Progress and Prospects of Research on Physical Soil Crust

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors 

The article 'Progress and prospects of research on the physical crust of soil' is a review article. Based on numerous publications and studies, the authors carefully present the achievements to date in China and around the world. In the basic sense, PSC in the context of literature presents mainly types and classifications, characteristics and dynamics of development, and factors influencing the formation of the crust, including hydrological processes and erosion. In addition, they indicate the need for further research in the field of soil structure dynamics, the use of erosion models in the context of crust development or improving the scale of the degree of crust development and structural features and optimizing the soil structure. In the paper, the content was referred to 115 items in the literature. Therefore, the material could be considered sufficient. In the following chapters, the content is supported by tables and graphs. In my opinion, there should be more tabulated summaries. This makes it easier for readers to generally understand the subject matter. So in this sense, the paper could be improved by adding new elements. Despite numerous citations, the reader may feel a certain dissatisfaction when it comes to world literature. In relation to the total of 115 citations, 66 of them are related to studies and research in different regions of China. At this point, the authors could try to correct the material with additional global knowledge on this topic.

Author Response

Comment 1：The paper could be improved by adding more tabulated summaries to help readers better understand the subject matter.

Responses: We sincerely thank you for this suggestion. To enhance the readability and clarity of the manuscript, we have added a new table summarizing the key findings on the main classifications of soil crust. This table is included in Section 1 (pages 2) of the revised manuscript. At the same time, we have added an image (Figure 2) that shows the four different stages of the quantified physical crust process (pages 8).

 

Comment 2: The literature review is dominated by studies from China (66 out of 115 citations), and more global perspectives should be included.

Response: We sincerely appreciate the reviewer’s observation regarding the predominance of studies from China in our literature review. We have downloaded and reviewed nearly all crust-related articles published since 1940, revealing that research on soil physical crusting in China commenced relatively late, with most studies concentrated post-2000, while pre-2000 literature predominantly consists of non-Chinese publications. In accordance with the structure of this paper and the exigencies of review article timeliness, we have not cited all available literature. Instead, we have predominantly referenced and analyzed recent publications alongside some seminal older works, resulting in a total of 131 cited articles, of which 66 are of Chinese origin. Nonetheless, adhering to your suggestions, we have incorporated several classic pre-2000 non-Chinese references and eliminated some redundant Chinese articles with overlapping content to maintain a balance.

Reviewer 2 Report

Comments and Suggestions for Authors

The article “Progress and Prospects of Research on Physical Soil Crust” by Huiyun Xu, Xuchao Zhu, and Mexia Mi presents a comprehensive, rigorous, and well-structured review of the development, characteristics, and environmental impacts of physical soil crust. The depth of the analysis, supported by an extensive review of the literature, provides readers with a clear and detailed overview of the evolution of research in this field.

One of the most noteworthy aspects of this work is its systematic approach, which encompasses conceptual foundations, formation mechanisms, environmental implications, and potential future research directions. The structured organization into well-defined sections facilitates content comprehension and its application in soil conservation and agricultural management. Furthermore, the inclusion of advanced technologies, such as computed tomography (CT) for quantifying crust development, introduces an innovative element that opens new opportunities for the precise monitoring of these processes.

Overall, this study represents a significant contribution to the understanding of physical soil crust, offering a comprehensive framework for future research and practical applications. Therefore, I recommend its publication in Soil Systems in its current form.

Author Response

Comment 1: The article presents a comprehensive, rigorous, and well-structured review of the development, characteristics, and environmental impacts of physical soil crust. The systematic approach, encompassing conceptual foundations, formation mechanisms, environmental implications, and future research directions, is particularly noteworthy. The inclusion of advanced technologies, such as computed tomography (CT), adds an innovative dimension to the study.

Response: We are delighted that you found our review comprehensive and well structured. Your emphasis on the systematic approach and the integration of advanced technologies, such as CT, is particularly gratifying. We aimed to provide a holistic perspective on physical soil crust, bridging foundational knowledge with cutting-edge methodologies to inspire future research and practical applications. Your positive feedback reaffirms that we have achieved this goal.

 

Comment 2: The structured organization into well-defined sections facilitates content comprehension and its application in soil conservation and agricultural management.

