Modeling the Spatial Dynamics of Soil Organic Carbon Using Remotely-Sensed Predictors in Fuzhou City, China

Round 1

Reviewer 1 Report

Your article examining the predictability of soil organic carbon from remotely sensed variables is compelling and well done. I have included a number of embedded comments in the attached PDF, with suggestions for improvement and clarification. My primary concern relates to the training data -- how representative was it, how geographically widespread in the study area is it, is it relevant for only a particular season of the year? I think this could be more fully explored, and it would make the paper much stronger. Many of your conclusions are in line with previous research, and you add novelty with your methods, so in general I would say that this is a valuable addition to the literature.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

we would like to extend our heartfelt thanks for your deep and insightful comments and suggestions and your favorable comments about our work.

We believe that the comments and suggestions you raised are very important, precise, will immensely improve our manuscript. We have carefully incorporated all comments and revised our manuscript accordingly.

The revised contents are highlighted with light blue background on the main manuscript file. We have also included a point-by-point response to the comments and suggestions on the PDF file.

Author Response File: Author Response.pdf

Reviewer 2 Report

 

Dear authors:

Thank you for your work. I think that it is of high interest and reflects a great effort of realization. The elaboration of this paper is very correct. However, I think that different considerations and specific improvements can be made, as well as recommendations or suggestions in some of the aspects of the document, which I show you below:

 

Abstract:

Line 23: “Since, variability of SOC…”

Please, remove comma.

Line 28: “…using the RF and Classification…” ; Line 30: “…RMSE values…”

Please, avoid using unspecified acronyms.

Line 33: “Furthermore, bauxite and laterite lithology had…”

It is more correct to express it as: “Furthermore, soils developed in bauxite and laterite…”, as well as for the rest of lithologies.

Lines 34-35: “…highest SOC content (14.69 mg/g) followed by fluvial (14.52 mg/g) and shale (13.57 mg/g), whereas the lowest was predicted in sandstone (5.53mg/g)…”

The SOC content is remarkably lower in soils developed in sandstones; however, the values obtained in soils from the rest of the lithologies can hardly be significantly differentiated between them. Perhaps this should not be highlighted in the abstract.

On the other hand, please modify the notation of the units always as “mg·g^-1.

Lines 35-36: “…where the majority of area was classified as humous and organo-humus, distributing in the mountainous regions”.

This sentence should be clarified, since "the majority of area" seems to refer to “the majority of soils in that area”, but then a soil classification should be used. If you are referring to a classification of "ranges of organic matter values in mg·g^-1", perhaps this should be clearly specified. The terms “humous” and “organo-humus” are apparently coined specifically for this work, but how they are used in this abstract is unclear. I understand that "humous" is a typo, it is "humus". On the other hand, the term "organo-humus" may not be appropriate, since "humus" is, by definition, organic. It can be assumed that the authors refer to the fact that the soils with the highest SOC are mainly located in the mountainous regions of the study area. Please clarify it as much as possible.

Lines 37-38: “…and soil spectral bands, respectively were the most influential predictors of SOC…”

This sentence may be modified as:

“…and soil spectral bands, respectively, were the most influential predictors of SOC in the study area.”

Line 40: “…and built-up indices added valuable…”

Please, add comma: “…and built-up indices, added valuable…”

 

Introduction:

Lines 51- 52: “However, its spatial distribution is influenced by landscape, lithologic and land use factors.”

This sentence may be replaced by:

“In a general context, SOC spatial distribution is influenced by climatic, geomorphological, lithological and land use factors.”

Line 67: “Even though, many previous…”

Please, remove comma.

Lines 68-73: “Moreover, there is a need to find the optimal set of suitable environmental predictors that may influence prediction of SOC distribution. As the spatial distribution of SOC is highly influenced by various environmental variables, assessing suitability of remotely-sensed predictors and other environmental covariates for mapping of the spatial variability of topsoil SOC in complex urban environment is imperative.”

As a suggestion, this sentence could be synthesized as follows (for example): “Moreover, there is a need to find the optimal set of environmental predictors to remotely determine the spatial variability of topsoil organic carbon, so that assessing the suitability of such predictors, as well as other environmental covariates, is an imperative task to SOC mapping in urban environments.”

Line 76-77: “...they ignored essential biophysical land surface variables that may add information to SOC distribution.”

The term "essential", referring to the variables, might not be appropriate here because it is excessively conclusive, since later it is indicated that such variables "...may add information ...". Maybe the phrase needs to be explained (for example): “...different biophysical land surface variables, which can provide information on the SOC distribution, were generally ignored. “

Line 78-79: “Since, SOC is highly influenced by soil temperature and moisture than any other factors, integrating them may provide new insight for SOC mapping...”

Please, remove comma in “Since, SOC is...”

In the other hand, I think it should be specified here that temperature and moisture values, certainly influencing SOC, refer to mean data from long-term observations, rather than single values obtained at a given time.

 

Materials and Methods:

Line 111-112 (Figure 1):

In general, the figures in this work are very appropriate, but I think that an improvement could be obtained from all of them if the size and resolution of the letters could be increased in order to facilitate their visualization. In particular, the information contained in figures 7, 8, 12 and others is difficult to read.

Line 127: “The soil samples were collected using purposively distributed sample points that represent the dominant land cover, soil types, and landform of the study area.”

