the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Nitrogen limitation information retrieved from data assimilation
Song Wang
Carlos Sierra
Yiqi Luo
Jinsong Wang
Weinan Chen
Yahai Zhang
Aizhong Ye
Abstract. Nitrogen (N) limitation greatly constrains terrestrial ecosystem carbon (C) uptake and its response to climate change and elevated carbon dioxide. Hence, accurate assessments of ecosystem N limitation are crucial for predicting C-N feedbacks, and vital for providing guidance for policy making or ecosystem management as well. This study aims to retrieve N limitation information by data model fusion from one field N addition experiment so that we can better understand N controls on the terrestrial C cycle. We estimated two sets of parameters with one C-only model and one coupled C-N model. Our results showed that the estimated leaf photosynthetic efficiency (LPE) and process rates (e.g., senescence and decomposition rates) of organic C from almost all pools were higher with the coupled C-N model than those with the C-only model at the ambient treatment. However, the differences in the LPE and the C exit rates between the coupled C-N model and the C-only model decreased with the increasing N addition rates. Both the C-only and coupled C-N models simulated similar C pool sizes as observed at every N addition treatment with their respective parameter estimates. However, simulated ecosystem C storage and gross primary productivity (GPP) decreased if we ran the coupled C-N model with the parameters estimated by the C-only model. This decrease was larger at the ambient treatment and became smaller with the increase of N addition. In general, we put forward a new method to retrieve N limitation information from observations by data model fusion. This method will make it possible to estimate the global nutrient limitation and benefit ecosystem management and policy making.
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Song Wang et al.
Status: closed
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RC1: 'Comment on bg-2023-33', Anonymous Referee #1, 31 Mar 2023
Peer Review for bg-2023-33
Summary
The paper “Nitrogen limitation information retrieved from data assimilation” by Wang et al. proposes a method to apply data from nitrogen addition experiments in combination with calibration of C-only and CN-coupled models to obtain information on nitrogen limitation. By comparing the calibrated values of either model setup and by comparing the predicted carbon pools, they claim to identify ecosystem processes that are responsible for nitrogen limitation in any given vegetation model. The paper tries to disentangle different carbon-nitrogen feedbacks to guide future model development regarding potential nitrogen limitation under elevated CO2.
However, the paper shows three major issues: (1) The methodology and results are not suitable for inferring main conclusions. (2) The paper has multiple insufficiencies regarding the quality of science and presentation (e.g., unclear use of concepts and missing definitions, imprecise wording, and hard-to-read display items). (3) The use of references is inadequate (e.g., methodology and Figure 1 are largely copied from a published but not referenced paper, referenced publications are mostly from authors, lack of references to remaining literature, and some references in the main text are not in the bibliography).
Moreover, most of the criteria of Biogeosciences are not fulfilled. Even if these major issues are addressed, the paper is unlikely to be worthy of publication. Therefore, this paper is recommended to be rejected.
Criteria of Biogeosciences Journal
Does the paper address relevant scientific questions within the scope of BG?
- Yes. It combines modelling and data assimilation to inform on relevant carbon-nitrogen feedback processes.Does the paper present novel concepts, ideas, tools, or data?
- No. The methods are largely the same as in Wang et al. 2022.Are substantial conclusions reached?
- No. The two main conclusions (the potential of global upscaling and that current ESMs overestimate the effect of nitrogen limitation) cannot be derived from the presented results.Are the scientific methods and assumptions valid and clearly outlined?
- Insufficient. The applied GECO model is described only at a high-level, and relevant carbon-nitrogen dynamics of the model are not explained (response to N-deposition, changes in carbon-to-nitrogen ratios of plant tissues, changes in allocation, nitrogen uptake). Additionally, parameters from structurally different models (C-only and CN-coupled models) cannot be compared directly, which is a basic assumption made in the paper.Are the results sufficient to support the interpretations and conclusions?
- No. As argued above, the results do not support the main conclusions.Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?
- No. The methodology is insufficiently explained: Some references are not listed in the list of references (e.g., Shi et al. 2016), and there is no reference to access the code to reproduce the results (data is accessible, but the figshare-link is broken).Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
- No. Methodology and Figure 1 are largely the same as in Wang et al. 2022 without reference.Does the title clearly reflect the contents of the paper?
- Partially. The title could be improved to indicate that the paper compares C- and CN-Models.Does the abstract provide a concise and complete summary?
- Partially. The abstract misses a connection from stating multiple results to the final conclusion.Is the overall presentation well-structured and clear?
- No. Part of the methodology is only introduced in the results section. Subsections in the Discussion are not separated as indicated by their headings. The discussion holds copied parts from the introduction.Is the language fluent and precise?
