Review Reports - Biogeochemical Processes of C and N in the Soil of Mangrove Forest Ecosystems

Round 1

Reviewer 1 Report

This paper aims to review the current state of knowledge regarding the carbon and nitrogen cycle in the soil of mangrove forest ecosystems. It seems like a fairly brief article for such a broad topic, but it does manage to at least touch on most major topics concerning these two biogeochemical cycles in mangrove soils. When reviewing studies that provide pools or fluxes of these elements in mangrove soils, the authors provide some non-mangrove wetland numbers for comparison, which is a good thing. However, there are some items I would expect from such a review that I do not see here.

For one, while the difference in climate forcing between different GHGs (CO₂, CH₄, N₂O) is mentioned on line 210, Table 2, which summarizes fluxes of these three GHGs does not provide a comparison in climate forcing terms. I would encourage the authors to do this or create another table doing this, using a widely-accepted framework, such as that of Neubauer and Megonigal (2015).

Secondly, while the effect of the presence of SO₄^2- on CH₄ emissions is briefly mentioned on line 116, it is not represented in Fig. 1 (the conceptual figure of the C cycle in mangroves) and no effort is made to compare CH₄ emissions across studies with different salinities. Given the wide range of CH₄ emissions cited (0.1-184.8 mg C m-2 d-1) here, some attempt at describing the variability would be of great interest to the readers. An effort similar to that of Poffenbarger et al. (2011) for tidal marshes would be an excellent addition to this review, especially if it allowed a direct comparison of salinity responses between tidal marshes and mangroves.

The final major point that seems to be lacking is the connection between soil C pool sizes/accumulation rates and the balance between primary production and decomposition. In many places, the difference between rates of primary production and net accumulation of C in soil are unclear. For example, the paragraph on Lines 69-77 discusses C stocks, plant primary production, plant biomass, soil C and C accumulation with no clear equations or definitions relating the concepts to one another. Line 71 states that coastal ecosystems ‘accumulate at a rate up to 100 times faster than terrestrial forest ecosystems,’ but it is not indicated whether this is due to high productivity, low decomposition rates or both.

Some smaller issues include:

Title: I would change the last words in the title to “…in the soil of mangrove forests.” Soil is part of an ecosystem, technically there is no such thing as a “soil ecosystem.”

Table 1: the last entry is a study from Maryland, USA, which is abbreviated “MD, USA” not “ML, USA.” Note that unlike the other entries in this table, this is not a tropical or subtropical climate, but a temperate one. The authors could try to find examples of the same species from Florida or Louisiana in the US, which would be more subtropical.

Table 2: Study [58] is located in Georgia, USA (which is abbreviated “GA, USA”) not in Louisiana (LA, USA), although the lead author is stationed there.

Reference 5: The second author is Bernhardt, E.S not “Bernhardlt, W.S.”

While the language is understandable to a native English reader, the grammar is incorrect or deviates from common usage in several places. Particularly noticeable are words made singular or plural or given a definite article, which are not commonly used in such a manner. I appreciate this is difficult to identify and English grammar conventions are very inconsistent in this manner. Also, there are many unnecessary phrases that could be omitted without any loss of meaning. Some examples include:

Line 31: “…to complete the biogeochemical cycles and support…” could be rephrased as “in biogeochemical cycles that support...”

Line 31: “higher animals” could simply be “animals”

Line 46: the phrase “…are an important ecosystem because they…” could be omitted without changing the meaning of the sentence. If it remains, there is a number disagreement in the phrase and it should be “…are important ecosystems because they…”

Line 61: “net blue C storages” could be rephrased “net blue C storage”

Line 71: “plant biomass accumulations” could simply be “plant biomass” or it could be rephrased “accumulated plant biomass”

Line 72: “…the coastal ecosystem is generally believed...” could be rephrased as “coastal ecosystems are generally believed…”

References:

Poffenbarger, Hanna J., Brian A. Needelman, and J. Patrick Megonigal. "Salinity influence on methane emissions from tidal marshes." Wetlands 31.5 (2011): 831-842.

Neubauer, Scott C., and J. Patrick Megonigal. "Moving beyond global warming potentials to quantify the climatic role of ecosystems." Ecosystems 18.6 (2015): 1000-1013.

Author Response

Please check the attached file, thanks.

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript reviewed the C and N dynamics in the mangrove forest ecosystem from a soil microbial perspective. The mangrove forests are unique ecosystems, which covered a very small area but can play a very important role in mitigating global climate change by its soil carbon sequestration. The topic of this manuscript is very important and the reader of Forests would be very interested. The authors of the paper are very knowledgeable in biogeochemistry and coastal ecosystem. Many important results from high-quality papers were cited and summarized in this review. So I think this manuscript can be accepted after some major revision.

