Organic Carbon Storage and 14C Apparent Age of Upland and Riparian Soils in a Montane Subtropical Moist Forest of Southwestern China
Round 1
Reviewer 1 Report
The authors have satisfactorily addressed all the comments. Good job.
Author Response
Dear reviewer and editor,
Thank you very much for your time and effort in reviewing and commenting our manuscript entitled “Organic carbon storage and 14C apparent age of upland and riparian soils in a montane subtropical moist forest of southwestern China”.
Best Wishes!
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear Authors,
I find the MS significantly improved. There are a few minor comments. Please see the comments in the attached file.
Best regards
Comments for author File: Comments.pdf
Author Response
Dear reviewer and editor,
Thank you very much for your time and effort in reviewing and commenting our manuscript entitled “Organic carbon storage and 14C apparent age of upland and riparian soils in a montane subtropical moist forest of southwestern China”. Please check our responses to reviewer’s suggestions and comments point-by-point as below.
Point 1: Line 74: Rev 13C/12C.
Response 1: Good suggestion! We accepted it and revised “13C : 12C” as “13C/12C”.
Point 2: Line 103: Rev higher.
Response 2: Good suggestion! We accepted it and replaced “older” with “higher”.
Point 3: Figure 1: Rev scale missing in both figures.
Response 3: Good suggestion! We added the scales in both figures.
Point 4: Figure 3: Rev Break the y axis so the top part from 80 to 100 percent can be seen better.
Response 4: We broke the y axis and found it was still hard to observe the top part from 80 to 100 percent (down left), so we suggest that start the y axis from 80 (down right).
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript is much improved by clarification of the procedure for the AMS dates, and I can now accept the possibility of vertical mobility of SOM. Some questions remain. I am going to recommend publication subject to consideration of the points below – this still constitutes major revisions.
The English is about as it was in the last version, and will need some editorial attention.
General comments: The revisions have been done rapidly, without attention to detail, particularly in the figures. I will give details below.
More references are required, in particular for the work on stable C isotopes.
In interpreting the SOC age profiles, the authors have taken the approach of putting a linear regression through the data and interpreting the regression as the age profile. The is valid only as a first approximation, and is better in the case of the upland soils than for the riparian soils. In detail, it is not valid. This is not a matter of filtering out noise by means of the regression; all of the dates provide valuable information, and all must be considered. For the upland soils, the apparent age reversals with depth are small, and may be within error – the authors should comment on this. For the riparian soils, the age reversals with depth are large, and require interpretation. As the authors have observed elsewhere, transport of charcoal (or of other, old SOC) may affect the ages of riparian soil.
Particular points, referenced by line number of the version showing tracked changes, or by figure number:
In methods, say explicitly that the SOC fraction dated includes all SOC, except possibly for a small fraction of acid-soluble SOC.
Fig. 3: Panel a shows one set of columns labeled a, b, b – should this be a, b, c?
Figs, 3, 5 and 6: Say what the error bar represents -- 1σ?
Fig. 6: It isn’t clear what A, a and b mean. Do they relate to Fig. 3?
Figs. 8 and 9: The soil depths (range 0-20 cm) do not correspond to those in other diagrams (range 2-200 cm).
Fig. 9: The error bars don’t help much with the clarity of this figure. It should be possible to give an analytical precision (1σ) in the methods section.
281-282: Better wording is required. Possibly: In each plot, results are shown for three different profiles, distinguished as a, b and c, or A, B and C? Why not have just a, b and c (no capitals?)
337: Better wording: …are probably controlled by additional factors?
339: “all-time” is the wrong term. I suggest consistently or permanently.
342: “stock” is the wrong word. How about “bulk SOC per unit area”?
355: Better wording: Soil age is one of five principal controls of soil development…
356-361: Suggested wording: Riparian and upland soils differ in soil-age profile. Riparian soils are deposited sequentially as soil is transported from the upland, so that soil age increases with depth. Upland soils develop by weathering, so that soil aage decreases with depth.
