Ecological Safety and Spatial Distribution of Mercury and Arsenic in Qinghai Spruce Ecosystems in Remote Plateau Mountains, Northwest China
Round 1
Reviewer 1 Report
The submitted manuscript deals with the spatial distribution of trace elements in Qilian spruce ecosystems, NW China. The authors point out the ecological significance of their work in the title, which is poorly presented in the manuscript. The significance and contribution of their work is not highlighted. Some major points are:
a) the number of samples are far too little compared to the study area. The selection of sampling sites is not justified by the objectives of the study.
b) The source of As and Hg is not discussed.
c) A number of references are listed within the text, but the discussion is poorly developed.
d) The importance of elevation in risk assessment and ecological significance of the area is not discussed.
e) The risk assessment analysis is not adequately presented, although it is highlighted as one of the main objectives.
A list of point-to-point comments can be afound in the attached file.
Generally, the manuscript seems more like an initial technical report, although the topic is interesting.
Comments for author File: 
Comments.pdf
Author Response
a) the number of samples are far too little compared to the study area. The selection of sampling sites is not justified by the objectives of the study.
The Qinghai spruce forest covers an area of approximately 3.25 km2 and is distributed on north-facing slopes at elevations from 2600 to 3540 m (Zang et al., 2018). Based on this, twenty-eight pairs of soil and moss samples were collected from 2687 m–3450 m in the summer 2018 and the altitude interval of the two sampling points were approximately 25 m. For soil samples (S1-S28), the topsoil represented 0–20 cm of soil layer including litter layer, and each soil sample was formed from five different locations at the same altitude. For plant samples, moss, Qinghai spruce tissues (bark, litterfall, foliage and cone), and the above-ground parts of Salix gilashanica and Caragana jubata corresponding to the sampling points were collected 28, 21, 4, and 5, respectively. To ensure the representative of each sample of soil and moss, 3~5 subsamples at the same altitude were mixed as one sample. Therefore, we are confident that the sample we collected is representative and can justify the objectives of the study.
b) The source of As and Hg is not discussed.
We have added a description of the pollution sources and discussed in detail the sources of Hg and As in the study area (lines 77-85, 216-230, 237-248, 250-253, 266-268, 283-285, 288-292 and 314-316).
c) A number of references are listed within the text, but the discussion is poorly developed.
We have provided an in-depth discussion of the references utilized.
d) The importance of elevation in risk assessment and ecological significance of the area is not discussed.
We have added the effect of altitude on plant enrichment of Hg and As and on the spatial distribution of Hg and As in the soil (lines 372-376 and 381-385).
- e) The risk assessment analysisis not adequately presented, although it is highlighted as one of the main objectives.
We have added the risk assessment analysis of Hg and As in study area in lines 392-408.
f ) A list of point-to-point comments can be a found in the attached file.
Changed as requested.
Reviewer 2 Report
Comments for author File: Comments.pdf
Author Response
Dear Editors and Reviewers,
Thank you for your letter and for the reviewer’s comments on our manuscript. Those comments are all valuable and very helpful for revising and improving our manuscript. We have made correction and checked the Author’s Checklist, which were marked in yellow in the revised paper. The responses are presented as follows:
Responses to Reviewer 2:
This original article has a good topic. The authors need to conduct minor revision before reconsideration.
1. Abstract: Delete sentence line 19-20.
Changed as requested.
2. Introduction: Delete sentence lines 38-39.
Changed as requested.
3. Revise English in sentence line 43.
Changed as requested (lines 40-42).
4. Line 46: start with “mercury”and not th2 abbreviation. Follow this suggestion in the entire article.
Changed as requested (lines 43, 46, 241, 317, 322 and 328).
5. Line 78: Explain why mining should be the responsible for the high Hg. Which kind of mining activity do you have? Are they close to the study area?
