the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Feb 2024
Root zone in the Earth system
Abstract. The concept of “root zone” is widely used in hydrology, agronomy, and land surface process studies. However, the root zone still lacks a precise definition. More essentially, the importance of root zone in the Earth system science is largely under explored. Furthermore, the methodology to estimate root zone is still controversial. In this study, we firstly attempted to clarify the definition of the root zone by comparing with various “similar” terms, such as rooting depth, soil depth, vadose zone, rhizosphere, and critical zone, to bridge the gaps within and between traditional disciplinary boundaries. Secondly, we found that, from a hydrological and thus water-centric perspective, the root zone holds profound implications across all the spheres of the Earth system, including biosphere (living organisms), hydrosphere (water), pedosphere (soil), lithosphere (rock), and atmosphere (air), through various exchange fluxes of mass and energy. The role of the root zone in the Anthropocene is elaborated as well, including the intensifying impacts of climate change and agriculture, along with implications for nature-based solutions and planetary stewardship. Thirdly, for root zone estimation, we underscore that the theoretical foundation of the traditional reductionist approach to understand and model the root zone is problematic due to the complex and dynamic nature of root zone functions. We advocate for a shift towards a holistic ecosystem-centered perspective, which offers a more realistic, simplified, and dynamic representation of the root zone in Earth system science.
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RC1: 'Comment on egusphere-2024-332', Anonymous Referee #1, 21 Mar 2024
Thank you for the opportunity to review this manuscript. It is evident that the authors have made significant contributions to this field, notably through their work on remote-sensing methodologies for monitoring root zone water storage. The concept of emphasizing the root zone as a distinct entity is commendable. However, the manuscript often reads more like a textbook chapter or a commentary rather than a comprehensive review article.
The structure and content of this paper are reminiscent of the work by Klos et al. (2017), which, surprisingly, is not cited. The authors seem to be aiming to present a similar vision, albeit with a lesser focus on hydrology. Nevertheless, the level of detail presented does not match that of the referenced work. The manuscript successfully delineates what the root zone is not, through numerous "not equals" subsections. Yet, it falls short in providing a clear conceptual figure or a definitive definition of the root zone. The reliance on a definition from Sprenger et al. (2019) – "The root zone is the upper part of the vadose zone" – raises questions about the exclusion of various types of vegetation, such as phreatophytes, from having a root zone. This definition, alongside the conceptual figures provided, does little to advance the discourse beyond the existing literature.
Conversely, the authors demonstrate a deep understanding of calculating remotely sensed water storage deficits. It is curious why this expertise was not leveraged to further explore the potential advancements in this methodology. Questions regarding the nature of the deficit, methods of measurement, spatial and temporal variations, subsurface implications, and the potential for methodological failures warrant a more thorough investigation.
Furthermore, the manuscript does not adequately address a range of root zone variables, which are crucial given the ambitious title "Root Zone in the Earth System". The section titled "Root zone estimation approaches" particularly falls short of expectations, focusing solely on storage rather than exploring other vital aspects such as carbon, weathering processes, and water transit times within the root zone.
In summary, while the authors' expertise is evident, the manuscript could greatly benefit from a more structured review of the literature, a clearer definition and conceptualization of the root zone, and a more comprehensive exploration of the methodologies and variables pertinent to the root zone's role in the earth system.
Citation: https://doi.org/10.5194/egusphere-2024-332-RC1 - AC1: 'Reply on RC1', Hongkai Gao, 17 Apr 2024
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RC2: 'Comment on egusphere-2024-332', Anonymous Referee #2, 26 Mar 2024
The authors highlight the importance of the root zone in different disciplines, reviewing the concepts that can be easily confused with it and the methods used to estimate it. Although I am convinced of the importance of understanding the processes occurring in the root zone to advance in the solution of a wide variety of problems of high research interest, I do not see a clear objective in this manuscript. The authors mentioned a possible confusion in defining the root zone but did not contrast them or show examples. Most importantly, they did not define the root zone clearly but posed many “root zone is not”. Besides, I did not find clearly which type of article this manuscript pretends to be since it is not detailed enough to be a review and is too general to be a perspective or opinion paper. In my opinion, the manuscript will be highly improved if the authors delineate the intention of the paper and focus on describing and discussing it. If the objective is to provide a unique and clear definition of the root zone, the structure and much of the information written in the manuscript could be inadequate. If the objective is to claim a holistic perspective for calculating the root zone, it should be better described this perspective and compared with the reductionist one. Moreover, I found several affirmations confusing, repetitive and not adequately referenced, and most of the figures were not well connected to the text. Please see the attached pdf for more details.