This might be a good place to introduce the idea that SOC will not necessarily have the same age distribution, because of transport of soil bearing SOM or charcoal that may be older than SOM added from plants after deposition.
I note that the upland soils are on 30° slopes, which raises the possibility of downslope transport. Does vegetation preclude this?
393: sandy loam, with sand predominant.
438-439: This is true at a particular depth, not in general.
452: even though…
454-455: SOC age is independent of soil age in the upland soils, but not in the riparian soils.
458-460: This is an over-simplified view, because SOM is provide from plants at the top of the profile continuously as the profile develops. One also needs to consider the depth of the root zone. If roots extend to the soil-rock interface, they can provide young SOM throughout the soil profile.
467-468: Mechanism 2 is not really distinct from mechanism 1, if both are a matter of transport of clay-size particles, actual clay on the one hand, and microbial remains on the other. An alternative second mechanism would be transport of SOC in aqueous solution. People who treat soil samples for C-14 dating will tell us that acid leaching indeed removes some SOC.
In the upland soils, are we looking at an age distribution of SOC governed by a kind of diffusive downward transport of particulate or dissolved SOC? How does the age of the oldest SOC measured relate to the age of the soil profile, according to the residual soil concept?
487: If upland soils are residual, how does charcoal manage to occur at depth? It can be added only at the surface.
494-499: This is not well expressed. It is the soil, not the decomposition rate, etc., that acts as the sink. It is not clear why reducing the rate of carbon emissions leads to an underestimation of the function of riparian soils. Here and elsewhere, please do not say “It is obvious…..”. For many readers, it may not be obvious; please explain the reasoning. One could say: Riparian soils can serve as temporary sinks for some of the C that would otherwise be released quickly into the atmosphere.
529: It is the soil, not the rocks, that has a pH of 4.2
509-511 and 538: References are needed for each of the isotope effects listed.
526-541: An estimate of δ13C for live biomass would help in this argument. Given the supply of SOM from the top of the upland (or of all four) profiles, does the relatively constant δ13C in the upper parts of the profiles provide such an estimate, say -26 to -28‰? Then, how much change could be attributed to water stress? This can be found in the literature. Beyond this, the larger changes are most likely attributable to microbial decomposition.
Author Response
Dear reviewer and editor,
Thank you very much for your time and effort in reviewing and commenting our manuscript entitled “Organic carbon storage and 14C apparent age of upland and riparian soils in a montane subtropical moist forest of southwestern China”. Please check our responses to reviewer’s suggestions and comments as below.
Point 1: The manuscript is much improved by clarification of the procedure for the AMS dates, and I can now accept the possibility of vertical mobility of SOM. Some questions remain. I am going to recommend publication subject to consideration of the points below – this still constitutes major revisions.
Response 1: Thank you for your comments!
Point 2: The English is about as it was in the last version, and will need some editorial attention.
Response 2: We checked our manuscript word for word from title to references, substantially polished the English.
Point 3: The revisions have been done rapidly, without attention to detail, particularly in the figures. I will give details below.
Response 3: Thank you for your patience.
Point 4: More references are required, in particular for the work on stable C isotopes.
Response 4: We added 5 more references for the work on stable C isotopes.
Point 5: In interpreting the SOC age profiles, the authors have taken the approach of putting a linear regression through the data and interpreting the regression as the age profile. The is valid only as a first approximation, and is better in the case of the upland soils than for the riparian soils. In detail, it is not valid. This is not a matter of filtering out noise by means of the regression; all of the dates provide valuable information, and all must be considered. For the upland soils, the apparent age reversals with depth are small, and may be within error – the authors should comment on this. For the riparian soils, the age reversals with depth are large, and require interpretation. As the authors have observed elsewhere, transport of charcoal (or of other, old SOC) may affect the ages of riparian soil.
