General comments:

This manuscript (bg-2021-127), entitled "Difference of SPAC composition and control factors of different vegetation zones in north slope of Qilian Mountains",  investigated the differences in SPAC composition of three different vegetation zones  of the period of April 2018 to October 2019, and attempled to find the controlling factors. This paper is the one that I have seen so far in the research on SPAC water cycle with relatively rich data and very reasonable argumentation. This manuscript provides a lot of valuable isotope data for understanding the water cycle in arid and semi-arid regions. In this paper, the micro-SPAC water cycle is well correlated with the basin water cycle, and the case studies are compared and discussed at the regional and global scales. It is a writing style that I admire a great deal. So I support the publication of the article. However, there exist some problems, including English language, basic format, and the description of the research samples is not enough. In addition, many explanations in the text are not clear enough. A good article must be very scientific and well expressed. Therefore, I think the article can only be published after having solve the following problems.

Specific comments:

1. Why choose these three sampling points as the research area of this manuscript?How long is the sampling interval? The author has made a lot of descriptions on sample collection and experimental analysis, but if the author can more clearly describe how to select samples, it will be more reasonable, for example, in Section 2.2: We have collected samples of precipitation, groundwater, soil, and plant at Lenglong (alpine meadow), Hulin (forest), and Xiying (arid foothills) in the Shiyang River Basin from April 2018 to October 2019.
2. Section 3.1: With the continuous progress of vegetation restoration, the vegetation coverage of alpine meadows will continue to increase. How do you know that the vegetation coverage of alpine meadows has increased? Is the conclusion drawn from the relevant literature or based on actual data?
3. Section 3.2: “Along the three vegetation zones of alpine meadow-forest-arid foothills, soil water isotope gradually enriched.The coefficient of variation of the arid foothills is the largest”.Why is the coefficient of variation of the isotope of soil water in the dry mountain foothills the largest?
4. Section 3.2: “The soil water content of alpine meadows is higher than that of forests and arid foothills, and the soil water content of alpine meadows increases with the soil depth”. Why does the soil water content of alpine meadows increase with the increase of soil depth? The author did not give an explanation in the manuscript. Does this characteristic of change appear in different soil depths?
5. Section 3.3, line 228-229: “High temperature is related to groundwater level exposure”.What does this sentence mean?
6. In the discussion section, why only discuss the impact of temperature and altitude on SPAC? Why are other factors not discussed?
7. Line 240-241:This phenomenon shows that precipitation plays a major control role in the water cycle of precipitation-soil-plants. What does this phenomenon mean?
8. Line 241-242: “Previous studies have shown that local factors, especially local temperature mainly control the stable isotope changes of precipitation in mid-latitudes”. The author does not mention any information about the previous research in the manuscript, such as the source of the research.
9. Line 248-250: “From alpine meadow to arid foothills, the correlations between temperature and soil are 0.41, 0.30, and 0.19, respectively, and the correlations with plants are 0.24, 0.27, and 0.25, respectively”.How are these correlation coefficients obtained?
10. Section 4.2: In this part, the author mainly discusses altitude and precipitation isotope, but the description of the relationship between soil water isotope and plant water isotope and altitude is too little.
11. Line 255-256: “...the correlation between the isotope of precipitation in the arid mountain foothills and the temperature fails the significance test.”Why fails the significance test?

Minor comments:

1. In the abstract, the author directly proposed SPAC, the author did not give any explanationabout wahst is SPAC.
2. Line 28:the reference is incorrectly cited.
3. Part 2.3, the font size of the formula (1-1) , (2-1) to (2-4) are different.
4. Line 47: change “The content of the SPAC hydrological cycle research are dramatically enriches and expands”to “The content of SPAC hydrological cycle research has been greatly enriched and expanded”.
5. In the manuscript, some formats are not standardized, and some letters should be superscripts, such as in line 34, 18O; line 57, δ18O; line 88, 15.75×
6. The scientific counting method of temperature in this manuscript is inconsistent, as in section 4.1: 8℃, 23.92℃.
7. Line 88: change 75×108 km3to 1.58×10⁸ km³.
8. Lin 89-91: please add the corresponding references.
9. Improve the clarity of Figure 1 to Figure 4 to make it easier to obtain relevant information from the diagram.
10. Line 226 and line 228: In order to maintain consistency, please change “arid piedmont”to “arid foothills”.
11. Line 256: Please pay attention to the spaces between the text , such as change “0°Cand”to “0°C and’.

