the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 May 2026
Urbanization drives coupled shifts in soil-carbon stocks, sources, and stability across natural and restored mangroves and tidal flats
Abstract. Urbanization reshapes coastal blue carbon, but its effects on how much carbon is stored, who supplies it, and how long it persists remain poorly integrated. We investigated natural and restored mangroves and adjacent tidal flats along an urbanization gradient, and quantified soil organic carbon stocks, burial from 210Pb profiles, source composition using isotope end-member mixing, and stability from turnover metrics. Our results showed a coordinated triad response to urbanization. Carbon stocks and burial declined, sources shifted away from mangrove detritus toward planktonic and algal inputs, and turnover accelerated, lowering stability. Responses were habitat dependent and nonlinear. Natural mangroves in low urbanization settings maintained the highest sequestration with mangrove-dominated inputs and slower turnover. Restored stands and tidal flats showed steeper stock losses, stronger source substitution, and faster cycling under higher urban pressure. We introduced a triad framework that treats stocks, sources, and stability as coupled state variables along the urbanization gradient and identified two reproducible system states: carbon anchors in low urbanization natural mangroves and instability fronts in restored stands and tidal flats. Shifts in sources and stability precede stock losses, providing clear early warnings of urban impact. A simple diagnostic that combines connectivity and accretion with source composition and a composite stability index guides anchor protection and stability-first restoration. These results link urban growth to blue-carbon performance and define actionable thresholds for sustaining coastal carbon.
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Status: open (until 03 Jul 2026)
- CC1: 'Comment on egusphere-2026-2512', Li Gang, 27 May 2026 reply
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RC1: 'Comment on egusphere-2026-2512', Anonymous Referee #1, 01 Jun 2026
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In my opinion, this paper addresses relevant scientific questions within the scope of BG and presents novel ideas and tools. I believe the topics addressed are well represented in the manuscript title and abstract and are important for the field of environmental science as a whole. I suggest accepting this manuscript with some revisions.
General comments:
Figures: All of your figures look very different from each other. Similar categories (Natural mangroves, Restored mangroves, Tidal flats or High, Moderate, Low-urbanization) are shown in several different colors or layouts throughout your figures. It might be easier for the reader if you picked a consistent color scheme throughout (example: applying the green, yellow, red scheme you used to depict High, Moderate, and Low-urbanizations in Figure 6 to all of your relevant figures) or consistent layout (like the clearly differentiated column and row headings in Figure 5).
Specific comments:
line 28: Do you want to include this image as Figure 1? If so, please add a figure description and adjust figure numbering throughout the manuscript accordingly.
line 72: Both “blue-carbon” and “blue carbon” have been used in the manuscript so far. Please choose one format and apply throughout the whole manuscript.
line 90: Figure 1. Please make some adjustments to Figure 1. Inset C is very clear and easy to understand. Insets B and D are not as clear. It would be helpful to have the first views for Insets B and D zoomed out more so we can tell where we are (as you did with Inset A). It would also be helpful to keep the color scheme consistent throughout all of the maps (water always shown as blue, adjoining land always the same gray or white as they are in Inset A).
lines 91-92: Figure 1 description. I do not see urbanization gradients depicted in your Figure 1. Please either add urbanization gradients to the figure (or make them more obvious if I have missed them) or adjust this figure description.
lines 106-107: Is this soil bulk density dry or wet? Based on information further down in the manuscript, it seems like you looked at dry bulk density, specifically. If this is the case, please clarify here.
lines 107-108: Please explain your leaf sample methodology further. What do you mean by “according to the local mangrove community composition”?
line 110: Please provide information on what kind of soil corer you used as you mention compaction issues further down.
lines 111-112: Please provide details on why you adjusted your soil core section intervals as you got deeper.
line 117: At what temperature did you refrigerate the samples?
lines 120-121: Please provide an explanation and a citation for a publication to support this assumption.
line 121: Please provide more information in this section of the manuscript on why you did not assess sediment accumulation rates for tidal flats.
lines 125-135: Please provide citations for your soil parameter measurement protocols.
lines 134-135: It might be worthwhile to define SBD as DBD (dry bulk density) to be more specific about the kind of soil bulk density your measured.
lines 158-159: I think this methodology is probably fine. Ideally, data exploration would involve a few more steps than listed here. A paper that provides a detailed protocol for data exploration with ecological data is Zuur, A. F., Ieno, E.N., and Elphick, C. S. 2010. A protocol for data exploration to avoid common statistical problems. Methods in Ecology and Evolution. https://doi.org/10.1111/j.2041-210X.2009.00001.x.
line 191: Table 1, line 1, column 3 (High-urbanization natural mangroves, Salinity): 52 g kg ^-1 seems high for this part of the world. Please double-check this is not a typo. If this is correct, please discuss the high salinity in more depth in your discussion section. If this salinity level is a known feature of this particular area, please include that in section 2.1 Study area.