Response 5: It is not clear to us what the points of the reviewer by saying are “it is not valid”. It is apparently a valid linear correlation between apparent age and soil depth with significant P values. Nor we understood the reviewer’s comment that the correlation “is better in the case of the upland soils than for the riparian soils”—In fact, the R2 values for the correlation of riparian soils are just as good as, or even better than, those for upland soils. If the reviewer meant a smaller correlation slope for the upland soils than for the riparian soils, it is true and we have already mentioned and discussed this phenomenon in our manuscript. In addition, this would indicate that soil deposition in the riparian soils is faster than the soil formation in the upland soils—we added this point in the discussion.
Point 6: In methods, say explicitly that the SOC fraction dated includes all SOC, except possibly for a small fraction of acid-soluble SOC.
Response 6: Thank you! We modified our text in the method.
Point 7: Fig. 3: Panel a shows one set of columns labeled a, b, b – should this be a, b, c?
Response 7: No. it shall be a, b, b to show statistical differences or lack of differences among the three soil layers (0-5 cm, 5-15 cm and 15-25 cm) in the SU site.
Point 8: Figs, 3, 5 and 6: Say what the error bar represents -- 1σ?
Response 8: the error bar in Figs 3, 5 and 6 represents standard error of the corresponding measured item. We modified our legends accordingly.
Point 9: Fig. 6: It isn’t clear what A, a and b mean. Do they relate to Fig. 3?
Response 9: Good point. We changed all As to lower cases-there is indeed no need to use uppercase letter.
Point 10: Figs. 8 and 9: The soil depths (range 0-20 cm) do not correspond to those in other diagrams (range 2-200 cm).
Point 10: We revised Figs. 8 and 9 soil depth (0-200 cm).
Point 11: Fig. 9: The error bars don’t help much with the clarity of this figure. It should be possible to give an analytical precision (1σ) in the methods section.
Response 11: We revised our Fig. 9 and gave an analytical precision (2σ).
Point 12: 281-282: Better wording is required. Possibly: In each plot, results are shown for three different profiles, distinguished as a, b and c, or A, B and C? Why not have just a, b and c (no capitals?)
Response 12: We used a, b and c to indicate significant difference of soil moisture and SOC among three different soil layers in each soil profile. We used A, B and C to represent different soil layer where A represents 0-5 cm, B represents 5-15 cm and C represents 15-25 cm.
Point 13: 337: Better wording: …are probably controlled by additional factors?
Response 13: Good suggestion! We revised “……are not always controlled by soil clay.” as “……are probably controlled by additional factors.”
Point 14: 339: “all-time” is the wrong term. I suggest consistently or permanently.
Response 14: Good suggestion! We revised two “all-time” words as “permanently”.
Point 15: 342: “stock” is the wrong word. How about “bulk SOC per unit area”?
Response 15: Good suggestion! We accepted it!
Point 16: 355: Better wording: Soil age is one of five principal controls of soil development…
Response 16: Good suggestion! We accepted it and revised the original sentence as “Soil age is one of five principal controls of soil development including the accumulation of SOC”.
Point 17: 356-361: Suggested wording: Riparian and upland soils differ in soil-age profile. Riparian soils are deposited sequentially as soil is transported from the upland, so that soil age increases with depth. Upland soils develop by weathering, so that soil age decreases with depth.
This might be a good place to introduce the idea that SOC will not necessarily have the same age distribution, because of transport of soil bearing SOM or charcoal that may be older than SOM added from plants after deposition.
I note that the upland soils are on 30° slopes, which raises the possibility of downslope transport. Does vegetation preclude this?
Response 17: Good suggestion! (1) we revised the text as “Riparian and upland soils differ in soil-age profile. Riparian soils are deposited sequentially as soil is transported from the upland, therefore soil age increases with depth. The deposition processes in the riparian alluvial soils create a soil profile in which the youngest soils are located on the surface and the oldest soils are located in the bottom of the soil profile. In contrast, the upland residual soils are developed from the parent materials where the youngest soils are formed at the bottom of the soil profile and the oldest soils are those located on the surface, therefore soil age decreases with depth.”; (2) we added “SOC and soil will not necessarily have the same age distribution along soil profiles because of transport of soil bearing SOC or charcoal from surface to bottom layers.” in the end of this paragraph; and (3) the SU site is located at the middle slope with little insignificant accumulation of eroded soil from upper slope although the slope is 30°. Furthermore, the heavy vegetation cover also minimize sthe rate of soil erosion.
