Molecular Composition of Stream Dissolved Organic Matter in Cool-Temperate Forest Headwaters with Landslides, Northern Japan

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigated the molecular composition of dissolved organic matter (DOM) in streams within headwater landslide areas in the northern cool-temperate forest of Japan. The findings revealed that vegetation conditions can influence the quality of DOM in streams. Through hydrological observations and sampling of stream water from several small forest catchment basins, researchers discovered that the catchment with no disturbance had the highest AIwa index, followed by catchments with landslide coverage of 16% and 52%. These results indicate that the quality of DOM depends not only on its concentration but also on the level of vegetation disturbance in headwater landslide areas of the cool-temperate forest. Furthermore, the study found that although landslides impact water quality and biogeochemical cycles, the recovery of vegetation can mitigate these effects. This is a manuscript of high quality, and I agree that it can be considered for publication after minor revisions.
Comments
1)Introduction:"Therefore, a simple and comprehensible criterion is required to evaluate the recovery of forest ecosystems." Please summarize and sort out what's the evaluation criteria?
2)The study primarily focused on the relationship between DOM quality and vegetation conditions, but may not have fully considered other factors that could potentially affect DOM characteristics, such as climate change, soil properties, and microbial activities.  These factors may exert significant impacts on the quality and recovery process of DOM.
3)Although the study conducted sampling and analysis in three forest watersheds with different degrees of landslide coverage, the sample size may still be limited, which may affect the statistical significance and reliability of the results.  Increasing the sample size may help enhance the conclusions of the study.
4)The article mentioned that the recovery process of forest ecosystems in cold regions is complex and difficult to perceive, which implies another potential deficiency in research methods: it may oversimplify the process and mechanisms of ecological recovery.  In fact, the recovery of forest ecosystems may involve the interaction of various biological and non-biological factors, and the complex relationships between these factors may be difficult to fully reveal through a single research method.
5)Although FT-ICR-MS was used to analyze the molecular composition of DOM, any technology has its limitations.  It would be more effective if DOM parallel factor analysis could be combined with FT-ICR-MS results for analysis.
6)A conclusion section needs to be added.

Author Response

We greatly appreciate the reviewer’s positive comments and suggestions, which have been helpful for our revision. Detailed responses are provided below.

 

Comment: 1)Introduction:"Therefore, a simple and comprehensible criterion is required to evaluate the recovery of forest ecosystems." Please summarize and sort out what's the evaluation criteria?

Response: Based on the comment, we changed "criterion" to "indicator" in Line 50 to avoid readers' confusion and correspond to the sentences, "Therefore, a detailed evaluation of the quality of stream DOM could be a useful indicator of the recovery of forest ecosystems. However, few studies have examined the molecular composition of stream DOM as an indicator of the recovery of forest ecosystems after disturbances induced by massive disasters, due to the limitations of analytical techniques" in Lines 61–65.

 

Comment: 2)The study primarily focused on the relationship between DOM quality and vegetation conditions, but may not have fully considered other factors that could potentially affect DOM characteristics, such as climate change, soil properties, and microbial activities.  These factors may exert significant impacts on the quality and recovery process of DOM.

Response: As the reviewer suggested, changes in soil properties and subsequent flow paths caused by disturbances, such as landslides, may alter the molecular composition of stream DOM in forest headwaters. Therefore, exploring the relationships between DOM molecular parameters, such as aromaticity index, and observation items regarding the physical environment, such as soil properties, water and sediment runoff, could enhance the evaluation of the recovery of vegetation and subsequent ecosystem services following a disturbance. We added these descriptions in the Discussion section (Lines 328–334). We also added the description, "further research is needed to understand how vegetation conditions affect changes in the molecular composition of stream DOM during rain events in cool-temperate forest headwaters" because several studies have implicated that the magnitude and frequency of precipitation change in response to climate change in cold-region forests (Lines 334–339). Because of these additions, we cited the literature in Lines 330, 335 and 336. The relationship between microbial activities and DOM has been described in the Introduction section (Line 55).

 

Comment: 3)Although the study conducted sampling and analysis in three forest watersheds with different degrees of landslide coverage, the sample size may still be limited, which may affect the statistical significance and reliability of the results.  Increasing the sample size may help enhance the conclusions of the study.

Response: The quality of stream DOM can dramatically change depending on runoff during rain events, as described in Wagner et al. (2019). Therefore, hierarchical differences in molecular parametes of stream DOM among the three catchments could be more prominent during rain events than during base flow conditions. As the reviewer suggested, increasing the sample size by conducting the intensive water sampling during rain events may enhance our understanding of how vegetation conditions affect the quality of stream DOM in cool-temperate forest headwaters. We added this in the Discussion section (Lines 336–340).

 

Comment: 4)The article mentioned that the recovery process of forest ecosystems in cold regions is complex and difficult to perceive, which implies another potential deficiency in research methods: it may oversimplify the process and mechanisms of ecological recovery.  In fact, the recovery of forest ecosystems may involve the interaction of various biological and non-biological factors, and the complex relationships between these factors may be difficult to fully reveal through a single research method.

