Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

Dear Authors,

The manuscript is interesting, below my comments:

Title: delete the point at the end, and report 2 of CO2 in subscript throughout the manuscript.

Abstract: report 2 of R2 in superscript throughout the manuscript.

Lines 215: specify how you measure the OM content, by CHN analyzer?

Regards

Author Response

Comment 1: Title: delete the point at the end, and report 2 of CO2 in subscript throughout the manuscript.

Response 1: The title was changed to the following “The variation and driving factors of soil organic carbon stocks and soil СО2 emissions in urban infrastructure: case of a university campus”

Comment 2: Abstract: report 2 of R2 in superscript throughout the manuscript

Response 2: The abstract was edited following the reviewer’s recommendations

Comment 3: Lines 215: specify how you measure the OM content, by CHN analyzer

Response 3: SOC was measured by loss-on-ignition (LOI) method (Pribyl, D. W. A critical review of the conventional SOC to SOM conversion factor. Geoderma. 2010, 156, 3–4. P. 75–83. https://doi.org/10.1016/j.geoderma.2010.02.003)

Reviewer 2 Report

Comments and Suggestions for Authors

The authors measure and model surface soil organic carbon (SOC) stocks, depth-profile soil properties to 1 m, and soil carbon dioxide (CO2) emissions on a university campus in the Netherlands. The authors use direct measurements paired with predictive digital soil mapping to generate campus-wide maps of SOC stocks and explanatory statistical analysis to understand driving factors in SOC stock and CO2 flux variation. Key results are that SOC stocks were highest in older woodland cover type soils and lowest in younger and intermediate age (since creation) herbaceous and lawn cover type soils. The opposite pattern was true for CO2 emissions due primarily to temperature differences between these cover types (lower under tree canopy). The authors conclude that shrub and tree covered urban green infrastructure may provide more carbon sequestration via higher litter inputs and productivity, while lawns may actually lose carbon over time unlike natural counterparts.

The study will be a nice contribution to the emerging urban ecosystem science focus on carbon cycle biogeochemistry and natural climate solutions. I enjoyed reading the manuscript as it is well-written and generally nicely presented though figures are quite simple and sometimes rely on bar plots that hide the underlying data distribution unlike violin plots of boxplots. The methods are described in detail and are credible. Notably, a large number of replicate surface soil samples were collected and I commend the authors on this effort. The results are presented logically and support the conclusions. 

I only have a few minor questions and comments listed here:

Introduction: Great introduction but please make sure in the introduction is aligned with the fact that this investigation and study is focused on mitigation, otherwise it would require a broader consideration of the moisture-temperature and cover type patterns in the discussion in the context of things like evaporative cooling and other adaptation-related topics.

Methods-and-Discussion: Did you explore ANOVA/regression models allowing for an interaction effect between moisture and temperature on CO2 emissions? This is an important dual-constraint trade-off observed across natural ecosystems. In either case - and it does appear that temperature effects dominated - please refer to and explain in the discussion the surprising result that you observed no obvious moisture constraint on CO2 emissions. Please in this refer to the area of weakness of your study in not measuring fluxes during the later dry season (Jul-Sep) when  moisture may be limiting. This lack of coverage is important to consider when modeling/scaling your estimates in time and space.

Minor comments:

(Line 2) The title could be: Driving factors of variation in soil organic carbon stocks and soil CO2 emissions in urban infrastructure: case of a university campus. Because urban green infrastructure is very prevalent throughout the manuscript, it would be nice to put it in the title for a nice connection with the rest of the paper.

(Line 48) replace 1 to 20 t C ha -1 year-1 by 1 to 20 t C ha^-1 year^-1

(Line 131) replace intro to into

(Lines 154-155) The figure.1 caption could define the combination as they are in the legend instead of the single vegetation type , for example, instead of having H: herbs , you would have HI: herbs intermediate, HR: herbs recent, HO: herbs old and replicate for the other vegetation type.  The caption will be longer but clearer. Also, the orange color coded for herbs and green for lawns make it difficult to see the difference between intermediate-recent and old. Replacing the color with a more distinctive color bands would improve the visibility.

(Line 175) You said 10 UGI types but based on Fig.1 it’s 11 UGI types.