Response: Thank you for highlighting the clarity and organization of our manuscript. We carefully designed the sections to ensure a logical flow of information, from conceptual frameworks to practical implications. Your acknowledgment of its applicability to soil conservation and agricultural management is especially encouraging, as these are key areas where we hope our work will make a meaningful impact.

 

Comment 3: Overall, this study represents a significant contribution to the understanding of physical soil crust, offering a comprehensive framework for future research and practical applications.

Response: We are truly honored by your assessment of our work as a significant contribution to the field. Our goal was to not only summarize existing knowledge but also to identify gaps and opportunities for future research. Your endorsement of the manuscript in its current form is a testament to the effort we invested in achieving this balance.

Reviewer 3 Report

Comments and Suggestions for Authors

The article is an interesting proposal that is a review of research on the formation and properties of soil physical crust. In general, the article - although not very advanced scientifically - is very well written, but requires some corrections:

1. Introduction: 

   There are many other scientific articles (older and more fully characterizing soil crust) available - the authors of these articles dealt with this problem much earlier and should be cited in this type of article. For example, Soil crust and its impact on crop establishment: A review 1985. N.K. Awadhwal  G.E. Thierstein

   Weber, Bettina (2022). "What is a biocrust? A refined, contemporary definition for a broadening research community". Biological Reviews. 97 (5): 1768–1785.

2. Table 1 - table should start in next page;
3. Chapter 3.1. - The authors focused only on China - in this type of review article a broader spatial context of the occurrence of such soil crusts should be provided - with an explanation of what environmental conditions (not only in China) lead to the formation of such crusts.
4. Figure 1 - In parts a and b of this figure the scales of the photos were not given. The letters a and b are faintly visible (black - should be white).
5. Line 384, Figure 2 - If this is a separate experiment (not cited from the literature), then it is very poorly described - we do not know what the soils were like, what the intensity of rainfall was (not only the length), hoew many replications wwere made etc. Authors should not "mix" review articles with experimental ones.
6. The article lacks the spatial aspect of the research - in which countries, regions has this phenomenon already been observed and researched - It should be added at the beginning of chapter 4. - Factors influencing crust formation.
7. Chapter 6.3 - However, erosion models (such as RUSLE) confirm what was stated in this article - surface horizons containing up to several dozen percent of clay seal up the fastest during rainfall (lower structural resistance), which leads to an acceleration of surface runoff and increases the erosion rate. This is confirmed by studies in specific locations (e.g. Agronomy 2022, 12(11), 2595; https://doi.org/10.3390/agronomy12112595). The thread of connections between the sealing of arable soil surfaces and their grain size expressed by the soil erodibility factor (K) values ​​could be added to this article - I believe that these phenomena are strongly related.

Author Response

Comment 1: There are many other scientific articles (older and more fully characterizing soil crust) available - the authors of these articles dealt with this problem much earlier and should be cited in this type of article. For example,

(1) Soil crust and its impact on crop establishment: A review 1985. N.K. Awadhwal; G.E. Thierstein.

Response: Thank you very much for your detailed review and valuable suggestions on our manuscript. We fully agree with your point regarding the importance of early literature on soil crust research. During the revision process, we have conducted an in-depth analysis of the 1985 review article by N.K. Awadhwal and G.E. Thierstein, titled "Soil crust and its impact on crop establishment: A review," and have cited it in Section 5.3 of our manuscript.

This review is indeed a high-quality review that thoroughly discusses the formation mechanisms, physical properties, and impacts of soil crust on crop seedling emergence. The viewpoints presented by the authors align closely with our research, particularly in terms of selecting crop varieties with strong emergence capabilities and modifying planting methods (such as ridge planting) to improve seedling emergence rates. Additionally, they emphasized the need for future research to focus on developing more economical soil conditioners and more efficient crust-breaking equipment to mitigate the negative effects of soil crust on crop establishment. These perspectives not only align with our research direction but also provide a more comprehensive consideration of the issue, offering significant theoretical support and reference for our work.

Based on this, we have supplemented and refined the relevant sections in the revised manuscript, ensuring that this important early literature is cited, and its viewpoints are thoroughly discussed. We believe these revisions have further enhanced the scientific rigor and completeness of our article.

