 times.Response: we have rephrased it.
The relevant section now reads (Line 71-74): The Okavango Delta is a current and historically important source of water during drought and dry seasons for both wildlife and livestock. The Delta has economic value through tourism associated with its ecological significance supporting high biodiversity, accounting for a large proportion of Botswana’s wildlife. It has an impressive paleoclimatic record contained in its sediments (Shaw, 1988).
- l. 75-76: Two occurrences of “big”, for which the authors might prefer “large” or some other word.
Response: we have replaced one “big” with “large”.
The relevant section now reads (Line 77-78): The 𝛿30Si in dissolved silica (DSi, hereafter) in the Delta’s surface water does not show large variations over time, ranging from 0.36 to 1.19‰.
- l. 77: “uptakes” -> “takes up”.Response: we have corrected it.
- Figure 1 could be made more informative, with DEM as background for example.Response: DEM = Digital elevation model. The purpose of Figure 1 is to show the sampling location in the Panhandle. We agree that a more detailed map would be more informative, but it would not add to the understanding of this research.
- l. 88: It says here that 10 plant samples were collected, but it remains unclear to me whether each of these samples was split for the different plant parts and analysed separately, or whether a selection only was analysed (and if yes, all from the same plant or across individuals?), or whether a “composite” was made…? This is important to understand what the numbers reported in the Results section (and in e.g. Fig. 3) mean (are they average? If yes this should be mention together with S.D. and n). Maybe this is something I missed in the text.Response: During the sampling of papyrus, 3 papyrus plants were harvested from a 1-meter radius, and then divided per organ. To make 1 sample, the 3 same organs (3 umbels, 3 culms, 3 scales, 3 rhizomes, 3 roots) of the 3 plants were combined together. Finally, the composite was dried and ground.
The relevant section of the manuscript now reads (Line 95-99): Ten samples were collected at a 60-m distance from the river channel (called the backswamp, hereafter). Every sample is a combination of three papyrus plants, collected from a 1-metre radius. Papyrus plants were harvested and separated into the different organs with respect to the age of the plants in the field. The organs of papyrus are umbels, culms, scales, roots, and rhizomes, and all of them were collected both as juvenile and mature tissues except for roots (Fig. 2).
- l. 111: Use subscripts in “Na2CO3” (as l. 107).Response: we have corrected it.
- l. 177: Shouldn’t “C. papyrus” be italicised?Response: we have corrected it.
- l. 182-186: Is it reasonable to have two digits for the TOC numbers here?Response: we have removed 1 digit for TOC data.
- Figure 4: The way the Frings et al. (2014) data is represented in panel c is slightly misleading: I think it should just be a rectangle extending ~55 -> ~110 on the X-axis and ~-0.5 -> 1.1 on the Y-axis.Response: we have replaced the shaded rectangle with a horizontal line in panel b and c, after another comment by the other reviewer.
- l. 266: I don’t understand this statement: how could there be a “linear relationship between d30Si and plant part”? The latter is not a quantitative parameters… Also, why would such a linear relationship tell us that the transport os Si in papyrus is “conservative”, and what is meant by “conservative” in this context?Response: we have clarified what we meant by linear relationship and conservative.
The relevant section now reads (Line 281-291): Si isotopes can be used to investigate how Si is transported from the roots up to the umbel. If we consider the roots and the umbel as end members, the DSi enters papyrus plants through the roots with the umbel as the most distant organ and the last organ forming. If DSi travels passively from the roots to the umbel then we would expect fairly consistent changes in isotopic composition from organ to organ. In juvenile parts, it appears that Si is transported from the roots to all the other organs. In mature parts, one hypothesis is that after precipitation in the roots, heavy Si follows the transpiration stream moving from rhizome to scales to culm and to umbel. A second hypothesis is that there are two different transport pathways (Fig. 4c): one going from the roots up to culm and umbel, and one going from the rhizome to the scales. These two different pathways could be explained by both roots and rhizomes being underwater and therefore directly in contact with water. A third hypothesis is that the roots represent a constant supply to both pathways. Si enters the papyrus plant from the roots, and the lightest isotopes are deposited in the rhizome. Then, in one pathway Si moves from the rhizome to the scales and the culm, while in the other one the roots supply Si to the culm and umbel.
- l. 275: Two occurrences of “numerous” in the same line.
Response: we have replaced one “numerous” with a synonym.
The relevant section now reads (Line 294-295): This could be due to degraded samples, possibly due to the several cleaning steps undertaken to remove the numerous diatoms we found on the samples, which are also siliceous.