Point 18: 393: sandy loam, with sand predominant.
Response 18: Good suggestion! We revised “……sand loamy with much sand fraction,……” as “……sandy loam with sand predominant”.
Point 19: 438-439: This is true at a particular depth, not in general.
Response 19: Good suggestion! We added “……in the corresponding same soil layer of the two watersheds,……”.
Point 20: 452: even though…
Response 20: Good suggestion! We accepted and revised “……even……” as “……even though……”.
Point 21: 454-455: SOC age is independent of soil age in the upland soils, but not in the riparian soils.
Response 21: Good suggestion! We added “……..in the upland soils” in the end of this sentence.
Point 22: 458-460: This is an over-simplified view, because SOM is provide from plants at the top of the profile continuously as the profile develops. One also needs to consider the depth of the root zone. If roots extend to the soil-rock interface, they can provide young SOM throughout the soil profile.
Response 22: Agree! We modified our sentence to “Our data showed a trend of continuous downward movement of old SOC towards the bottom of soil profile regardless of the relative soil age at the bottom (young in residual soil and old in alluvial soil) or the input of root carbon throughout the rooting zone.”
Point 23: 467-468: Mechanism 2 is not really distinct from mechanism 1, if both are a matter of transport of clay-size particles, actual clay on the one hand, and microbial remains on the other. An alternative second mechanism would be transport of SOC in aqueous solution. People who treat soil samples for C-14 dating will tell us that acid leaching indeed removes some SOC.
In the upland soils, are we looking at an age distribution of SOC governed by a kind of diffusive downward transport of particulate or dissolved SOC? How does the age of the oldest SOC measured relate to the age of the soil profile, according to the residual soil concept?
Response 23: Great point. We modified our mechanisms of SOC downward movement and added the DOC movement as the third mechanisms: (1) solid SOC with mineral clay (organo-mineral complex) or without mineral clay moves downward along soil profile [84,85]; (2) DOC moves from ground surface down to deep soil layers [86,87]; and (3) root inputs (litter and exudates) occurs in deep soil layers. The linear correlation between SOC 14C apparent age and soil depth suggests that SOC down movement is most likely accompanied by the clay downward movement rather than microbial necromass alone because the movement of necromass alone would be clay-content dependent resulting in a non-linear correlation. If SOC were passed to bottom soil through DOC or root inputs, SOC 14C apparent age would decrease with soil age that conflicted with our observation in upland soils. Therefore, it is most likely that the soil organic carbon moves downward in the solid form of organo-mineral (or necromass-mineral) complex. This organo-mineral movement was attributed to DOC movement in Michalzik et al. study [87] because they did not consider the possibility of this new mechanism.
Point 24: 487: If upland soils are residual, how does charcoal manage to occur at depth? It can be added only at the surface.
Response 24: Good question! Severe forest fire did not only produce massive carbon on the ground surface, but also did combust tree coarse root under the ground surface and produce massive charcoal at depth. Tunnels made by decayed/burned roots are pathways of surface charcoals to deep soils too.
Point 25: 494-499: This is not well expressed. It is the soil, not the decomposition rate, etc., that acts as the sink. It is not clear why reducing the rate of carbon emissions leads to an underestimation of the function of riparian soils. Here and elsewhere, please do not say “It is obvious…..”. For many readers, it may not be obvious; please explain the reasoning. One could say: Riparian soils can serve as temporary sinks for some of the C that would otherwise be released quickly into the atmosphere.