Method section has the data that allows to answer the research questions

(Lines 428 and 430) Replace 0.26±0.05 gCO2 m-2h-1 by 0.26±0.05 gCO2  m^-2 h^-1

Figure 6. I'm having a hard time reconciling the narrow error bars on panels B and D with the variance observed in panels A and C. Can you confirm that the error bars are being computed on the original fluxes pooled by the cover type, rather than on already-computed averages.

Figure 7. panel C is not defined in the caption and the color legend is missing.

Text: Please include "City, Country" for each mention of another UGI study.

Replace CO2 by CO₂  throughout the entire document except for the line 63

 

Author Response

Comment 1: Introduction: Great introduction but please make sure in the introduction is aligned with the fact that this investigation and study is focused on mitigation, otherwise it would require a broader consideration of the moisture-temperature and cover type patterns in the discussion in the context of things like evaporative cooling and other adaptation-related topics.

Response 1: We have edited Introduction and specified the focus on climate mitigation rather than climate adaption or a more general C neutrality (L44 and L116)

Comment 2: Methods-and-Discussion: Did you explore ANOVA/regression models allowing for an interaction effect between moisture and temperature on CO2 emissions? This is an important dual-constraint trade-off observed across natural ecosystems. In either case - and it does appear that temperature effects dominated - please refer to and explain in the discussion the surprising result that you observed no obvious moisture constraint on CO2 emissions. Please in this refer to the area of weakness of your study in not measuring fluxes during the later dry season (Jul-Sep) when  moisture may be limiting. This lack of coverage is important to consider when modeling/scaling your estimates in time and space

Response 2: The dual effect of soil temperature and moisture was tested by multiple linear regression (L 254-257). The effect of soil moisture as well the combined effect of soil moisture and soil temperature, on soil CO2 emissions were not significant at any of the sites (based on both linear and quadratic models ). Therefore, soil moisture was not included in the further spatial modeling. the lack of observations during the driest period (July and August) may lead to an underestimation of total effluxes and could particularly overlook the effect of soil moisture. Although soil moisture did not show a significant effect on soil CO2 emissions based on available observations, it is difficult to confirm this assumption without considering the extreme values in the soil moisture distribution. The explanation was added to the Discussion section (L 592-599)

 

Comment 3 (minor comments)

(Line 2) The title could be: Driving factors of variation in soil organic carbon stocks and soil CO2 emissions in urban infrastructure: case of a university campus. Because urban green infrastructure is very prevalent throughout the manuscript, it would be nice to put it in the title for a nice connection with the rest of the paper.

(Line 48) replace 1 to 20 t C ha -1 year-1 by 1 to 20 t C ha^-1 year^-1

(Line 131) replace intro to into

(Lines 154-155) The figure.1 caption could define the combination as they are in the legend instead of the single vegetation type , for example, instead of having H: herbs , you would have HI: herbs intermediate, HR: herbs recent, HO: herbs old and replicate for the other vegetation type.  The caption will be longer but clearer. Also, the orange color coded for herbs and green for lawns make it difficult to see the difference between intermediate-recent and old. Replacing the color with a more distinctive color bands would improve the visibility.

(Line 175) You said 10 UGI types but based on Fig.1 it’s 11 UGI types.

Method section has the data that allows to answer the research questions

(Lines 428 and 430) Replace 0.26±0.05 gCO2 m-2h-1 by 0.26±0.05 gCO2  m^-2 h^-1

Figure 6. I'm having a hard time reconciling the narrow error bars on panels B and D with the variance observed in panels A and C. Can you confirm that the error bars are being computed on the original fluxes pooled by the cover type, rather than on already-computed averages.

Figure 7. panel C is not defined in the caption and the color legend is missing.

Text: Please include "City, Country" for each mention of another UGI study.

Replace CO2 by CO₂  throughout the entire document except for the line 63

Response 3: All the minor comments were taken into account and the manuscript was edited following the reviewer’s recommendation

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript reported the changes in soil organic caron (C) stocks and CO2 emissions at a campus-based scale using field-based measurement and spatial analysis approach. The results would improve the understanding the variations and driving factors of soil organic C stocks and CO2 emissions under urban green infrastructures. Some special comments were shown at the bottom for the authors' consideration.

 

1) The tile would change into "the variations and driving factors of soil organic carbon stocks and carbon dioxide emissions at a university campus".

 

2) Abstract would be refined to highlight the importance of this study.