- l. 274-279: Do the authors mean that if present, phytoliths were removed by the cleaning steps? It is not stated clearly.Response: we meant that, since in the roots we could not find any phytoliths and we had to undertake several cleaning steps to remove the diatoms present in the root samples, the root samples were degraded and this could be one reason why we did not find phytoliths.
The relevant section now reads (293-295): The high BSiO2 concentration in the roots was expected to reflect phytolith formation, however, we were not able to find phytoliths in the roots. This could be due to degraded samples, possibly due to the several cleaning steps undertaken to remove the numerous diatoms we found on the samples, which are also siliceous.
- l. 316: What does “shift from decreasing to increasing available DSi” mean?Response: we have deleted that sentence.
Citation: https://doi.org/10.5194/egusphere-2024-225-AC2
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AC2: 'Reply on RC2', Giulia Lodi, 12 Jul 2024
Status: closed
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RC1: 'Comment on egusphere-2024-225', Damien Cardinal, 24 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-225/egusphere-2024-225-RC1-supplement.pdf
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AC1: 'Reply on RC1', Giulia Lodi, 12 Jul 2024
This paper reports the Si isotopic compositions together with TOC, TN, TP and BSi concentrations in papyrus from the Okavango Delta. The data were measured from different plant parts (umbel, scales, culm, rhizome, roots) and for both mature and juvenile plants.
The Okavongo Delta is a unique environment where the silicon cycle has already been studied, including with Si isotopes, but not on this type of samples. The topic is therefore original and suitable for publication in Biogeosciences.
The number of data reported is relatively limited (10 in total: 5 plant parts for juvenile and mature), which partly justifies this short article. However, there are a significant number of missing information, approximations and limitations to general statements that make the paper not ready for publication yet and would require another round of review. The main points - detailed below – are: 1) explanations of the number of samples collected and how they were processed; 2) some issues related to the methods: cleaning for diatoms as well as dealing with the DOC matrix effect; 3) lack of reporting and discussion of the standard deviation / range of variation throughout the paper (tables, figures, results and discussion sections); 4) some vague statements and inappropriate references that makes the study mostly descriptive limiting its interest despite the originality of data.
Response: Thank you very much for your positive and constructive comments about the manuscript. We are glad you found the topic original and suitable for publication in Biogeosciences. The main criticisms were associated with the methods. We have clarified the section about the sampling, and also made terminology more consistent throughout the text. We have also elaborated on standard deviations and the supplementary table.
Detailed comments
L51-53, please refer at least to Opfergelt et al. 2008 biogeochemistry which precisely reports BSi isotopic compositions of different plants parts in Africa.
Response: We have referred to Opfergelt et al. 2008 and cited this reference.
The relevant section now reads (Line 50-52): There is a lack of isotopic studies on plants (Opfergelt et al., 2008) from natural environments and from species on the African continent. In addition, apart from a study on banana plants (Opfergelt et al., 2008), no other studies have measured different parts of plants including reproductive structures.
L52-54 The last sentence of this paragraph is vague, and the specific questions the authors wish to address could be more appropriately listed here. There is already a body of field work on Si isotopes in plants that has proven to be a useful tool for Si cycle (papers by Ding et al., Opfergelt et al., Riotte et al. ….).
Response: We have re-written according to your suggestions.
The relevant section now reads (Line 52-57): Si isotopes have been identified as a promising tool to resolve outstanding questions around plant Si use and biogeochemical studies (Ding et al., 2005, 2008; Opfergelt et al., 2006, 2008; Riotte et al., 2018; Sun et al., 2008; Y. Sun, Wu, and Li, 2016; Y. Sun, Wu, Li, et al., 2016), but more studies on natural plants are needed to understand their natural Si uptake and fractionation, and their role in biogeochemical cycles. In this study we analysed the different plant parts of natural, juvenile and mature Cyperus papyrus, from the roots up to the inflorescence, from the Okavango Delta, Botswana.
L93-94 The wording is unclear. It appears as if high temperature combustion has been used for Si isotopes as well as for TOC and TN. However, in section 2.3.3. on Si isotope digestion, there is no mention of combustion to remove organic matter. This sentence should be removed as it is confusing and should not appear in the sampling section since the methods are detailed elsewhere.
Response: We have deleted that sentence.
Sections 2.2, 3 and supp mat.