Response: Based on the comment, we added the sentence, "exploring the relationships between molecular parameters and observation items regarding the physical environment, such as soil properties, water and sediment runoff, could enhance the evaluation of the recovery of vegetation and subsequent ecosystem services following a disturbance" in the Discussion section (Lines 330–334) as I mentioned earlier. We also added the description related to the fact that the combination of the physical environment observations with ultrahigh-resolution mass spectral analyses for detecting the molecular composition of stream DOM will improve our understanding of the relationship between the DOM molecular composition and vegetation conditions associated with the recovery processes of forest ecosystems in cold regions (Lines 337–340).

 

Comment: 5)Although FT-ICR-MS was used to analyze the molecular composition of DOM, any technology has its limitations.  It would be more effective if DOM parallel factor analysis could be combined with FT-ICR-MS results for analysis.

Response: Based on the comment, we added the sentence, "Conversely, the combination of optical spectral analyses with AI would provide more complementary information for DOM characterization in stream water" in the Discussion section (Lines 318–319). Because of this addition, we cited the literature in L319 and changed "Therefore" to "Taken together" in Line 320.

 

Comment: 6)A conclusion section needs to be added.

Response: In accordance with the comment, we added the Conclusions section (Lines 341–357).

 

Cited literature:

Wagner, S., Fair, J. H., Matt, S., Hosen, J. D., Raymond, P., Saiers, J., Shanley, J. B., Dittmar, T., & Stubbins, A. (2019). Molecular hysteresis: hydrologically driven changes in riverine dissolved organic matter chemistry during a storm event. Journal of Geophysical Research: Biogeosciences, 124(4), 759-774.

Reviewer 2 Report

Comments and Suggestions for Authors

This study clearly shows that DOC and vascular plant-derived DOM differ among different vegetation conditions.
The manuscript is well written and I have no specific comments to correct regarding minor points.

However, as the authors mention in the Introduction section,
"Therefore, the restoration of forest headwaters is an urgent issue to be addressed in the areas damaged by massive disasters", and "Therefore, it remains unclear how long and how much the recovery of vegetation and subsequent ecosystem services can be expected in cold-region forests", it would be good if the last sentence of the Introduction section and the Discussion section described what role this study plays in them.

Author Response

We greatly appreciate the reviewer's positive comment and suggestion, which have been helpful for our revision. A detailed response is provided below.

 

Comment: This study clearly shows that DOC and vascular plant-derived DOM differ among different vegetation conditions.
The manuscript is well written and I have no specific comments to correct regarding minor points.

However, as the authors mention in the Introduction section,
"Therefore, the restoration of forest headwaters is an urgent issue to be addressed in the areas damaged by massive disasters", and "Therefore, it remains unclear how long and how much the recovery of vegetation and subsequent ecosystem services can be expected in cold-region forests", it would be good if the last sentence of the Introduction section and the Discussion section described what role this study plays in them.

 

Response: As described in the Introduction section, the role of this study was to investigate whether a detailed evaluation of the quality of stream DOM could be a useful indicator of the recovery of forest ecosystems. Based on the comment, to emphasize the role of this study, we added the sentence, "Water chemistry in a headwater stream is the product of the natural ecosystem, the control of which can be altered by disturbances" in the Introduction section (Lines 59–61). Because of this addition, we cited the literature in Line 61. Furthermore, we added the Conclusions section, which describes that molecular species in stream DOM reflect vegetation conditions and therefore may be a useful indicator for evaluating the recovery of vegetation following a disturbance in cold-region forest catchments (Lines 341–357).

Reviewer 3 Report

Comments and Suggestions for Authors

The title reflects the aim addressed by the research: to examine whether vegetation conditions were reflected in the molecular species of stream dissolved organic matter DOM in cold-region forest catchments, in particularly in cool-temperate forest headwaters in northern Japan. This work is not original. The study is documented, well designed, the scientific explanations at each chapter/subchapter are clear, concise, the views regarding database were used accordingly. The authors respect the requirements of the journal.

Suggestions: although the journal's requirements do not consider it mandatory, I recommend for authors to prepare a short material using the ideas/sentences from the Discussion chapter for a last chapter: CONCLUSIONS, with emphasizing the particularities of their experiment compared to others experiments from the same research area. 

Author Response

We greatly appreciate the reviewer's positive comment and suggestion, which have been helpful for our revision. A detailed response is provided below.

 

Suggestions: although the journal's requirements do not consider it mandatory, I recommend for authors to prepare a short material using the ideas/sentences from the Discussion chapter for a last chapter: CONCLUSIONS, with emphasizing the particularities of their experiment compared to others experiments from the same research area. 

Response: In accordance with the comment, we added the Conclusions section (Lines 341–357). No studies have applied ultrahigh-resolution mass spectral analyses, i.e., FT-ICR-MS analyses, to the detection of the molecular composition of stream DOM from cool-temperate forest headwaters with landslides. Therefore, in the Conclusions section, we emphasized the particularity of our research design, that is, the application of FT-ICR-MS analyses to stream DOM in three cool-temperate forest catchments with different landslide coverage to detect changes in the quality of DOM associated with vegetation conditions. Our research design could overcome laborious and time-consuming observations to evaluate the extent to which forest ecosystems and their services recover over time in cold-region forest headwaters, where the growth of vegetation is typically slow and often imperceptible.