The criteria established for soil sampling are perfectly selected and include the soil type. I think that this factor, obviously decisive in the study of SOC, should also be included in the Introduction (Lines 93 to 95) together with the variables “landform”, “land-use”, and “lithology”.

Line 169: “...Additional landform characteristics of the study area was described...”

Please, replace “was” by “were”.

Lines 173 and 177; line 351-352; line 478, 488: “...land-use...”

Please, replace always by “land use”.

Line 191 (Figure 4, legend):

Please, correct “Temprature”

Lines 194-195: “...values of 6.25(°C), 15.15(°C), 10.65(°C)...”

Please, remove the parentheses.

 

Results and Discussion

Line 278: “3. Results and Discussion”

The Section 3, named “Results and Discussion” in this work, is very appropriately written as a "Results" section, so I think it can be called that way. Section 4 is effectively an appropriate "Discussion" section.

Lines 284-288; lines 300-301; line 340 (Table 4); line 357 (Table 5); lines 359-376; lines 379-380:

Please, modify the notation of the units as “mg·g^-1.

Line 313: “Humous SOC level was covering...”

It is probably "Humus" SOC level.

Line 374 and 376: “...inland water (lakes) (7.47 mg/g)...”,

“Inland waters or lakes” should not be considered as soil surfaces; therefore, they are not areas in which their SOC content can be evaluated. I understand that this requires modifying the values assigned to each of the land uses, but it is essential to ensure consistency of the results from the physiographic and, in particular, pedological points of view.

Line 376: “Table 7. SOC (mg/g) distribution across lithology”

I would like to propose to change the title of the Table 7 to: “SOC (mg·g^-1) distribution across soils developed in different lithologies.”

 

Discussion

Lines 403-405: “...since soil moisture is an essential element for microbial growth, it may facilitate the degradation of plant and animal residues that improves SOC contents.”

This is correct, but it must also be taken into account that an intense microbial activity is what determines a rapid evolution of organic matter, and therefore its mineralization, leading in the long term to a low SOC, which is precisely characteristic of tropical soils. A similar consideration can be made regarding the sentence in lines 463-464.

Lines 411-412: “...enhanced vegetation index (EVI) 411 has low soil, and atmospheric effects...”

Please, remove comma between “soil” and “...and atmospheric effects”

Lines 434-435:  “Additionally, a significant positive correlation between the NDBI and SOC (r = 0.26,  p < 0.05) also shows strong relationships.”

Perhaps the term "strong relationships" is excessive here, considering the many factors involved.

Lines 435-439: “The high extent of impervious surface may impact SOC contents since increased human disturbances and a mix with artificial materials from buildings causing variability. Similarly, Yan et al. [68] confirmed the importance of impervious surface on SOC quantification and stated that SOC from impervious surface accounted for over half of the city’s SOC stock in their study."

It is very important to highlight the role of "impervious surfaces" in the evaluation of the SOC by means of remote sensors. However, for the quantification of the real stocks of SOC, the extremely relevant role of the organic carbon contained in the subsoil, should not be overlooked. In this sense, this work is of high interest as it includes variables that, more or less directly, are related to the C contained in the subsoil.

Lines 473-474: “Additionally, the dominant soil types were paddy-soil, with a small proportion of red soil and plaster fields.”

When describing the soil typology, it is important to use terms of universal use such as those established by WRB-FAO or Soil Taxonomy, or at least internationally recognized national classifications. For example, the term “paddy-soil” refers to the use of the soil, which is very relevant, but it is not an acceptable term as a soil taxon. In the same way it would be necessary to cite "red soil" and of course the term "plaster fields", which I think perhaps it refers to something like "exposed gypsiferous soils". In any case "plaster fields" is not at all an adequate term to define a type of soil, natural or artificial. I think this should be clarified somewhere in the document, perhaps methodology.

Lines 500-505: “Additionally, weathered residuum had the highest content of SOC. It may be due to possession of aluminosilicate red soil (acid red soil), Fe, and permeates paddy soil. The Fe leaching, coupled with the high activities of microorganisms, may lead to the increased content of SOC in the weathered residuum, bauxite, and laterite lithology. The previous study reported increased SOC in bauxite lithology due to increased pyrite owing to the redox reactions.”

In general, the increase in soil acidity limits the mineralization of organic matter, precisely by reducing bacterial activity. On the other hand, the low oxygenation of soils such as "paddy-soils" also limits bacterial activity, slowing down the mineralization of organic matter. The complexity of the mineralization processes is extremely high, and although the discussion in this paragraph is of interest, it is nevertheless clearly insufficient to justify the higher SOC content of the weathered residuum, a lithology that, on the other hand, can be highly variable in regarding the soils that develop from that parent material. A similar reflection could be made regarding the soils developed in sandstones and other lithologies: lines 511-521.

 

Conclusions

The Conclusions section is clear and is very consistent with the main findings of this study; however, the first paragraph (lines 525-531), which is perfectly correct, in my opinion refers more to the content of an abstract.

 

Thank you.

 

Author Response

Dear Reviewer,

We would like to sincerely thank you for your deep and insightful comments and suggestions and your favorable comments about our work.

We believe that the comments and suggestions you raised are very important and will immensely improve our manuscript. Therefore, we have carefully integrated your comments and suggestions and revised our manuscript accordingly.

The revised contents are highlighted with light blue background color on the main manuscript file. We have also included a point-by-point response to the comments and suggestions raised here under.

Author Response File: Author Response.pdf