Response 25: Good suggestion! (1) we revised the original sentence as “Deforestation can often cause soil erosion in the upland areas and deposit these eroded upland soil and accompanying organic debris in the riparian area”; (2) because the small areas of riparian soils in forest ecosystems, most ecologists estimated carbon emission from residual soils in the upland forests, neglecting alluvial or colluvial soils. Therefore, we addressed that riparian soils can potentially reduce the emission of carbon to atmosphere after deforestation, resulting in an underestimated function of riparian soils; and (3) we revised the last sentence of this paragraph as “Therefore, riparian soils can serve as temporary sinks for some of the soil carbon that would otherwise be released quickly into the atmosphere after a severe disturbance in forest ecosystems”.
Point 26: 529: It is the soil, not the rocks, that has a pH of 4.2.
Response 26: Good comment! We modified the sentence as ”both the upland and riparian soils with a pH of 4.2”.
Point 27: 509-511 and 538: References are needed for each of the isotope effects listed.
Response 27: Good suggestion! We added all the required references (in total 4 ) for each of the isotope effects listed in these two paragraphs.
Point 28: 526-541: An estimate of δ13C for live biomass would help in this argument. Given the supply of SOM from the top of the upland (or of all four) profiles, does the relatively constant δ13C in the upper parts of the profiles provide such an estimate, say -26 to -28‰? Then, how much change could be attributed to water stress? This can be found in the literature. Beyond this, the larger changes are most likely attributable to microbial decomposition.
Response 28: Good suggestion! Unfortunately, we did not measure δ13C for live biomass of vegetation. However, variation of SOC δ13C values was between –26.7 ‰ and –27.1 ‰ in the two upland soil profiles and between –27.6 ‰ and –27.9 ‰ in the two riparian soil profiles in the top 15 cm soil layer, indicating that small difference (between –0.5 ‰ and –1.2 ‰) of SOC δ13C values between the upland and riparian soils. We also added “Large difference in SOC δ13C values (up to 10 units) both within the riparian soils and between the upland and riparian soils occurred in deep (>15 cm) and old soil layers. This difference exceeded substantially the regular variation of about 4 units in SOC δ13C values due to water stress, suggesting a predominant microbial (anaerobic) influence on the fractionation of carbon isotopic values.”
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
The authors' response to previous reviewer comments is fairly minimal, but sufficient for acceptance. The following minor matters still need attention:
Fig 3: there is still a set of columns labeled a, b, b
Fig. 6: The caption fails to clarify the use of lower-case letters as labels
line 456 "that conflicted" should be"which conflicts"
466-468: There is still no explanation of how charcoal could penetrate a residual soil.
Author Response
Dear reviewer and editor,
Thank you very much for your time and effort in reviewing and commenting our manuscript entitled “Organic carbon storage and 14C apparent age of upland and riparian soils in a montane subtropical moist forest of southwestern China”. Please check our responses to reviewer’s suggestions and comments as below.
Point 1: Fig 3: there is still a set of columns labeled a, b, b
Response 1: Lowercase letters indicate statistical differences of soil moisture among three soil layers (0-5 cm, 5-15 cm, and 15-25 cm). In the SU site, letters “a, b, and b” indicate no statistical difference between 5-15 cm and 15-25 cm soil layer, but soil layer 0-5 cm was significantly greater in soil moisture than the other two layers.
Point 2: Fig. 6: The caption fails to clarify the use of lower-case letters as labels
Response 2: We revised the original sentence as “Same lowercase letters (a and b) indicate no significant difference of SOC content or mean SOC concentration in the whole soil profile among the four soil pits.” Please see lines 293-294.
Point 3: line 456 "that conflicted" should be "which conflicts"
Response 3: We revised "that conflicted" as "which conflicts". Please see lines 421-422.
Point 4: 466-468: There is still no explanation of how charcoal could penetrate a residual soil.
Response 4: We added the sentence “Charcoal might be present in deep soil layers through either incomplete combustion of coarse roots or the washdown of surface charcoal along soil tunnels originated from dead roots or faunal activities.” Please see lines 434-436.