 

(2) Weber, Bettina (2022). "What is a biocrust? A refined, contemporary definition for a broadening research community". Biological Reviews. 97 (5): 1768–1785.

Thank you for your valuable advice. In response to your suggestion regarding the introduction, we acknowledge the existence of numerous pioneering articles that have extensively characterized biological crusts, some of which predate our current understanding. Notably, the review by Weber, Bettina et al. (2022), titled "What is a biocrust? A refined, contemporary definition for a broadening research community", published in Biological Reviews, has significantly contributed to refining and updating the definition of biological soil crusts by synthesizing knowledge from various historical and contemporary sources. This review encompasses a comprehensive framework, including physical structure, functional characteristics, habitats, and taxonomic composition, thereby enhancing our ecological understanding and appreciation of the services provided by these crusts.    Recognizing its academic merit and relevance, we have already incorporated a citation of this seminal work in our introduction to provide a robust theoretical foundation for our discussion on the definition of biological soil crusts. This addition aims to honor the contributions of earlier researchers and strengthen the coherence of our narrative within the existing scientific discourse. It is important to clarify, however, that while the forementioned review by Weber et al. focuses broadly on biological soil crusts, our article primarily aims to delve into the specific aspects of physical crusts. Therefore, in the introduction, we only added the cited literature without elaborating on it in detail.

 

Comment 2: Table 1 should start on the next page.
Response: We apologize for this formatting oversight. In the revised manuscript, Original Table 1 (now Table 2) has been adjusted to ensure proper layout and readability.

 

Comment 3: Chapter 3.1 focuses only on China and should be expanded to include a broader spatial context of soil crust occurrence and the environmental conditions leading to its formation.
Response: Thank you for your constructive feedback on our manuscript, particularly regarding the focus of Chapter 3.1. We appreciate your suggestion to expand the spatial context of soil crust occurrence and the environmental conditions conducive to its formation beyond China. In response to Comment 3, we acknowledge that our initial focus on China in Chapter 3.1 may have limited the broader applicability of our discussion. However, it is important to note that our decision to emphasize China was informed by the extensive literature review conducted during the preliminary stages of this review. This review revealed that China, with its diverse climatic zones, exhibits significant variations in soil distribution types and precipitation patterns, which are crucial factors influencing soil crust formation.

Furthermore, in Section 3.2, where we delve into the formation mechanisms of different types of soil crusts, we have enhanced our discussion by integrating insights from representative studies worldwide (summarized in Table 3). This comprehensive approach allows us to draw conclusions that are potentially applicable to all soil types globally, rather than being confined to a few specific regions.

 

Comment 4: Figure 1 lacks scale bars in parts a and b, and the labels are faintly visible.
Response: We appreciate your observation. Based on your suggestions, we have replaced Figure a with a more explicit image of the caliper measurement of the crust. Additionally, we have provided detailed descriptions for Figure 1a and Figure 1b, clearly stating that Figure 1a is a photograph taken with a camera, while Figure 1b shows a 4*3.3 cm crust slice observed under a polarizing microscope. These adjustments aim to help readers better understand the source and content of the images, avoiding potential misunderstandings.

Label Color Optimization: We have changed the label colors in the original figures to white, enhancing the visibility and clarity of the labels. This ensures that readers can more intuitively grasp the information presented in the figures.

These modifications are intended to improve the interpretability of the images and the clarity of the labels, preventing any potential confusion for readers. We believe these adjustments will further enhance the readability and scientific rigor of the manuscript.

 

Comment 5: Figure 2 (Line 384) is poorly described if it is an independent experiment. Details such as soil properties, rainfall intensity, and replication are missing.
Response: Thank you for highlighting this issue. We fully understand and agree with your concerns regarding the insufficient description of the experiment in Figure 2. In fact, Figure 2 is based on an independent experiment conducted in the red soil region of southern China, focusing on Quaternary red clay and simulating local rainfall conditions with a conventional intensity of 30 mm/h lasting for 90 minutes. However, as the related research has not yet been formally published (the revised manuscript has just been submitted and we have not yet received an official acceptance notice), we are unable to provide specific references. Following your suggestion, we have supplemented the manuscript with a detailed description of the relevant content (Line 386-392).