- AC3: 'Reply on RC2', Hongkai Gao, 26 Apr 2024
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CC1: 'Comment on egusphere-2024-332', H. Jochen Schenk, 28 Mar 2024
This review addresses the importance of the root zone in hydrology and discusses the root zone concept in relation to related hydrological concepts, such as soil, rhizosphere, rooting depth, critical zone, etc. The authors argue for adopting a holistic perspective of the root zone, but this argument appears to be based on a flawed application of the concepts of reductionism and holism. Much of this review reads like an extended chapter in a hydrology textbook and lacks the new perspective that one would expect from a review article. The only exception to that is section 5 with the discussion of inverse modeling towards the end of the review, which does provide a distinct perspective for avenues of future research. This manuscript could be greatly improved by focusing on that perspective and reducing the first four sections to a fraction of their current length, based on the justified assumption that readers will already be familiar with much of what is being presented in those sections.
The authors contrast their view of the root zone with a variety of related concepts and largely focus on what the root zone is not (with extended use of the unequal sign ≠ in headings), which is not that helpful to the reader and sometimes confusing. For example, what is the difference between the root zone and the soil? Illustrations like Fig. 1 are often used to define the concept of soil, which is what the central circle normally represents. The authors label that circle as the root zone, but nothing in that image implies that there must be plants or roots present in that circle. It could be just microbes.
The choice of calling the root zone a “zone” also invites confusion. A zone is an area or volume existing at a particular time. A point in space and time is either within the zone or it is not, because a zone is a discrete concept. The root zone in this manuscript is mostly discussed as having a size, but at one point (line 574), the authors assert the zone consists of a gradient, which implies a continuous distribution that does not have a definable size. They criticize researchers for attempting to determine rooting depth (lines 590-591), but in the next sentence argue for determining the root zone’s size (line 591). Surely, depth is one dimension of size. Some of this discussion about what the root zone is and how it compares to related concepts simply boils down to semantics and is not that helpful to the reader.
Also, none of this discussion about the nature of the root zone is new. The authors’ view of the root zone appears to be quite similar to that of a plant’s belowground zone of influence discussed by Casper et al. (2003), which the authors do not cite, or the much older concept of phytogenic fields in Russian ecology (Uranov 1965 and many others), also not mentioned in this review. If it was the aim of this review to introduce a new concept then it would be useful for the reader to learn how this differs from previous discussions, based on a thorough review of the literature.
The problems with semantics extend to the contrast that the authors make between a reductionist and a holistic approach to root zone modeling. A reductionist approach implies that a complex system can be explained mechanistically by the behavior of all of its parts, while a holistic approach assumes that the combination of parts of a complex system leads to emergent properties that cannot be explained as the sum of the parts’ behavior. The authors criticize what they call the reductionist approach, but not for neglecting complex interactions but for making erroneous or over-simplified assumptions (section 5.2). That is not an argument against reductionism, just an argument against bad reductionism. They argue for measuring and modeling properties of entire ecosystems through inverse modeling, which is an empirical rather than mechanistic approach. Thus, the argument is against using a bad mechanistic approach and in favor of using a good empirical ones. This is not a contrast between reductionism and holism.
It is always challenging for any review to offer a proper balance between simply reviewing and providing a new perspective. This manuscript offers very little perspective for a lot of reviewing and the perspective that is offered is largely based on a strawman argument. That said, the topic is important and worth addressing, and I hope that the reviewers’ comments will help to improve this manuscript.
Literature Cited
Casper, B. B., H. J. Schenk, and R. B. Jackson. 2003. Defining a plant's belowground zone of influence. Ecology 84:2313-2321.