Author Response File: Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Organic Carbon Storage and 14C Apparent Age of 2 Upland and Riparian Soils in a Montane Subtropical Moist Forest of Southwestern China
Abstract: The abstract is clear and concise and the authors have successfully brought out the novelty of their research work.
Introduction:
Line 54: Moreover, it is widely accepted that SOC storage varies greatly across various 54 landscapes and thus large uncertainties exist in estimating soil carbon storage over large areas
Please add results of global meta-analysis on soil carbon stocks across various land use practices. That ones that the authors have cited are not a compilation of global studies. References to meta-analyses hold more weightage: De Stefano et al., 2018, Chatterjee et al., 2018,
Articles:
Chatterjee, N., Nair, P.K.R., Chakraborty, S., Nair, V.D., 2018. Agriculture , Ecosystems and Environment Changes in soil carbon stocks across the Forest-Agroforest-Agriculture / Pasture continuum in various agroecological regions : A meta-analysis. Agric. Ecosyst. Environ. 266, 55–67.https://doi.org/10.1016/j.agee.2018.07.014
De Stefano, A., Jacobson, M.G. Soil carbon sequestration in agroforestry systems: a meta-analysis. Agroforest Syst 92, 285–299 (2018). https://doi.org/10.1007/s10457-017-0147-9
Guo, L.B., Gifford, R.M., 2002. Soil carbon stocks and land use change: A meta analysis. Glob. Chang. Biol. 8, 345–360. https://doi.org/10.1046/j.1354-1013.2002.00486.x
Line 65-66: Add more relevant citations:. The following studies have explicitly used used soil texture as covariates to determine the variations in SOC concentration within different land management systems.
Percival, H.J., Parfitt, R.L., Scott, N.A., 2000. Factors Controlling Soil Carbon Levels in New Zealand Grasslands : Is Clay Content Important ? Soil Biol. Biochem. 64, 1623–1630.
Chatterjee, N., Nair, P.K.R., Nair, V.D., Bhattacharjee, A., Melo, E. De, Filho, V., Muschler, R.G., Noponen, M.R.A., 2020. Do Coffee Agroforestry Systems Always Improve Soil Carbon Stocks Deeper in the Soil ?— A Case Study from Turrialba , Costa Rica. Forests 1–22. https://doi.org/10.3390/f11010049
Line 67-68: SOC contents in the top 20 cm soil layer of shrubland, grassland and forest can respectively reach to 33%, 42% and 50% of total SOC storage stored in the entire soil profile, suggesting the key roles of climate and vegetation in SOC formation
Please add more references. There are far better studies than the one referred to.
Chatterjee, N., Nair, P.K.R., Chakraborty, S., Nair, V.D., 2018. Agriculture , Ecosystems and Environment Changes in soil carbon stocks across the Forest-Agroforest-Agriculture / Pasture continuum in various agroecological regions : A meta-analysis. Agric. Ecosyst. Environ. 266, 55–67.https://doi.org/10.1016/j.agee.2018.07.014
Line 98-108: Reframe your statement. Clearly state the research objectives, hypothesis and research gap if any.
Materials and Methods:
Great description. If possible, add the Koppen climate classification of this particular region.
Line 155: We collected all the soil samples in August 2010. At each site, we vertically dug a soil pit down to the parent material.
Please mention the soil depth.
I am glad that the authors have been honest about mentioning the limitations arising from research fund availability. This is a reality and scientific bodies must acknowledge it and reviewers must be considerate.
The authors have conducted the data analysis with full clarity. It was easy to comprehend why they fitted linear regression model between SOC concentration and soil clay percent, as well as between SOC 14C apparent age and soil depth.
Results:
Results were well presented. My only question would to the authors would be what took them so long to analyze the soil samples as they have mentioned that the soil samples were collected in August 2010.