The actual number of samples/replicates is unclear. In the sampling section, the authors refer to the collection of 10 samples at 60 m distance, but when looking at the Supp. mat. there is only mention of 2 ID samples per category. Does this mean that leaves from 5 plants at the same site were split into 5 different parts (umbel, culm, scale, rhizomes and roots) and then combined to measure a composite geochemical and isotopic characterisations for each plant part at one site? And then the same method for the second site which is 60m from the first? If this is the case, please clarify and detail how were the composites prepared? Or is it that only two plants were collected at each site, so the 10 samples would mean only 5 different parts of 2 plants? In this case the number of samples is minimal and may not meet the standard for a journal such as Biogeosciences.
Response: We agree this was confusing. The sampling was performed at the sampling location Sepopa. The papyrus plants were collected at a 60-m distance from the river channel (referred to as backswamp in the text). During the sampling of papyrus, 3 papyrus plants were harvested from a 1-metre radius, and then divided per organ. To make 1 sample, the 3 same organs (3 umbels, 3 culms, 3 scales, 3 rhizomes, 3 roots) of the 3 plants were combined together. Finally, the composite was dried and ground.
The relevant section now reads (Line 94-100): Naturally-grown papyrus plants were sampled in 2017 from Sepopa, a site located in the Panhandle region of the Okavango Delta, Botswana (Fig. 1). Ten samples were collected at a 60-m distance from the river channel (called the backswamp, hereafter). Every sample is a combination of three papyrus plants, collected from a 1-metre radius. Papyrus plants were harvested and separated into the different organs with respect to the age of the plants in the field. The organs of papyrus are umbels, culms, scales, roots, and rhizomes, and all of them were collected both as juvenile and mature tissues except for roots (Fig. 2). Harvesting of plants was executed during flood recession or low flood (August-November 2017). The samples were rinsed, dried, and ground.
This confusion extends to every method used. In sections 2.2, 2.3.1, 2.3.2, 2.3.2, 2.3.3: the term subsample is used: what is a subsample of 2-3 mg e.g. for TOC? Is it an aliquot of a composite of leaves (or other parts) from 5 plants? Or is a subsample a 2-3 mg aliquot of leaves from 1 plant at 1 location? How the representativeness of a 2-3 mg (composite or not?) subsample has been ensured?
Response: With the term “subsample” we mean, e.g. for TOC, the 2-3 mg of sample have been weighed and analysed for TOC and TN. The amount of sample used for the analysis is a standard amount. As we clarified what we mean by sample (See Line 96), we hope you now consider the term subsample clearer in the manuscript.
Similarly, the Supp Mat table is unclear.
- There is no need to have three columns that are the same for every row (sampling date, location and site),
- In contrast, there is no explanation in the sample ID, with _2 and 'dup'? What do the 2 duplicates in the SuppMat stand for? Also, there is no duplicate for Si isotope, is there?
- The data are given as % SiO2 or uM SiO2. See also comment on Fig. 4: what does the unit uM represent in a plant part? Moreover, it's written BSi in the text and figure and % SiO2 in the table. If the unit of mass is BSi, the concentration is about half that of BSiO2, so is it SiO2 or Si? This needs to be homogenised and clarified. If the concentration is given for silica mass unit then use BSiO2, if it's silicon then use BSi.
Response: we have deleted the 3 columns (sampling date, location and site) and added this information in the supplementary table caption. The 2 duplicates were made by the machine itself while analysing the samples. Every sample, except for 1 root, was analysed twice for BSi. There is no replicate for Si isotope. uM was the BSi concentration measured by the machine, and that is afterwards converted to %wt SiO2. BSi for us was representing biogenic silica, so it has been changed into BSiO2 and homogenised throughout the text.
The relevant table caption now reads: Supplementary Table 1. Table with raw data from samples collected in May-June 2017, in the backswamp environment at the sampling site Sepopa. Every sample is a combination of three papyrus plants, collected from a 1-metre radius.
2.3.3
If high-temperature combustion was not performed prior to alkaline digestion, did the authors check for residual DOC that could induce a matrix effect (Hughes et al. 2011)? If this was done, it needs to be written more clearly.
Response: As described in the section 2.3.3, phytoliths materials were treated with HCl, HNO3 and H2O2 before being digested for Si isotopic analysis to remove carbonates and organic matter. Also before the digestion step, images of the phytoliths were made to check the purity of the sample. The matrix effect due to DOC mentioned by Hughes et al., 2011 concerns the river waters and unfortunately we have not measured river waters.