In this manuscript, Liu et al. explored oxygen and hydrogen isotope signals in precipitation, groundwater, soil water and plant water in different vegetation zones within the upper reach of Shiyang River Basin, and aimed to elucidate internal linkages between various water bodies. Such investigation could deepen our understanding on mechanisms in water cycle and facilitate ecosystem management of water-limited areas. The topic of manuscript falls into the scientific scope of Biogeosciences. The authors selected three representative vegetation zones in the study area, and worked hard to collect many samples last for more than one year. I believe their data are informative and interesting. However, in my view, the manuscript is not well written, including the whole structure, data analysis, interpretation of results, discussion, and language issues. Thus, the current version of this manuscript is beyond the standard of BG, and I suggest the authors resubmit the manuscript after major revision.

Major concerns

1. It is difficult to figure out the background and necessity of this study from the current version. In the initial paragraph of Introduction, the authors only told us that water isotopes were useful with so many sentences, but they didn’t show us the latest developments and trends of isotopic research in soil-plant-atmosphere continuum. In the second paragraph, the authors described soil water, precipitation, and plant water. But they didn’t raise any scientific problems. Until the last paragraph, the authors still didn’t state the reasons to perform research in Shiyang River Basin.
2. In the current version, Introduction, Results, and Discussion sections were mismatched from each other. From Introduction, I understand the authors intend to analyze the isotopic differences in different bodies and the potential controlling factors. However, no data on the controlling factors were shown in Results, while temperature and altitude were discussed in Discussion, which were not mentioned before. I suggest to reorganize the manuscript and closely link each section.
3. A subsection on data analysis is also necessary in the Materials and Methods section. In Results, I found many words like “…smaller than…”, “…greater than…”, “…the smallest…”, and “…closer to…”. Such presentations need solid evidence of statistical analysis. So do the comparisons of slopes and intercepts between different water lines. It will also be better to show the quantitative relationships between isotopic information of SPAC and potential controlling factors.
4. The manuscript needs to be modified to avoid ambiguity and obscurity of expression. Such as the sentence in L52-54, “Because the isotope ratio in soil moisture obviously changes with depth, when water is transported between plant roots and stems, it reaches the leaves or young unbolted branches before its isotopic composition has not changed”. I am confused about what is the intention of such presentation, what is the logic here. There are other sentences like this through the manuscript.

Specific comments

L1: As first mentioned in the title, the abbreviation SPAC should be written as its complete form, soil-plant-atmosphere continuum. Also, the authors may want to say “Isotopic differences of…”.

L15: “The results showed…”.

L26: “…changes of oxygen and hydrogen isotopes in water…”.

L41: “…, but also…”.

L43: Nie et al., 2014.

L46: It seems that the “SPAC” appears suddenly here. Necessary conversions of words and content are needed.

L49: Both “soil water” and “soil moisture” are used in the manuscript. I suggest use one of them.

L51: Is sampling of the current study involving desert vegetation?

L56-58: “The source of plant water use can be determined by measuring…”.

L66-70: The sentence is too long, and specific subjects before “can” and “lay” lack.

L70: “plant transpiration” or “vegetation transpiration”.

L90: What kind of the drought index?

L90: “…classified as…”, not “divided into”.

L91-93: Replace these descriptions with exact data.

L96-97: Add to the previous sentence.

L98: Did the authors investigate vegetation in the study area? If yes, please show the data of vegetation coverage; if not, relevant references should be added. In addition, “relatively good” is not a proper expression in scientific papers.

L102: “Samples of…were collected…”

L109: “telling the date”?

L111: Are there any replicates for soil samples of each soil layer?

L117: How many plant species are sampled? How about the position of sampled stems in the canopy? What is the size of stem samples? “xylem stem” should be “stem”.

L120: How is the groundwater sampled? What is the depth of water table at each sampling point.

L126: How many isotope standards were used?

L133: “Due to the existence of methanol and ethnol in plant water samples…”

L148: Since different water lines have been defined here, I suggest the revised manuscript used their abbreviations hereafter.

L148-150: These information should be mentioned in the Introduction section.

L152-159: Sentences are repeated here.

L163: Is there any data or references for such statements.

L169-171: These results should be based on proper statical analysis.

L190-192: Any data or references?

L192-194: References are also needed here.

L192-198: Since this is the Results section, I suggest move these content to Discussion.

L197: “The dry foothills…”.

L209, L212: “affluent” and “abundant” are not proper words here.

L216-217: This is not a convincing conclusion.

L244: Why use 8℃ as turning point?

Fig.1: “Shiyang River syste”? Is it “Shiyang River Basin”? The letters (a, b, and c) should be explained in figure caption.

Fig.2: Please rearrange the graphs in a single column or row.

Fig.3: “θ” should be defined here. I also suggest the graphs on the left and right panel using a same x-axis range, respectively.

Fig.4: Where are the standard deviations or errors?

Fig.6: Please define M1, M2 and M3 in the figure caption. How is the situation of hydrogen isotope?

Table 1: Are the comparisons including significance testing?