 

3) In the introduction, the authors have given a related background for performing this study. In the M&M section, the authors mentioned the measurement of POM-C and MAOM-C in soil profiles. The relationships between the two C fractions and soil C stocks or CO2 emissions would be nicely explained in the introduction section, particularly in lawn and forest ecosystems of urban.

 

4) Line 159, topsoil sampling was performed inside this campus. What about the depth of soil sampling?

 

5) Lines 160-164: the authors mentioned the measurement of POM-C and MAOM-C in the soil profiles across eight key plots. The relationships between the two C fractions and soil C stocks or CO2 emissions should be explained in the results section and discussion section, respectively.

 

6) In the M&M section, the biomass of roots across various land uses in this campus would be mentioned, which is useful for reasonably explaining the relatively high CO2 emission and POM-C content in the young lawn plots.

 

7）In Figure2, there were 17 monitoring days for measuring soil CO2 emissions inside this campus site. Can the low-frequency measurement of soil CO2 emission present an effective method to obtain an annual soil CO2 emission across various land uses inside this campus?

 

8) In the section 2.2, the relationships between POM-C and MAOM-C with soil CO2 emissions and soil carbon stocks should be fully explained. It is nice to incorporate with the biomass of fine roots. For the sub-section, lines 206-216 belong to introduction section, and should be transferred into the introduction.

 

9) In lines 217-227, the authors mentioned the soil CO2 emissions were done during September 2022 - Jan 2023 and April 2023 -May 2023. The contents of sampling dates are not obviously different from the contents in lines 162-165, and 366-267. The authors should take care to make some necessary revisions.

 

10) In the section 2.5 data analysis and mapping, some statistical analysis would be reasonably explained here.

 

11) In lines 351-359, the authors mentioned the contents of POM-C and MAOM-C. The contents should be nicely incorporated into the soil CO2 emissions and soil C stocks. Given the authors could use the carbon content along the soil profile, a nice pattern for soil carbon stocks at 0-100 cm depth across varying land uses in the campus would improve the quality of this study,

 

12) Figure 6 contained some data about soil CO2 emissions. Here there is no message about the measurement of soil CO2 emission, which was obviously different from the M&M section and other contents in the text. The authors should take care to perform this revision.

 

13) For section 3.4, the authors use a relationship between soil temperature and soil CO2 emission to perform a spatial and temporal change in soil CO2 emission inside the campus area. Given the absence of field measurement in summer with the relatviely high soil CO2 emissions, the authors would take care to perform this extrapolation analysis.

 

14) Line 424, ...for each of 24 days?

 

15) In lines 487-496 and 540-542, the authors gave some discussion based on the POM-C and MAOM-C data. The related data would be nicely incorporated into the text, to explain the changes in soil carbon stocks and CO2 emissions, particularly those in lawn areas.

 

16) In lines 565-569, the authors mentioned soil respiration components, and their relative contribution to total soil CO2 emission. Also, the CO2 emission to soil carbon stock ratios were used to explain the turnover of soil carbon across various land uses in the campus area. Here, soil heterotrophic respiration would be taken into account, via considering the contribution of root respiration to total soil CO2 emission. 

Author Response

Comment 1: The tile would change into "the variations and driving factors of soil organic carbon stocks and carbon dioxide emissions at a university campus".

Response 1: The title was changed to the following “The variation and driving factors of soil organic carbon stocks and soil СО2 emissions in urban infrastructure: case of a university campus”

Comment 2: Abstract would be refined to highlight the importance of this study

Response 2: The abstract was edited following the reviewers’ recommendations

Comment 3: In the introduction, the authors have given a related background for performing this study. In the M&M section, the authors mentioned the measurement of POM-C and MAOM-C in soil profiles. The relationships between the two C fractions and soil C stocks or CO2 emissions would be nicely explained in the introduction section, particularly in lawn and forest ecosystems of urban

Response 3: The text linking UGI management to SOC fractions and CO₂ emissions was added (L79-89 and L94)

Comment 4: Line 159, topsoil sampling was performed inside this campus. What about the depth of soil sampling?

Response 4: The sampling depth was 0-10 cm (added to L 170) and shown on Fig. 2

Comment 5: Lines 160-164: the authors mentioned the measurement of POM-C and MAOM-C in the soil profiles across eight key plots. The relationships between the two C fractions and soil C stocks or CO2 emissions should be explained in the results section and discussion section, respectively

Response 5: Relationships between SOC fractionations and CO2 emissions were highlighted through the manuscript and discussed on L501-528

Comment 6:  In the M&M section, the biomass of roots across various land uses in this campus would be mentioned, which is useful for reasonably explaining the relatively high CO2 emission and POM-C content in the young lawn plots.