L135-140. The authors here honestly refer to some diatom contamination in the samples and describe their method to remove it, which consists of 2 hot alkaline leachings (0.1M Na2CO3 then 0.2M NaOH). Such leaching should dissolve not only diatoms but also some plant BSi, but no mention is made of whether this was considered and tested. How many samples were affected by the presence of diatoms? Are they mainly from root/rhizome samples? Is it possible to estimate the potential loss of plant BSi by this method and its effect on the Si isotope, e.g. if the BSi pool of the plant organ is not isotopically homogeneous? Can we also neglect the diatom contribution to plant BSi estimated by another digestion (§2.3.1)?
Response: All samples were visually inspected and we found diatoms only in root samples. We cleaned them thoroughly as described and re-inspected finding no diatoms presented. We limited the digestion to 20 min which is known to only dissolve the diatoms. Phytoliths are much more resistant (Fraysse et al., 2006, 2009) than diatoms and need hours before being attacked by NaOH like for spicules and radiolarians. The visual inspection was not only made to look at the presence of diatoms but also the surface of the phytoliths.
The relevant section now reads (Line 146-148): We limited the digestion to 20 min which is known to only dissolve the diatoms. Phytoliths are much more resistant than diatoms, with dissolution rates between those of quartz and vitreous silica (Fraysse et al., 2009) and are less reactive than diatom frustules (Fraysse et al., 2009).
Results sections 3.1, 3.2 and Figures 3 and 4 give average concentrations per tissue type and d30Si, but do not give the st dev and number of samples for each category. Please provide these - and keep the figures to significant digits only. St. dev. is only given for TN and TOC in roots in Fig. 3. For the other items, is the st. dev. within the symbol size? This must be mentioned in the caption. Significant differences in concentrations between juvenile and mature plants and between tissue types must be appropriately reported.
Response: We have added st.dev. for BSiO2 and 𝛿30Si. We do not have st.dev. for TN, TOC, and TP. In Figure 3, we provide the st.dev. for the roots, as the point in the graph represents the average of the 2 measurements of the 2 roots samples. For TP, the st.dev. is within the symbol size and therefore not visible. The same is true for Figure 4 where the st.dev. is not visible.
The legend for Figure 4 is incomplete, e.g. panel (c) is not listed. It is unclear what BSi refers to in the abscissa of panel c: units are uM, does it refer to BSi concentration in water? Why would this be relevant? This unit is different from panel (a) where the BSi concentration is given as % of dry weight. The reference to Frings et al. 2014 data is also unclear, particularly the shaded rectangle in (c). It would be more appropriate to present the Frings et al. data as a single point representing the mean +/- st dev in panel (c) and add the mean as a horizontal line in panel (b). Finally, the choice of different symbols (size, color, type…) could be improved to better differentiate the series.
Response: We have corrected and updated the Figure 4 caption. We have changed Figure 4. In panel (c), the x-axis is now Biogenic Silica (1/wt% SiO2), and not [BSi] (uM). We have added the water 𝛿30Si as a horizontal line in panels (b) and (c). We have changed the symbols: they are now all dots in a colour-blind-friendly combination of colours representing the different age categories. We have also added 2 lines to better explain the relationship between points which is explored in the discussion. Lastly, the data on [DSi] in water in the Panhandle region are now added as a caption only and and not in panel (c).
The relevant figure caption now reads (Line 218-220): Figure 4: Scatter plots showing (a) BSiO2 divided by plant part and age, (b) 𝛿30Si divided by plant part and age, and (c) 1/BSiO2 vs 𝛿30Si divided by plant part and age. 𝛿30Si and [DSi] water data are taken from Frings et al. (2014) as a reference. The st.dev. is within the symbol size where it is not visible.
L195 this section should be 3.2, not 3.1
Response: we have corrected it.
Section 4 – Discussion
Differences should only be discussed if they are significant, so either mention / add st dev and limit the numbers to the significant digits.
L234: There is another discrepancy between the text, figure and table. Here the authors refer to a BSi concentration of 6.61% in roots corresponding to the Supp mat, but in the figure it is less than 5%? This again causes a lot of confusion.
Response: we have deleted the reference to Fig. 4. In the figure, an average of the 2 roots samples with its relative st.dev. represented, while in the text we referred to the individual measurements and to the table in the supplementary material.
Fig. 5: It's a very nice illustration to identify the location of the phytolith, but it's a pity there's no close-up, as we miss a focus on a single phytolith scale to see the "conical morphotypes with satellites".
Response: Yes, a close up of a single phytolith would be good, but we do not have it. We have added the best close up we have.
L241-244. Unclear, please rephrase.