Discussion:
The discussion lacks relevant literature citations. In line 304the authors talk about soil clay and how it is regarded as the main controller of SOC. But the references are dates back to 1987 . Indeed the Parton et al., 1987 is a classic and should be kept intact but please add newer citations as well. Please beef up the discussion section by citing recent publications.
Reviewer 2 Report
Review on the paper forests-758157
by
Xianbin Liu, Xiaoming Zou, Min Cao and Tushou Luo
Titled:
Organic Carbon Storage and 14C Apparent Age of Upland and Riparian Soils in a Montane Subtropical Moist Forest of Southwestern China
The paper compares soil organic carbon content, concentration, 13C composition and radiocarbon age of upland and riparian soil types at two nearby located stations in the Ailao Mountains of the southwestern China. The aim of the paper is to assess the carbon turnover time in the upland soils vs the riparian soils. The conclusion is that riparian soil contains more organic carbon and has higher turnover time, and should be considered as an important unknown carbon sink.
I suggest publishing the paper after major revision. Here are my comments:
Major comments:
Paper is generally well written, however is has some issues.
The construction of paper should be reconsidered. As the paper is divided in Results and Discussion part in certain places it is difficult to follow, so I suggest merging these two parts.
SOC content and concentration data should be presented in more details, not just as mean values.
The carbon isotope data should be presented and discussed in more details, especially when 13C and 14C values are compared. From the authors carbon isotope 13C data, it is clear that there is freshwater/hardwater reservoir present that influenced the 13C data and, consequently radiocarbon data, making thus radiocarbon ages seemingly too old. The data should be reconsidered and conclusions should be modified in the light of this information. It should be stressed that the upland soils are much more under influence of the terrestrial biota, while riparian is under large influence of aquatic vegetation/biota, especially the Sankeshu watershed riparian soil. It would be interesting if the authors could provide chemical parameters of the lake/river water (like pH, water hardness).
Also, I suggest combining all available data (SOC content, concentration and isotope composition) further to try to propose the model of the sedimentation processes of the upland and riparian soils and explain in more details why the authors reach certain conclusions (like e.g. line 387-388).
Minor (technical) comments:
Place the map with marked sampling locations and water bodies.
Round all the numbers on the first digit after the decimal point. Round the radiocarbon ages on zero or five, whichever number is closer, as that is the standard for reporting the radiocarbon age. Also, report radiocarbon age as BP (i.e. before present).
In all figures which are separated as a, b, c and d I suggest placing LU, LR; SU and SR instead.
Do not write “more negative” (or positive) when you are talking about d13C, but use “lower” or “higher”.
All other comments are placed as stick notes, notes to the text or striketrough text in the PDF file.
Comments for author File: Comments.pdf
Reviewer 3 Report
The authors present a set of data (14C, d13C, SOC content and concentration, soil fraction parameters) for soil profiles in a tropical forest in Yunnan, and offer interpretations of the differences between residual and riparian soils. The interpretation depends heavily on 14C data that cannot easily be interpreted by the reader at present because no information is given on the pre-treatment applied to the samples in the Xi’an AMS laboratory. Therefore it is not certain which fraction of soil carbon was dated.
I suggest rejecting the manuscript in its present form, with the option of re-submitting following reconsideration of the data. The data may well make a useful, but minor, contribution to the literature on soil carbon in forests.
In view of this recommendation, most of my comments will be general in nature.
General comments:
- The English would need considerable editing before publication. It is wordy, and incorrect in detail of grammar and idiom. Note carefully the correct use of articles (a, the, or no article) – they change the meaning subtly, but significantly. In the Introduction, the writing is spoiled by the incorporation of spurious pairs like “ecologists and researchers”, “sinking and cycling”, “factors and measures”, “essential and important” and “reforestation or revegetation”. Are the terms really different in the context of the study, or are the duplicates just padding?
- Organization of ideas is important, particularly in the Introduction.
- Some statements appear false. g. line 135: soil age does not decrease with depth at LU and SU, according to the 14C data – but only according to a hypothesis that is not supported by the data. Lines 336 and following: This seems to be discussing the opposite of what was observed, that SOC storage is the same in the U and R profiles.