General comments

The authors mentioned blue C storage in the abstract, keywords, and many other places in the manuscript. Blue C is definitely a very important concept in mangrove forest C and N, and the soil is the largest reservoir of the blue C. However, the term of blue C was not clearly defined in the manuscript, and the use of this term seems misleading. As a review paper, we should not assume that all the readers have the same level of understanding of such an important concept. I would suggest the authors introduce more about the blue carbon in the introduction section. Moreover, the author seems to take the blue C storage as kind of ecosystem service of mangrove forests. This is correct. But more importantly, blue C storage is the key that links mangrove soil C and N cycle with global climate change. Once this linkage is established, a lot of more results can be discussed about the interaction between mangrove soil and climate change.

Both of the two figures of the manuscript are modified from the same book (Vepraskas and craft 2015). These major processes of C and N cycles are very basic information that can be found in all the textbooks. Actually the authors can put more information in the figures using the general structure of the two existing figures. For example, the authors mentioned many environmental factors that can control the fluxes of C and N, like the presence of sulfate and anaerobic conditions. These factors can be added next to the arrows in the figure, with “+” or “-“, indicating the positive or negative effect of the factor for the C or N fluxes. This way, the impact of climate change can also be present in the figures. In my opinion, a review paper should not only list the facts that we already know but also provide some more comprehensive understanding of the topic. A diagram like this is a very useful tool for such a review paper. But they can be more informative.

The ending part of this manuscript is not very strong. As a review paper, the author should draw some general conclusions and give some future research directions by the end of the discussion. Based on what we know, what we need to do next? What are the questions that we still cannot answer based on the current results? Which type of research (Field, lab, modeling, etc.) should be encouraged in the future? In addition, there are still many opening questions (at least I’m still curious after reading this paper). For example, if the mangrove forests die due to severe environmental change, how much CO2, CH4, N2O, or other greenhouse gases will be released to the atmosphere? Is there any seasonal or inter-annual variation of the C and N fluxes in mangrove soil? Can the change of sea-level have any impact on mangrove forest C and N cycles? I hope these questions can help the author extend the dimension of the discussion in the manuscript.

Specific comments

10 driven by the biogeochemical cycles of C and N,

14 damaging -> detrimental

14-15 as air temperature warm -> with increasing air temperature

19 climate change may affect related biogeochemical properties-> these processes may respond to climate change

27 average ->mean

24-28 The first paragraph is about the general fact about climate change. More citation is needed to support the greenhouse effect and the linkage between climate change and human activity. Expect the IPCC report, there are still many other books or report that should be cited.

31 higher animals -> animals and humans.

33 “Soil is believed to be a net sink of atmospheric CO2.” Globally, it is correct, but the soil is not always a net C sink everywhere. A lot of wetland soil could become a C source during the dry periods, and this is likely to happen more frequently in the future with the changing climate.

47-49 very confusing sentence. It looks like the authors want to compare the costal wetlands with other upland ecosystems. But for what? A wider range? Then what is the meaning of a wider range along the spatial scale, what’s that along the temporal scale? Please be more precise and specific.

52-53 change to “More than one-third of the global mangrove forests are inhabited in South-East Asia.”

54 change to “Globally, they only cover a small area,”

55 wide->valuable

56 “, and” -> “. They”

54-57 ecosystem services and functions are always combined together. In most of the case, the function is the reason for the service. So I suggest rearranging the sentence here, like: “Although the mangrove forest covers a small area, however, they had many ecosystem functions, for example, dissipating excess nutrients from the upland watershed (can add other functions, like storage of blue carbon). Therefore they can provide valuable ecosystem services which worth XXX money.”

60-61 remove “highlighted above”

62 Start a new paragraph from “Many reviews…”

75-76 Change to “where most organic C stocks are stored in soil”

76 in both ->between

88 short C turnover speed-> fast C turnover rate

89 “for a long period of time” For how long? What is the general turnover rate for mangrove forest biomass and soil?

102 change to “ a considerable source of greenhouse gas”

104 fluxes -> efflux

101-109 This paragraph is focusing on CO2 emission. However, the first sentence can be misleading and make me feel like the mangrove forest can be a net source of CO2 due to the anaerobic soil and high respiration rate. Please clarify

116 is this comparing with inland wetland?