Mercury (Hg) is a pollutant of global concern owing to its long-range atmospheric transport, persistence, bioaccumulation, and significant adverse effects on human health and the environment (Wang et al. 2016). Coal combustion, Hg, Au, Ag, Cu and Zn mining and smelting activities, sewage irrigation, and industrial and agricultural uses of Hg have led to rising mercury contamination in soils (Clarkson, 2002; Du et al., 2005). Anthropogenic metals adsorbed on suspended fine particles can be deposited on remote and inaccessible high mountains by long-range atmospheric transport (Bing et al., 2018). Meng et al. (2014) measured Hg soil concentrations in four mining districts and found that the mean total mercury concentrations in soils ranged from 2287.7±1466.5 μg/kg in Fankou to 101,734.3 ± 60,220.7 μg/kg in Wanshan. Our previous study pointed out that long-range transport of trace elements (Cu, Zn, Cd, Pb, Ni, Cr, Hg and As) has a significant impact on the Qinghai spruce ecosystems in the Qilian Mountains (Zang et al., 2021). The location of the catchment is close to Zhangye City, Sunan County, Minle County and Qilian County. Zhangye City is rich in mineral resources, with the highest concentrations of iron, copper, tungsten, molybdenum, coal, metallurgical auxiliary materials, and chemical raw materials in the whole province. Three major sources-industrial activities and waste incineration, traffic emissions and coal combustion, and biomass burning-are important sources of trace elements in wet deposition in Qinghai spruce forest (Zang et al., 2021). Moreover, we have also added some related content in the manuscript (lines 77-85).
M&M
6. Line 96: Add author name abbreviation after the species latin name.
Changed as requested (lines 97-98 and 101-102).
7. Line 106: were designed?
Changed as requested (line 111).
8. Line 117: Which part of these species was sampled? Explain it
We have added an explanation (lines 122-124).
9. Line 134: I do not understand why you dilutedsamples with HNO35% after digestion.
There are two reasons we used 5% HNO3 as the diluting solution in the experiment: (1) Reduce interference and improve the accuracy of the measurement; (2) Maintain acidic environment to prevent hydrolytic precipitation of elements and facilitate long-term preservation of samples.
10. Line 142: were used
Changed as requested (lines 150-152).
11. Delete sentence line 144-145
We have deleted these sentence.
12. Line 175 explain why you used correlation analysis
Correlation analysis is a statistical approach to quantitatively describe and analyze the degree of correlation between two or more variables. Simple correlation analysis is suitable for correlation studies between two variables (Xiang et al., 2021; Zang et al., 2021). In this study, we use pearson correlation coefficient analysis to explore the correlation between the distribution of Hg and As in soil and soil physicochemical properties (pH, SOM, CaCO3, nitrogen and phosphorus), and between the concentration of Hg and As in soil and plants (Wang et al., 2016; Li et al. 2019).
Results and Discussion
13. Line 191: was significantly
Revised as requested (line 200).
14. Line 193: The input and decomposition
Changed as requested (lines 203-205).
15. Line 236: Some studies
Changed as requested (line 258).
16. Line 290: Write the species in italics. Follow it in all the text
Revised as requested (lines 319 and 322).
17. Line 291: Are these differences statistically different? Add this information also in table 2.Add in statistical analysis how you calculate these differences between samples.
The statistical differences among Hg and As contents in plants were assessed using One-way analysis of variance (ANOVA) followed by Tukey’s test at p < 0.05 (line 188 and Table 2).
18. Line 303: use some more recent literatures.
Thank you very much for your comment, we have added references in line 334 .
19. Line 329: Add this parameter calculation in M&M.
We have added the parameter calculation of the bioconcentration factor (BCF) in M&M (lines 156-161).
20. Line 353: Delete the first part and start with“More focus should be given……”
Changed as requested (lines 407-408).
21. Line 355: Delete “was higher that As”
We have deleted these sentence.
References used in the responses:
Bing, H J., Zhou, J., Wu, Y H., Luo, X S., Xiang, Z X., Sun, H Y., Wang, J P., Zhu, H., 2018. Barrier effects of remote high mountain on atmospheric metal transport in the eastern Tibetan Plateau. Sci. Total Environ. 628, 687-696. https://doi.org/10.1016/j.scitotenv.2018.02.035.