Response: we have rephrased it.
The relevant section now reads (Line 253-256): The Cyperaceae are prolific producers of phytoliths. Phytoliths have been reported from the leaves and culms of multiple species of Cyperus (Murungi and Bamford, 2020) and the culms and inflorescences of C. papyrus (Albert et al., 2006). To the best of our knowledge, phytoliths have not been isolated from Cyperus spp. roots (Murungi and Bamford, 2020).
L267-260 Since it's unclear what the x-axis of Fig. 4c represents, this sentence is also unclear. How could we expect a linear relationship between d30Si and 'plant part', which is not a numerical value, and/or why would we expect a linear relationship between d30Si and BSi in a plant?
Response: we have re-written this section to better explain what we meant with linear relationship.
The relevant section now reads (Line 281-291): Si isotopes can be used to investigate how Si is transported from the roots up to the umbel. If we consider the roots and the umbel as end members, the DSi enters papyrus plants through the roots with the umbel as the most distant organ and the last organ forming. If DSi travels passively from the roots to the umbel then we would expect fairly consistent changes in isotopic composition from organ to organ. In juvenile parts, it appears that Si is transported from the roots to all the other organs. In mature parts, one hypothesis is that after precipitation in the roots, heavy Si follows the transpiration stream moving from rhizome to scales to culm and to umbel. A second hypothesis is that there are two different transport pathways (Fig. 4c): one going from the roots up to culm and umbel, and one going from the rhizome to the scales. These two different pathways could be explained by both roots and rhizomes being underwater and therefore directly in contact with water. A third hypothesis is that the roots represent a constant supply to both pathways. Si enters the papyrus plant from the roots, and the lightest isotopes are deposited in the rhizome. Then, in one pathway Si moves from the rhizome to the scales and the culm, while in the other one the roots supply Si to the culm and umbel.
L290-292. The statement that ‘heavy Si isotopes were found to be more mobile than light isotopes in plants, contradicting the belief that the transport of light isotopes is favoured in plant biological processes (Dawson et al. 2002)’ is seriously flawed.
- First, contrary to what is written, there are no Si isotope data in Dawson et al. 2002, which focuses on C, N, O, H isotopes.
- Secondly, after a quick look at the paper, it doesn’t seem there is reference to mobility and associated isotopic fractionation within the plant. There is a discussion of C transport and bidirectional exchange between root and fungi, but nothing related to preferential transport of heavy isotopes within the plant. Note that comparison of C and O with Si isotopic systematics is difficult and should be justified because of the multiple processes and sources at stake in a plant for C and O. Comparison with N isotope could perhaps be more straightforward and useful if done properly since the N source is acquired via the roots as Si.
- Third, it's not a "belief" that biological processes favour the transport of Si light isotopes. There is plenty of data on Si isotopes (as well as on other isotopic systems) and the rationale is based on physical theory and isotopic fractionation data which clearly show that light isotopes move and react faster. Enrichments of heavy isotopes do exist, but they are related to bidirectional exchange with preferential incorporation of the heavier isotopes into the product due to the formation of more stable chemical bonds.
So either delete this sentence or provide appropriate references and discussion.
Response: We have deleted that sentence.
Table 1 and associated discussion should provide ranges or st. dev. to be useful
Response: We have checked the original sources, and added any measures of variance where it was available.
L302-303. It is true that this study provides evidence for a high concentration of BSi in the roots, which could lead to this pool being overlooked in previous studies. The authors could strengthen this statement by calculating the BSi fraction in each plant part relative to the total BSi content of the plant, if the biomass of the plant parts is known. Root biomass may be difficult to obtain accurately, but at least a range could be known. Do we expect the root biomass of papyrus to be significant compared to other parts?
Response: We do not have any data on biomass, but it will be interesting in the future to expand on this. However, we have found that Mnaya et al. (2007) reported some biomass percentages for this species, in a study from Tanzania. Hence, we can’t calculate the BSi fraction of each plant part but we have incorporated this new reference into the text to strengthen our arguments. Line 248-250
The relevant section now reads (Line 318-322): Other data for C. papyrus BSiO2 concentrations are limited, enabling only some comparisons. As a consequence, Si cycling calculations made with existing literature would lead to an underestimation because, as found in this study, the most BSiO2-rich organs (i.e., roots, culms and umbels) have not previously been measured for BSiO2. Mnaya et al. (2007) reported roots account for 14% of the total plant biomass in a case study from Tanzania. We can’t calculate the BSiO2 fraction of each plant part but roots’ biomass is significant compared to the other plant parts and therefore likely to be an overlooked BSiO2 pool.