lines 194-196: Figure 2 description: Please include units in all of your figure descriptions.
lines 198-199: Figure 3 description: Please include units in all of your figure descriptions. What do you mean by "different letters indicated significant differences in soil organic carbon among urbanization levels within the same ecosystem type..."? Define what the letters are and what each of them means, exactly.
line 208: Figure 4: It seems you have two main rows for these figure insets (Natural Mangroves and Restored Mangroves) and three main columns (High-urbanization, Moderate-urbanization, and Low-urbanization). Can you adjust the figure so it is more obvious that these insets all fit inside these different categories? Simply making the row and column titles bigger may achieve this or you could add a background grid or ribbons like you did in Figure 6.
lines 209-210: Figure 4 description: Please provide more details in your figure description, such as units. This would also be a good place to clarify which of the Inset Figures are natural mangroves vs restored mangroves and high vs moderate vs low-urbanization.
lines 208-210: What locations do Inset Figures A, B, C, D, and E correspond with (ex: Shantou, Zhanjiang, or Huizhou)? Please provide the location names in either the figure or in the figure description.
lines 231-232: Figure 5 description: Please include more details in your figure description. What do the different colored boxes for your different depth intervals represent? Do Insets A, B, C, D, and E correspond with the same Insets in Figure 4? If so, it may be helpful to include the locations for each of these either in the figure or in the figure description.
lines 246-247: Figure 6 description: Please provide more details in this figure description. What do you mean by "different letters indicated significant differences in Beta values among urbanization levels within the same ecosystem type..."? Define what the letters are and what each of them means, exactly.
line 270: “Once those limits are crossed, previously buried horizons return to active cycling and carbon stock decline rapidly…” Provide citation.
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RC2: 'Comment on egusphere-2026-2512', Anonymous Referee #2, 01 Jun 2026
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This MS addresses an important and timely question and is supported by a valuable field dataset. The integration of carbon stocks, sources, and stability into a single framework is interesting, and the finding that shifts in carbon sources and stability may precede carbon stock losses is potentially important. Generally the MS is well written. However, some aspects of the conceptual framing and interpretation could be strengthened, particularly regarding the attribution of observed patterns to urbanization and the consideration of alternative explanations.
1. Abstract
1) The same underlying concepts are introduced using several different expressions (e.g., "how much carbon is stored, who supplies it, and how long it persists", "stocks, sources, and stability", and the "triad framework"). While these terms appear to refer to the same ideas, switching terminology within a short abstract makes the narrative harder to follow. I suggest introducing the preferred terminology early and using it consistently throughout.
2) L22–25 The transition from the main findings to the introduction of the framework feels somewhat abrupt. A brief linking sentence explaining how the findings led to the development of the framework would help improve the logical flow.
3) L25–27 The terms "connectivity" and "accretion" are introduced only in the final sentence of the abstract. Because they have not been mentioned previously, their role in the proposed framework is not immediately clear. Consider introducing them earlier or briefly explaining how they contribute to the diagnostic framework.
2. Introduction
This part does a good job of motivating the study. The authors clearly identify two important gaps: (1) stock, source and stability components of blue carbon have largely been studied separately, and (2) we still know relatively little about how these components vary across habitats along urbanization gradients. However, I think several aspects of the conceptual development could be strengthened.
Specific Comments:
1) One issue is that the Introduction discusses several specific stabilization mechanisms (e.g. MAOM and Fe/Al-bound carbon), but these mechanisms are not clearly linked to the hypotheses. As a result, the hypotheses feel somewhat disconnected from the framework developed in the preceding paragraphs. It would be helpful to add a short mechanistic explanation of why urbanization is expected to affect these stabilization pathways, and how this leads to the predictions being tested.
2) I also found the relationship between the urbanization gradient (low, moderate and high) and the habitat categories (natural, restored and tidal flat) somewhat unclear. These represent different ecological dimensions, and it would be useful to explain more explicitly how they fit together within the conceptual framework of the study.
Related to this, the three study regions are located in different parts of the Guangdong coastline and may differ in important environmental characteristics such as tidal regimes, hydrodynamics and sedimentation processes. A brief discussion of why these locations can be treated as an urbanization gradient, and how potential geographic confounding is addressed, would strengthen the rationale of the study.
3) I found H3 to be relatively broad compared with the mechanistic framework presented earlier in the Introduction. The prediction that lower urban pressure leads to greater stability is intuitively reasonable, but it does not fully exploit the habitat-specific framework developed in Paragraph 4) I think the hypothesis would be more interesting if it made a more specific prediction about how different habitat types are expected to respond to urbanization, particularly whether restored habitats are expected to converge towards natural reference systems under lower levels of urban pressure.