117 In terms of -> Comparing with

118-121 the numbers reported in the text are in the unit of mg C m-2 d-1, while the unit of Table 2 is mg c m-2 hr-1. The different units made it impossible to find the data in the table. The GHG efflux unit should be consistent throughout the manuscript.

132 lowered -> reduced

136 remove “other”

141 move “in mangrove forest soils” to following “Methylobacter”

143 What does it imply by “the deep-sea 5 clusters”? They are inactive? For readers not familiar with soil microbiology like me, this needs some clarification.

147 This paragraph seems to be continued with the previous one. So I suggest not to start a new paragraph here. However, this very long paragraph can break from line 137, before “compared to…”

157 for -> of

159 Fig 2 is referenced here. But N assimilation process is not indicated in fig 2. As N assimilation is the topic of this entire section, I think it worth to be illustrated clearly in the figure. The manuscript can be largely improved if the authors pay more attention to the link the figure with the text, considering the process in the figure can be label and referred at multiple places in the MS.

187 capitalize the first “Dissimilatory Nitrate Reduction to Ammonium (DNRA)”

194 change to “The major pathway of nitrate reduction may shift…”

199 “excessive N loading” or “excess N”?

199 What is the implication of higher denitrification potential in estuarine mangrove soil? This is interesting. More can be discussed here.

205 Don’t need to be a new paragraph.

210 the radiative forcing -> the global warming potential

211-212 I don’t think N2O efflux can drags down the blue C storage. It just changes the balance between GHG emission and C sequestration in the ecosystem. So it can reduce the climate mitigation effect of the blue carbon. Again, the authors should have a clear understanding of blue carbon storage before using the term in the manuscript.

215 Only estuarine?

221 How can the forest coverage migrate to temperate zones?

228 How did the results shown in the previous sentence imply the temperature sensitivity of mangrove soil denitrification? Need more explanation here, otherwise, the logic chain is not completed.

231 What do you mean by “shorten the C and N cycles”. Are there any processes that can be skipped by the changing microbial community? If not, maybe it just “accelerate” the C and N cycles.

233 Which effect? There are many more needs to be discussed.

565 Uncompleted reference information

Author Response

Please check the attached file, thanks.

Author Response File: Author Response.docx

Reviewer 3 Report

General comments

This review collects key literature and makes a clear synthesis of mangrove forest soil C and N biogeochemistry and GHG emissions. This work is important and useful, almost ready for publication in Forests. I found minor items for reconsideration.

Specific comments

L 110 Aerobic and anaerobic should refer to the respective processes, while oxic, hypoxic and anoxic should refer to the conditions throughout of the document. In Figure 1, the left hand terms should be Oxic and Anoxic, because they apparently refer to conditions. The processes in the figure are already correctly termed as aerobic or anaerobic. I understand that there are varying conventions in the use of this terminology, but please justify or reconsider.

L 116 Causality between CH4 and CO2 is dubious. CH4 emissions may be *mostly* lower than ecosystem respiration because less C passes the less effective anaerobic decay. CH4 emissions from freshwater systems could better compare with those from saline ones. Please justify or reconsider.

L 134. 90%. It is even possible to oxidize 100% of CH4 produced in soil, and even consume more CH4 from the atmosphere in well-drained organic soils such as freshwater peatlands, turning the system as a net atmospheric CH4 sink. Please reformulate the claim of 90%, based on the published observations you use.

L 162. Terminology as in Fig. 1.

L 210 Should this reference be 135?

Author Response

Please check the attached file, thanks.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors did a great job revising this manuscript. I appreciate the effort they made to reorganizing the materials and editing the figures. The manuscript has been largely improved. I think it can be accepted as the current version, except for a few small editions:

1. The blue arrows in the figures need to be explained in figure captions so that the readers will know they are different from the black arrows indicating the fluxes.

2. I still think the ending of the manuscript is not strong enough. Maybe the authors can try to use one short paragraph to summarize the "C and N dynamics under climate change" section and use another new paragraph to list the potential future directions. Bullet points may help.

Author Response

List of Point-by-Point Responses to Comments

Reviewer #2:

Response: Thank you very much for these comments, and for your valuable suggestions throughout the revision process.

The blue arrows in the figures need to be explained in figure captions so that the readers will know they are different from the black arrows indicating the fluxes.

Response: We followed the comment and added descriptions to the figure captions.

I still think the ending of the manuscript is not strong enough. Maybe the authors can try to use one short paragraph to summarize the "C and N dynamics under climate change" section and use another new paragraph to list the potential future directions. Bullet points may help.

Response: We followed this suggestion and revised the end of the manuscript to provide more specific suggestions for future research topics.