Uranov, A. A. 1965. Fitogennoe pole. (The phytogenic field. In Russian). Pages 251-254 in E. M. Lavrenko, editor. Problemy Sovremennoj Botaniki. Nauka, Moscow, Russia.
Citation: https://doi.org/10.5194/egusphere-2024-332-CC1 - AC2: 'Reply on CC1', Hongkai Gao, 17 Apr 2024
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RC3: 'Comment on egusphere-2024-332', Anonymous Referee #3, 01 Apr 2024
General comments
The paper, titled "Root zone in the Earth system," addresses the critical but underexplored concept of the root zone within Earth system science. It begins by acknowledging the historical significance of roots in plant evolution and their transformative impact on the biosphere and abiotic environment. The study aims to clarify the definition of the root zone, differentiate it from similar terms (e.g., rooting depth, rhizosphere, critical zone), and highlight its profound implications across Earth's spheres (biosphere, hydrosphere, pedosphere, lithosphere, atmosphere). The authors advocate for a shift from traditional, reductionist approaches to a holistic, ecosystem-centered perspective for understanding and modeling the root zone. This perspective considers the root zone as a dynamic entity influenced by and influencing hydrological processes, plant water uptake, and atmospheric moisture cycling. However, I believe there is no need to newly state anything regarding the comparison between holistic methods and reductionist approaches, as there are already many research examples on this topic. Especially in many Earth system models, the holistic method is predominantly used, and reductionist methods seem to be virtually non-existent. On the other hand, the concept of the root zone might differ between models with dynamic vegetation and those with static vegetation. In this study, it would be beneficial to review how the root zone is handled in dynamic versus static vegetation models. Furthermore, although Figure 7 explains using a single tree, in Earth system models, discussing on the scale of individual trees is nonsensical, and it is more common to discuss in terms of forest communities. Especially on these points, I feel the review is incomplete. Before publication, I believe the paper would benefit from revisions in Chapter 5, taking into account the above comments. I trust that making these revisions will improve the quality of the paper. My main concerns are as follows:
- I think it would be beneficial to include a review on the differences between dynamic vegetation models and static vegetation models, especially in terms of root zone estimation. Additionally, considering vegetation as a community is an approach that aligns with the holistic method, so it would be useful to mention and review this aspect as well.
- At line 111, Sumax suddenly appears, but I would like it to be expressed in an equation, specifying in which model this variable is defined. Doing so would allow for a better understanding of the content of the paper.
- I believe there are papers in the literature that have been overlooked. It would be appreciated if you could consider adding them. In the early studies mentioned at line 178, there is Tanaka et al. (2004, DOI: 10.1029/2004JD004865). In addition to the references at line 411, I find the lag mechanism caused by soil freeze-thaw cycles in permafrost regions interesting. I recommend citing literature such as Sugimoto et al. (2002, DOI: 10.1046/j.1440-1703.2002.00506.x) and Suzuki et al. (2021, DOI: 10.3390/rs13214389).
These comments would aim to enhance the paper's contribution to understanding the root zone's role in Earth system science and encourage further research in this important but underexplored area.
Citation: https://doi.org/10.5194/egusphere-2024-332-RC3 - AC4: 'Reply on RC3', Hongkai Gao, 26 Apr 2024
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RC4: 'Comment on egusphere-2024-332', Anonymous Referee #4, 07 Apr 2024
While this review paper demonstrates promise and importance, it predominantly focuses on listing previous research findings, with each section serving as a basic introduction, lacking deeper analysis and synthesis. Here are some suggestions for improvement:
In line 77, it is recommended to revise "Plants" to "Plant roots."
Section 2 should be reorganized to enhance coherence.
Section 3.2.2 would benefit from additional content to provide greater depth.
Figure 6 should have the first letter of "Capillary" capitalized.
Consider removing Section 4.3.3 from the manuscript, as it may not be directly relevant to the main topic.
Figures 1, 3, and 4 are cited from published papers, which may be excessive for this manuscript. Please consider creating some original figures for inclusion.Citation: https://doi.org/10.5194/egusphere-2024-332-RC4 - AC5: 'Reply on RC4', Hongkai Gao, 26 Apr 2024
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