5) L50-55 it’s better to add the references
3. Discussion
1) Throughout Section 4, many paragraphs begin with phrases such as "Our results suggest...", "Our data show...", or "Our results indicate...". While not problematic individually, their repeated use becomes noticeable and can make it difficult to identify which analyses support a given conclusion. Given the diverse evidence presented in this study (SOC stocks, ^210Pb dating, stable isotopes, and turnover metrics), I encourage the authors to refer more directly to the relevant indicators or analytical approaches when discussing specific findings.
2) In Sections 4.1 and 4.2, the discussion links the stronger decline in carbon stocks and the shift towards more labile carbon sources in restored stands and tidal flats largely to urbanization. However, some of the mechanisms invoked, such as young substrates, lower structural complexity, and underdeveloped root systems, are also characteristic of these habitats regardless of urban pressure. The discussion would benefit from more clearly distinguishing habitat maturity effects from urbanization effects.
3) The study design captures a clear urbanization gradient across the three regions. However, these locations also differ in their geographic and geomorphic settings and may vary in hydrodynamics, sediment characteristics, and natural accretion processes. Some observed differences in carbon burial and stability may therefore reflect regional environmental variation in addition to urbanization. A brief discussion of these potentially confounding factors would strengthen the interpretation of the results.
4) Section 4.4 introduces the concepts of "carbon anchors" and "instability fronts" as contrasting system states. While these categories provide a useful synthesis of the observed patterns, their broader applicability may currently be overstated. The study includes multiple habitat types and urbanization levels, and other combinations not represented by these end-members could potentially produce different responses. I suggest presenting these states as scenarios identified within this study system rather than broadly transferable categories.
5) The proposed framework is built around responses across habitat types and urbanization levels. However, the absence of natural mangroves at the moderate urbanization level leaves one part of the conceptual matrix unrepresented. It would be helpful to acknowledge this limitation and discuss how it affects interpretation of the proposed framework and its generality.
Citation: https://doi.org/10.5194/egusphere-2026-2512-RC2 -
RC3: 'Comment on egusphere-2026-2512', Anonymous Referee #3, 03 Jun 2026
reply
This manuscript examines how urbanization affects soil organic carbon stocks, source composition, and stability in natural mangroves, restored mangroves, and tidal flats across three Guangdong cities classified at low, moderate, and high urbanization levels. The topic is timely, and the three-habitat comparison is useful.
The study attributes observed differences in SOC stocks, sources, and stability to urbanization, but the Methods do not specify how the urbanization effect is identified or tested.
The MixSIAR model considers only mangrove leaf tissue and marine phytoplankton but overlooks terrestrial inputs (see the terrestrial end-member from https://doi.org/10.1021/acs.est.3c00012).
The phrasing "composition of organic-carbon sources" (H2, line 75) is awkward. The intended meaning is the relative contribution of different sources to soil organic carbon, not the composition of the sources themselves. Consider replacing with "source composition of soil organic carbon" or simply "organic carbon source contributions," and apply the revised phrasing consistently across the abstract, hypotheses, and Discussion.
The Introduction and Discussion invoke mineral-associated organic matter fractions, Fe/Al-bound carbon, thermal indices, and radiocarbon ages as relevant stability metrics, yet none were measured (see the stabilization mechanisms documented in https://doi.org/10.1111/gcb.70763). The Discussion should more clearly separate what was measured from what is inferred by analogy with the cited literature.
The linear mixed-effects model structure is unreported. The Methods state that LMMs were used but do not specify the fixed-effect structure, the random-effect structure, or the grouping variable.
The phytoplankton sampling (five 500 mL samples per site, line 116) is described, but the processing steps (filtration, drying, isotope measurement) are not. What organism-level end-member values were used in MixSIAR? Were they site-specific or pooled?
Citation: https://doi.org/10.5194/egusphere-2026-2512-RC3
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This well-written manuscript investigates soil organic carbon stocks, burial rate, source composition, and turnover rate along an urbanization gradient in natural (low urbanization) and restored mangroves (moderate urbanization), as well as adjacent tidal flats (high urbanization) in Zhanjiang, Huizhou, and Shantou, Guangdong Province, Southern China. The study found that urbanization reduces carbon stocks and burial rates and shifts carbon sources from mangrove detritus toward planktonic and algal inputs. Linking urban growth to blue carbon performance is interesting and useful for defining actionable thresholds to sustain coastal carbon.
My main concern is that the investigation in areas with low, moderate, and high levels of urbanization was conducted in three different cities in China. Whether or to what extent do the substantial differences in environmental factors among these cities affect the conclusions?
As shown in the manuscript, annual sunshine duration increases from less than 1,500 hours in Shantou (north) to over 2,100 hours in Zhanjiang (south), indicating that environmental conditions, climate, and biological processes differ greatly. The impacts of these differences on carbon sources cannot be ignored. The manuscript should address and discuss the extent to which these differences affect carbon sources.