Citation: https://doi.org/10.5194/egusphere-2024-225-AC1
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AC1: 'Reply on RC1', Giulia Lodi, 12 Jul 2024
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RC2: 'Comment on egusphere-2024-225', Anonymous Referee #2, 22 Jun 2024
I would first like to apologize to the authors for the delay in handing my review in. I also would like to clarify that at this current stage of submitting my own review, I haven’t read yet the other reviewer’s comment, which I saw was already posted.
To summarize, this manuscript by Lodi et al. reports the C, N, P and Si content and Si isotope composition of plants (Cyperus papyrus) collected in the Okavango Delta in Botswana. The study focuses on characterising those chemical signatures in various plant parts of both juvenile and mature individuals, with the aim to contribute to a better understanding of the Si cycling in wetland environments in general, and in particular in the Okavango Delta system, where this cycling is thought to be very dynamic in particular through the role of Si accumulating species.
The manuscript is properly organized, with acceptable phrasing (which could be improved at places still), and the analytical methods seem robust. Generally speaking, any study providing a refined understanding of Si biological cycling at the Earth surface, and/or of how Si isotopes fractionate during absorption by (and transport through) plants is timely and of interest to a wide audience, and potentially appropriate for publication in Biogeosciences.
However, I fail to identify the scientific question(s) this particular manuscript is asking. As an example, the end of the introduction states that the authors “sought to (1) Quantify the amounts of
macronutrients, including Si, in papyrus plant parts, and (2) Explore Si isotope distribution/fractionation in papyrus plants.” I don’t think either of the two makes for a sound scientific question, as (1) is merely about measuring concentrations of elements; and (2) is unclear (what does it mean “to explore” exactly?).I have more specific comments below, but I do think that as it stands this study is more of a data report. Note that this is not to dismiss the time and effort that went into this work, which I think features a very nice dataset that can be useful to the community. I think that if the authors still want to go ahead with a publication in something like Biogeosciences, this would require clarification of the scientific questions, and a more thorough investigation of e.g. a) the mechanisms at play in Si absorption and translocation in the targeted plants, and/or b) the ecosystem-scale Si fluxes involved in the context of the Okavango Delta.
***** Specific comments *****- l. 36: I don’t think “Silicon” needs to be capitalised.
- l. 39: “had been” - > “has been”.
- l. 67-69: The word “important” is repeated 3 times.
- l. 75-76: Two occurrences of “big”, for which the authors might prefer “large” or some other word.
- l. 77: “uptakes” -> “takes up”.
- Figure 1 could be made more informative, with DEM as background for example.
- l. 88: It says here that 10 plant samples were collected, but it remains unclear to me whether each of these samples was split for the different plant parts and analysed separately, or whether a selection only was analysed (and if yes, all from the same plant or across individuals?), or whether a “composite” was made…? This is important to understand what the numbers reported in the Results section (and in e.g. Fig. 3) mean (are they average? If yes this should be mention together with S.D. and n). Maybe this is something I missed in the text.
- l. 111: Use subscripts in “Na2CO3” (as l. 107).
- l. 177: Shouldn’t “C. papyrus” be italicised?
- l. 182-186: Is it reasonable to have two digits for the TOC numbers here?
- Figure 4: The way the Frings et al. (2014) data is represented in panel c is slightly misleading: I think it should just be a rectangle extending ~55 -> ~110 on the X-axis and ~-0.5 -> 1.1 on the Y-axis.
- l. 266: I don’t understand this statement: how could there be a “linear relationship between d30Si and plant part”? The latter is not a quantitative parameters… Also, why would such a linear relationship tell us that the transport os Si in papyrus is “conservative”, and what is meant by “conservative” in this context?
- l. 275: Two occurrences of “numerous” in the same line.
- l. 274-279: Do the authors mean that if present, phytoliths were removed by the cleaning steps? It is not stated clearly.
- l. 316: What does “shift from decreasing to increasing available DSi” mean?Citation: https://doi.org/10.5194/egusphere-2024-225-RC2 -
AC2: 'Reply on RC2', Giulia Lodi, 12 Jul 2024
I would first like to apologize to the authors for the delay in handing my review in. I also would like to clarify that at this current stage of submitting my own review, I haven’t read yet the other reviewer’s comment, which I saw was already posted.