 

Comment 6: The article lacks a spatial aspect of research. A discussion of where this phenomenon has been observed and studied should be added at the beginning of Chapter 4.
Response: We sincerely appreciate your valuable suggestion regarding the need to include a discussion on the global distribution of soil crust research at the beginning of Chapter 4. This perspective is indeed crucial for enhancing the comprehensiveness and universality of our study. However, since the spatial dimension was not fully considered during the initial drafting of this review, directly introducing this discussion might raise debates about the universality of our conclusions. Nevertheless, through a comprehensive review of the available literature on physical soil crusts, we observed that the factors influencing crust formation are highly diverse. For instance, climatic conditions, topographic features, and the resulting soil properties and farming practices in different regions (e.g., Italy, the UK, China, and the USA) significantly influence the crust formation process. Therefore, we believe that approaching the discussion from two broad perspectives—natural factors (e.g., climate, topography, soil properties) and anthropogenic factors (e.g., farming practices, land use types)—may enhance the universal applicability of our conclusions. Additionally, we found that different factors often interact and collectively influence crust formation, which led us to categorize them into natural factors, human factors, and comprehensive factors (e.g., interactions between tillage and rainfall or soil properties and slope gradient).

Nonetheless, we have fully incorporated your suggestion and added a brief discussion on the global distribution of soil crust research at the beginning of Chapter 4, along with the forementioned perspective, to address the reviewer's concern regarding the lack of a spatial dimension.

 

 

Comment 7: Chapter 6.3 should discuss the connection between soil sealing, grain size, and the soil erodibility factor (K), as supported by studies such as Agronomy (2022).
Response: We thank you for this insightful suggestion. By reading the literature you provided, we found that the researchers analyzed the stability of soil aggregates under different erosion levels in the young hummocky moraine landscapes of northern Poland, revealing the negative impact of erosion on soil structure. The study concluded that: (1) completely eroded and strongly eroded soils exhibited poor aggregate stability due to low organic matter content and high clay content, while soils with colluvial materials showed better aggregate stability due to higher soil organic carbon content; (2) soil aggregate stability was positively correlated with soil organic carbon content and negatively correlated with clay content. Based on these findings, the researchers suggested that in areas with highly heterogeneous soil cover, land management practices should be adjusted according to the actual soil conditions to reduce erosion risks and maintain soil health.

In response to your suggestion, we would like to address the following points: (1) In Section 5.2 of the manuscript, we have comprehensively analyzed the impact of soil sealing formation on the erosion process. We have explicitly identified key factors influencing soil erosion rates, including soil texture (particle composition), structural properties, aggregate stability, and bulk density [1-5]. (2) Existing research suggests that while soil properties can be used to construct erosion prediction models, the quantification of parameters such as aggregate stability and bulk density often depends on laboratory experiments, which may introduce subjective biases and compromise model accuracy. Therefore, in Section 6.3, we emphasize that quantifying soil pore structure using CT scanning technology can objectively obtain pore metrics, thereby reducing subjective errors and enhancing the precision of erosion prediction models. This approach not only provides a more accurate characterization of soil physical properties but also offers robust data support for the development of erosion models. Therefore, we think that if we are to refer to this document, it may be more appropriate in Section 5.2, which we have refined in the manuscript.

1. Cheng, Q.; Cai, Q. Erosion response of topsoil crust in loess under simulated rainfall. Journal of Soil and Water Conservation 2013, 27(4), 73-77.
2. Chen, L.; Xu, F.; Li, J.; Yang, C.; Wang, J. Aggregate stability in rainfall‐induced soil physical crusts on the Loess Plateau, Northwest China. Soil Sci. Soc. Am. J. 2022, 86, 528-539.
3. Zhang, X.; Miller, W.P. Physical and Chemical Crusting Processes Affecting Runoff and Erosion in Furrows. Soil Sci. Soc. Am. J. 1996, 60, 860-865.
4. Gao, Y.; Zheng, F.; Wang, B.; Li, G.; Zhang, J.; Jiang, Y. Influence of soil crust on flow and sand production on slopes in a black soil area. Soil and Water Conservation Research 2014, 21(4), 17-20.
5. Xu, D.; Wang, J.; Cai, X.; Yu, X.; Zhao S.; Zhao S. Progress of physical crust erosion in China. Henan Agricultural Science 2023, 52(4), 9-20.