- The methods section is incomplete (particularly C isotopes). Analytical precisions should be listed.
- Please consider alternative hypotheses for explanation of the data. The hypothesis about downward movement of old SOC is problematic, assuming that the AMS laboratory did the dates on immobile soil carbon (the usual protocol). If it is immobile carbon that was dated, then it is unlikely to have moved vertically. An alternative, and simple, hypothesis would be that the LU and SU soils are not in fact residual soils, but have been deposited sequentially at a rate exceeding the rate of weathering, retaining SOC from the time of deposition. Is this possible -- were the sites on slopes down which transport could have occurred? Under this hypothesis, soils in the L profiles were deposited over about the same time interval, and soils in the S profiles over a shorter interval, also about the same for the two sites. The age sequences in the R profiles suggest that orgainc matter was not transported, but deposited from organisms living in situ, except possibly at depths near 100 cm.
- Statistics: presenting the means of two sites, with standard error, is problematic. Why not just present the data for each site? Concerning weighted means, what was used to weight the data? How were errors calculated? Standard errors of weighted means are problematic, and a procedure for calculation is not agreed upon in the literature.
- Before re-writing the Discussion, please consider carefully what aspects of the data warrant detailed treatment. The differences in SOC between U and R samples/profiles seem minor; the lower concentrations in R profiles just signify more dilution by clastic sediment. This is not worth the page or more devoted to the topic in the present text. Some of the interesting topics for discussion might include:
- The disturbances in age profiles at about 100 cm in both R profiles.
- The d13C profiles at the S sites.
- Explaining the age profiles at the U sites.
- Implications for regional and more widespread carbon storage.
- Comparison with similar profiles from the Dinghushan study.
The authors may find it useful to consider what happens to soil OM during decomposition by microbes. Two of many references on the topic appear below.
Some details:
Accelerator Mass Spectrometry is the correct term.
Lines 395-399: If charcoal is present, it is unlikely to be transported vertically through the profiles.
Please don’t use “often” (a time word) where commonly is intended.
There’s a general statement about minimal human influence at the beginning of the paper, but multiple mentions of human influence thereafter – deforestation, and the vegetation of the valley bottoms.
Tables:
Table 1 needs units for soil depth.
Table 2 has too many significant figures – these are unjustifiable in relation to the usual errors of the measurements reported (e.g. can one really determine sand content to 0.01%, and were depths in the profiles really measured to 0.01 cm?). The surplus figures only make the table difficult to read, and contribute nothing to the interpretation.
Why aren’t the two sites distinguished in the table? This leads to loss of information for the reader. It isn’t clear that a particular depth range correlates from one site to the other.
Table 3 has no apparent relevance to the text and discussion.
There is no table of 14C and d13C data.
Figures:
Fig. 1: Give the length of the tape.
Fig. 2: Again, why aren’t all four profiles distinguished?
Figs. 3, 5 6: Labels a, b, c, d could just as easily be replaced by LU, LR etc, to make it easier for the reader. The captions could then be shortened greatly. It is not necessary to give the location of the study area in each caption.
Fig. 5: Please add error bars for the 14C ages, preferably 2σ.
A map figure would be desirable, showing the study area, and the area of similar forests in South China.
Ehleringer, J.R., Buchmann, N., Flanagan, L.B., 2000. Carbon Isotope Ratios in Belowground Carbon Cycle Processes. Ecol. Appl. 10, 412–422. https://doi.org/10.1890/1051-0761(2000)010[0412:CIRIBC]2.0.CO;2
Cerling, T.E., Solomon, D.K., Quade, J., Bowman, J.R., 1991. On the isotopic composition of carbon in soil carbon dioxide. Geochim. Cosmochim. Acta 55, 3403–3405. https://doi.org/10.1016/0016-7037(91)90498-T
Chris Eastoe, Tucson, Arizona, USA
March 26, 2020