To summarize, this manuscript by Lodi et al. reports the C, N, P and Si content and Si isotope composition of plants (Cyperus papyrus) collected in the Okavango Delta in Botswana. The study focuses on characterising those chemical signatures in various plant parts of both juvenile and mature individuals, with the aim to contribute to a better understanding of the Si cycling in wetland environments in general, and in particular in the Okavango Delta system, where this cycling is thought to be very dynamic in particular through the role of Si accumulating species.
The manuscript is properly organized, with acceptable phrasing (which could be improved at places still), and the analytical methods seem robust. Generally speaking, any study providing a refined understanding of Si biological cycling at the Earth surface, and/or of how Si isotopes fractionate during absorption by (and transport through) plants is timely and of interest to a wide audience, and potentially appropriate for publication in Biogeosciences.
However, I fail to identify the scientific question(s) this particular manuscript is asking. As an example, the end of the introduction states that the authors “sought to (1) Quantify the amounts of macronutrients, including Si, in papyrus plant parts, and (2) Explore Si isotope distribution/fractionation in papyrus plants.” I don’t think either of the two makes for a sound scientific question, as (1) is merely about measuring concentrations of elements; and (2) is unclear (what does it mean “to explore” exactly?).
I have more specific comments below, but I do think that as it stands this study is more of a data report. Note that this is not to dismiss the time and effort that went into this work, which I think features a very nice dataset that can be useful to the community. I think that if the authors still want to go ahead with a publication in something like Biogeosciences, this would require clarification of the scientific questions, and a more thorough investigation of e.g. a) the mechanisms at play in Si absorption and translocation in the targeted plants, and/or b) the ecosystem-scale Si fluxes involved in the context of the Okavango Delta.
Response: Thank you very much for the acknowledgement of the importance of contributions to the understanding of Si biological cycling and Si isotope fractionation in plants for a wide audience, as well as the time and effort that went into this work. We also appreciate the constructive criticism and specific comments, which are addressed below.
Both reviewers have asked for clarification on what is the important scientific question(s) to be addressed. We have rewritten the concluding paragraph in the introduction to focus on the important scientific question.The relevant section now reads (Line 82-91): Current understanding of Si and other nutrient cycles in the Okavango Delta needs further study, including the key drivers of these cycles and an understanding of the uptake and fractionation of Si during the growth of plants (Mendelsohn, 2010). This study aims to provide new knowledge on the aquatic plant communities living in the area, focusing on C. papyrus, one of the most abundant sedges in the Delta. The few studies on nutrient concentrations are particularly focused on Nitrogen (N) in papyrus plants. In order to understand more about Si cycling in this system and potential uptake by C. papyrus we sought to (1) Quantify the ability of papyrus plants to accumulate Si in all plant parts in an aquatic ecosystem, (2) Determine the Si isotope composition in different plant parts, and (3) Test if Si isotope fractionation in papyrus plants behaves similar to land plants (i.e., increasing 𝛿30Si along the transpiration stream, with the preferential deposition of lighter isotopes). To our knowledge this is one of the few studies applying Si isotopes on non-agricultural plants to enhance our knowledge of the biogeochemical Si cycle.
The reviewer has suggested that we could a) examine the mechanisms of uptake and translocation and b) extend the results to the entire Okavango Delta. Both of these suggestions require extensive research and essentially require a new study with many additional measurements. We should note that in the discussion we have addressed a number of issues regarding Si cycling in the papyrus plants providing important new knowledge regarding the application of Si isotopes to natural plant communities.
***** Specific comments *****- l. 36: I don’t think “Silicon” needs to be capitalised.
Response: we have removed the capital S.
- l. 39: “had been” - > “has been”.Response: we have corrected it.
- l. 67-69: The word “important” is repeated 3 times.Response: we have rephrased it.
The relevant section now reads (Line 71-74): The Okavango Delta is a current and historically important source of water during drought and dry seasons for both wildlife and livestock. The Delta has economic value through tourism associated with its ecological significance supporting high biodiversity, accounting for a large proportion of Botswana’s wildlife. It has an impressive paleoclimatic record contained in its sediments (Shaw, 1988).
- l. 75-76: Two occurrences of “big”, for which the authors might prefer “large” or some other word.
Response: we have replaced one “big” with “large”.
The relevant section now reads (Line 77-78): The 𝛿30Si in dissolved silica (DSi, hereafter) in the Delta’s surface water does not show large variations over time, ranging from 0.36 to 1.19‰.
- l. 77: “uptakes” -> “takes up”.Response: we have corrected it.