Clarkson, T W., 2002. The three modern faces of mercury. Environ. Health Perspect. 110, 11-23. https://doi.org/10.1289/ehp.02110s111.
Du, X., Zhu, Y G., Liu, W J., Zhao, X S., 2005. Uptake of mercury (Hg) by seedlings of rice (Oryza sativa L.) grown in solution culture and interactions with arsenate uptake. Environ. Exp. Bot. 54, 1-7. https://doi.org/10.1016/j.envexpbot.2004.05.001.
Li, Y., Wang S L, Nan, Z R, Zang, F., Sun, H L, Zhang, Q., Huang, W., Bao, L L., 2019. Accumulation, fractionation and health risk assessment of fluoride and heavy metals in soil-crop systems in northwest China. Sci. Total Environ. 663, 307-314. https://doi.org/10.1016/j.scitotenv.2019.01.257.
Meng, M., Li, B., Shao, J., Wang, T., He, B., Shi, J B., Ye, Z H., Jiang, G B., 2014. Accumulation of total mercury and methylmercury in rice plants collected from different mining areas in China. Environ. Pollut. 184, 179-186. https://doi.org/10.1016/j.envpol.2013.08.030.
Wang, S., Nan, Z., Prete, D., Ma, J., Liao, Q., Zhang Q., 2016. Accumulation, transfer, and potential sources of mercury in the soil-wheat system under field conditions over the Loess Plateau, northwest China. Sci. Total Environ. 568, 245-252. https://doi.org/10.1016/j.scitotenv.2016.06.034
Xiang, M.T., Li Y., Yang, J.Y., Lei, K.G, Li, Y., Li, F., Zheng D.F., Fang X.Q., Cao,Y., 2021. Heavy metal contamination risk assessment and correlation analysis of heavy metal contents in soil and crops. Environ. Poll. 278, 116911. https://doi.org/10.1016/j.envpol.2021.116911.
Zang, F., Wang, H., Zhao, C., Nan, Z., Wang, S., Yang, J., Li, N., 2021. Atmospheric wet deposition of trace elements to forest ecosystem of the Qilian Mountains, northwest China, Catena 197. https://doi.org/10.1016/j.catena.2020.104966.
Author Response File: 
Author Response.docx
Reviewer 3 Report
The topic of the paper is of general interest, the discussion of the results is clear but not enough extended. Previous studies are mentioned but the discussion of their prospects is limited. The text is well structured, but there are quite a lot of confusing moments. Some of them are mentioned bellow.
55-57- Please take note the “In SI, a tonne (t) equals 103 kg, or 1 Mg and is understood to mean metric ton.” and Pirrone et al., 2010 mentions “Anthropogenic sources, which include a large number of industrial point sources, are estimated to account for 2320 Mg of mercury emitted annually”, which can not be 2,32x106 t.
57-58: Outridge et al speak about Hg anthropogenic emissions in terms of 2.5+0.5 kt/y kt per year and not mg yr-1
150-151: The classification of EF and other parameters is not clear. Relevant literature is absent. Gasiorek et al. (2017) do not show the classification but refer to another work.
181-183: why the comparison with Gongga and Luoji Mountains is performed? Please explain the relevance of these two sites.
221: The high Hg concentrations or the highest?
483-484: https://doi.org/10.1016/j.chemosphere.2016.10.126 do not corresponds to Nriagu, J., Pacyna, J., 1988. Quantitative assessment of worldwide contamination of air, water and soils by trace metal(loid)s. Nature 333, 134-139.
Supplementary material: Table S1 and S4. Are you sure that you have to use world “lever” and not “level”?
Author Response
Dear Editors and Reviewers,
Thank you for your letter and for the reviewer’s comments on our manuscript. Those comments are all valuable and very helpful for revising and improving our manuscript. We have made correction and checked the Author’s Checklist, which were marked in yellow in the revised paper. The responses are presented as follows:
Responses to Reviewer 3:
The topic of the paper is of general interest, the discussion of the results is clear but not enough extended. Previous studies are mentioned but the discussion of their prospects is limited. The text is well structured, but there are quite a lot of confusing moments. Some of them are mentioned bellow.