Response 6: Root biomass was not measured. However, based on visual description of the soil profiles the density of fine roots in the layer of 0-30 cm under lawns was significantly higher compared to other UGI types. The explanation was added to L 331-333

Comment 7:  In Figure2, there were 17 monitoring days for measuring soil CO2 emissions inside this campus site. Can the low-frequency measurement of soil CO2 emission present an effective method to obtain an annual soil CO2 emission across various land uses inside this campus?

Response 7: With a limited dataset of in situ soil CO2 emission measurements (17 daily measurements within a calendar year), the extrapolation of annual CO2 emissions relied on several assumptions, which contributed to uncertainty. For example, the lack of observations during the driest period (July and August) may lead to an underestimation of total effluxes and could particularly overlook the effect of soil moisture. Although soil moisture did not show a significant effect on soil CO2 emissions based on available observations, it is difficult to confirm this assumption without considering the extreme values in the soil moisture distribution. The explanation was added to the text (L 592-599)

Comment 8: In the section 2.2, the relationships between POM-C and MAOM-C with soil CO2 emissions and soil carbon stocks should be fully explained. It is nice to incorporate with the biomass of fine roots. For the sub-section, lines 206-216 belong to introduction section, and should be transferred into the introduction.

Response 8: The text was edited following the reviewer’s recommendations

Comment 9: In lines 217-227, the authors mentioned the soil CO2 emissions were done during September 2022 - Jan 2023 and April 2023 -May 2023. The contents of sampling dates are not obviously different from the contents in lines 162-165, and 366-267. The authors should take care to make some necessary revisions.

Response 9: The CO2 observations were conducted during September 2022 - Jan 2023 and April 2023 -May 2023 (edited through the whole text). The period June 2022 – June 2023 refers to the meteorological observations

Comment 10: In the section 2.5 data analysis and mapping, some statistical analysis would be reasonably explained here.

Response 10: We added some clarification on the regression models. Apart from this, statistical analysis seems sufficiently described in 2.5 from our point of view.

Comment 11: In lines 351-359, the authors mentioned the contents of POM-C and MAOM-C. The contents should be nicely incorporated into the soil CO2 emissions and soil C stocks. Given the authors could use the carbon content along the soil profile, a nice pattern for soil carbon stocks at 0-100 cm depth across varying land uses in the campus would improve the quality of this study

Response 11: Comments on the relationships between SOC fractions and CO₂ emissions were added to the text (L 392-398)

Comment 12: Figure 6 contained some data about soil CO2 emissions. Here there is no message about the measurement of soil CO2 emission, which was obviously different from the M&M section and other contents in the text. The authors should take care to perform this revision.

Response 12: Fig. 6 reflects the results on soil CO2 emissions measured following the methodology described in M&M (2.4). Fig. 6 A shows seasonal dynamics in average (mean and standard error for all plots per observation date). Fig. 6 B shows average (mean and standard error for the observation period per plot) soil CO2 emission

Comment 13: For section 3.4, the authors use a relationship between soil temperature and soil CO2 emission to perform a spatial and temporal change in soil CO2 emission inside the campus area. Given the absence of field measurement in summer with the relatviely high soil CO2 emissions, the authors would take care to perform this extrapolation analysis.

Response 13: see the response 7

Comment 14: Line 424, ...for each of 24 days?

Response 14: Yes, the primary CO2 maps were developed for the days with available LST and at the next step they were averaged on a monthly basis

Comment 15: In lines 487-496 and 540-542, the authors gave some discussion based on the POM-C and MAOM-C data. The related data would be nicely incorporated into the text, to explain the changes in soil carbon stocks and CO2 emissions, particularly those in lawn areas.

Response 15: Comments on the relationships between SOC fractions and CO₂ emissions were added to the Results (L 392-398) and Discussion (L 513-28)

Comment 16: In lines 565-569, the authors mentioned soil respiration components, and their relative contribution to total soil CO2 emission. Also, the CO2 emission to soil carbon stock ratios were used to explain the turnover of soil carbon across various land uses in the campus area. Here, soil heterotrophic respiration would be taken into account, via considering the contribution of root respiration to total soil CO2 emission. 

Response 16: The explanation is given in L579-587