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.

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.

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.

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.