- Figure 1 could be made more informative, with DEM as background for example.Response: DEM = Digital elevation model. The purpose of Figure 1 is to show the sampling location in the Panhandle. We agree that a more detailed map would be more informative, but it would not add to the understanding of this research.
- l. 88: It says here that 10 plant samples were collected, but it remains unclear to me whether each of these samples was split for the different plant parts and analysed separately, or whether a selection only was analysed (and if yes, all from the same plant or across individuals?), or whether a “composite” was made…? This is important to understand what the numbers reported in the Results section (and in e.g. Fig. 3) mean (are they average? If yes this should be mention together with S.D. and n). Maybe this is something I missed in the text.Response: During the sampling of papyrus, 3 papyrus plants were harvested from a 1-meter radius, and then divided per organ. To make 1 sample, the 3 same organs (3 umbels, 3 culms, 3 scales, 3 rhizomes, 3 roots) of the 3 plants were combined together. Finally, the composite was dried and ground.
The relevant section of the manuscript now reads (Line 95-99): Ten samples were collected at a 60-m distance from the river channel (called the backswamp, hereafter). Every sample is a combination of three papyrus plants, collected from a 1-metre radius. Papyrus plants were harvested and separated into the different organs with respect to the age of the plants in the field. The organs of papyrus are umbels, culms, scales, roots, and rhizomes, and all of them were collected both as juvenile and mature tissues except for roots (Fig. 2).
- l. 111: Use subscripts in “Na2CO3” (as l. 107).Response: we have corrected it.
- l. 177: Shouldn’t “C. papyrus” be italicised?Response: we have corrected it.
- l. 182-186: Is it reasonable to have two digits for the TOC numbers here?Response: we have removed 1 digit for TOC data.
- Figure 4: The way the Frings et al. (2014) data is represented in panel c is slightly misleading: I think it should just be a rectangle extending ~55 -> ~110 on the X-axis and ~-0.5 -> 1.1 on the Y-axis.Response: we have replaced the shaded rectangle with a horizontal line in panel b and c, after another comment by the other reviewer.
- l. 266: I don’t understand this statement: how could there be a “linear relationship between d30Si and plant part”? The latter is not a quantitative parameters… Also, why would such a linear relationship tell us that the transport os Si in papyrus is “conservative”, and what is meant by “conservative” in this context?Response: we have clarified what we meant by linear relationship and conservative.
The relevant section now reads (Line 281-291): Si isotopes can be used to investigate how Si is transported from the roots up to the umbel. If we consider the roots and the umbel as end members, the DSi enters papyrus plants through the roots with the umbel as the most distant organ and the last organ forming. If DSi travels passively from the roots to the umbel then we would expect fairly consistent changes in isotopic composition from organ to organ. In juvenile parts, it appears that Si is transported from the roots to all the other organs. In mature parts, one hypothesis is that after precipitation in the roots, heavy Si follows the transpiration stream moving from rhizome to scales to culm and to umbel. A second hypothesis is that there are two different transport pathways (Fig. 4c): one going from the roots up to culm and umbel, and one going from the rhizome to the scales. These two different pathways could be explained by both roots and rhizomes being underwater and therefore directly in contact with water. A third hypothesis is that the roots represent a constant supply to both pathways. Si enters the papyrus plant from the roots, and the lightest isotopes are deposited in the rhizome. Then, in one pathway Si moves from the rhizome to the scales and the culm, while in the other one the roots supply Si to the culm and umbel.
- l. 275: Two occurrences of “numerous” in the same line.
Response: we have replaced one “numerous” with a synonym.
The relevant section now reads (Line 294-295): This could be due to degraded samples, possibly due to the several cleaning steps undertaken to remove the numerous diatoms we found on the samples, which are also siliceous.
- l. 274-279: Do the authors mean that if present, phytoliths were removed by the cleaning steps? It is not stated clearly.Response: we meant that, since in the roots we could not find any phytoliths and we had to undertake several cleaning steps to remove the diatoms present in the root samples, the root samples were degraded and this could be one reason why we did not find phytoliths.
The relevant section now reads (293-295): The high BSiO2 concentration in the roots was expected to reflect phytolith formation, however, we were not able to find phytoliths in the roots. This could be due to degraded samples, possibly due to the several cleaning steps undertaken to remove the numerous diatoms we found on the samples, which are also siliceous.
- l. 316: What does “shift from decreasing to increasing available DSi” mean?Response: we have deleted that sentence.
Citation: https://doi.org/10.5194/egusphere-2024-225-AC2
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AC2: 'Reply on RC2', Giulia Lodi, 12 Jul 2024
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