1. 55-57:Please take note the “In SI, a tonne (t) equals 103kg, or 1 Mg and is understood to mean metric ton.” and Pirrone et al., 2010 mentions “Anthropogenic sources, which include a large number of industrial point sources, are estimated to account for 2320 Mg of mercury emitted annually”, which can not be 2,32x106 t.
We really appreciate the reviewer’s careful reading of our manuscript. We have revised the contents (line 52).
2. 57-58: Outridge et al speak about Hg anthropogenic emissions in terms of 2.5+0.5 kt/y kt per year and not mg yr-1
We have corrected the contents (lines 55).
3. 150-151: The classification of EF and other parameters is not clear. Relevant literature is absent. Gasiorek et al. (2017) do not show the classification but refer to another work.
After careful verification, we have revised the EF categories in Table S1 according to Gujre et al. (2021) and added relevant literature to explain the definition and classification of EF (lines 163-172).
4. 181-183: why the comparison with Gongga and Luoji Mountains is performed? Please explain the relevance of these two sites.
Qilian Mountain, Gongga Mountain and Luoji Mountains are a famous mountainous region in China. These study areas share a degree of similarity in some aspects including vegetation evolution, topography, microclimate, etc. In addition, these areas are also low in human disturbance and far from pollution sources (Bing et al., 2014; Li et al., 2018; Rong et al. 2019). The comparison of their soil properties and pollution level facilitates the understanding of the enrichment and transport of trace elements in Qilian Mountains.
5. 221: The high Hg concentrations or the highest?
In the article, we wanted to convey that the Hg concentrations of the topsoil in the vicinity of the forest staggered zone are higher than other areas, not the highest concentration of mercury. Therefore, we chose to use the words of “the high Hg concentrations” in line 237.
6. 483-484: https://doi.org/10.1016/j.chemosphere.2016.10.126 do not corresponds to Nriagu, J., Pacyna, J., 1988. Quantitative assessment of worldwide contamination of air, water and soils by trace metal(loid)s. Nature 333, 134-139.
We have revised it (lines 525-526).
7. Supplementary material: Table S1 and S4. Are you sure that you have to use world “lever” and not “level”?
We are sorry that we made a mistake here. We have revised in Supplementary material Table S1 and S5.
References used in the responses:
Bing, H J., Wu, Y H., Zhou, J., Ming, L. , Sun S., Li X., 2014. Atmospheric deposition of lead in remote high mountain of eastern Tibetan Plateau, China, Atmos. Environ. 99, 425-435. https://doi.org/10.1016/j.atmosenv.2014.10.014.
Gujre, N., Mitra, S., Soni, A., Agnihotri, R., Rangan, L., Rene, E R, Sharma M.P., 2021. Speciation, contamination, ecological and human health risks assessment of heavy metals in soils dumped with municipal solid wastes. Chemosphere 262. https://doi.org/10.1016/j.chemosphere.2020.128013.
Li, R., Bing, H J., Wu, Y H., Zhou, J., Xiang, Z X., 2018. Altitudinal patterns and controls of trace metal distribution in soils of a remote high mountain, Southwest China. Environ. Geochem. Health 40, 505-519. https://doi.org/10.1007/s10653-017-9937-2.
Rong, Z., Zhao, C., Liu, J., Gao, Y., Zang, F., Guo, Z., Mao, Y., and Wang, L., 2019. Modeling the Effect of Climate Change on the Potential Distribution of Qinghai Spruce (Picea crassifolia Kom.) in Qilian Mountains, Forests. 10, 1. https://doi.org/10.3390/f10010062.
Round 2
Reviewer 1 Report
The authors made a great effort to improve the manuscript. However, I beleive the small number of samples still pose a serious problem to safely extract conclusions about a spatial distribution.
Reviewer 3 Report
The paper was significantly impoved.