- No. The text holds multiple unclear formulations and multiple redundant sentences.Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?
- Yes, with the exception of formula (1).Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
- Yes. Large parts should be revisited due to redundancy and lack of argumentation (see recommendations below).Are the number and quality of references appropriate?
- Partially. References are often from co-authors, and there is little connection to the remaining literature.Is the amount and quality of supplementary material appropriate?
- Partially. The supplementary material gives insight into some model dynamics, but a detailed model description should have been provided.Major Issues
- The paper lacks a sufficient description of its methodology and requires more explicit discussion and contextualisation of the results with respect to recent publications on nitrogen limitation processes, such as Zaehle et al. 2014, Jiang et al. 2019, Arora et al. 2020, Davies-Barnard et al. 2020. The text should be improved at the following positions:
- L94: For readers not familiar with the GECO model, it would be useful if its processes were explained in more detail. More importantly, to derive any information on nitrogen limitation, the model’s carbon-nitrogen feedback must be explained clearly (e.g., response to N-deposition, changes in carbon-to-nitrogen ratios of plant tissues, changes in allocation, nitrogen uptake). Also, without a process-based explanation of GECO’s carbon-nitrogen dynamics and without comparison to representations in other Earth System Models, one cannot draw conclusions on whether current Earth System Models simulate N limitation realistically or not.
- L157: The definition of the N-limitation matrix is presumably smaller than 1 (not clearly defined in the methodology). This implies that the calibrated parameters of the CN-model will be larger than the parameters of the C-only model. This behaviour is visible in the results, and therefore the discussed difference in model parameters is a methodological consequence and holds no information on N-limitation. A strong argument needs to be provided why calibrated parameters from models with different structures should be directly comparable.
- L325: The proposed upscaling approach is not convincing because it lacks a more detailed explanation. The method seems not to be generalizable across space, which weakens any inference on global nitrogen limitation effects (as done in the abstract and conclusion). The authors should clarify the following questions: What gives the confidence that other data assimilation studies do not need N addition data? How is a “nitrogen limitation degree” quantifiable? What are “basic C and N conditions” and “global data products”, and how can the latter hold information on the former? How can results from different models be inter-comparable to achieve “global nutrient limitation distributions”?
- L338: It is not apparent how the results can inform ecosystem services and management. This requires a more detailed explanation.
- L369: The authors state that some parameters are significantly different across different N addition treatments, whilst other parameters are not. This requires more detail and should address the following questions: Which are the exact parameters that are referred to here? Is it reasonable that the respective parameters are (not) influenced by nitrogen limitation? Does this agree with findings from other studies?
- L404: The final sentence of the paragraph makes a strong claim that current ESMs overestimate N constraints. This requires more argumentation than stating it is a consequence of not re-calibrating CN-models. What are the parameters that need to be re-calibrated most urgently? Are other studies also hinting towards too stringent N limitation in current ESMs?
- Quality of presentation (Figure listed here, specific sections in text are listed in under "Minor Issues" below):
- Figure 1: It is unclear what the light-blue “Regulation” arrow denotes.
- Figure 2b-g: Plots are generally too small. Axes are not legible. It is difficult to judge whether the predicted interannual cycle matched observations.
- Figures 4 and 5: Increase font size, respectively, size of the entire figure.
- Figure 7: Description of upscaling procedure should be more exhaustive because it is not evident from the concept figure alone.
- Regarding references
- Intransparent re-use of published methods: Large parts (e.g. Figure 1) of the methodology are identical to Wang et al. 2022, which requires clear referencing.
- The bibliography does not include all references from the main text (e.g. Shi et al. 2016, Koven et al., 2013; Sokolov et al., 2008; Zaehle and Friend, 2010)
- Friedlingstein et al. 2022 (Global Carbon Budget 2022) is a carbon accounting paper and is not a suitable reference for the uncertain feedback of nitrogen limitation to climate change across Earth System Models. For example, Chapter 5 of IPCC (2021) and the references therein are more suitable.
- The bibliography relies strongly on references from co-authors. A broader literature base is necessary to reflect critically on the results and to support conclusions.
Minor Issues
- The paper often mixes different types of statements that should be separated:
- L186: At the end of the model description, a key part of the method is described. This should appear towards the beginning of the section so that the reader knows why the model description is needed at all.
- L282: Here, a new methodology is introduced. This belongs to the methods and not to the results.
- L331: This is an exact repetition of the relevance statement from the introduction. This should either be removed or shortened and moved to the beginning of the discussion as a short reminder for the reader.
- The discussion holds multiple lines at which the same information (the finding that information on N-limitation is extractable from comparing calibrated parameters) is repeated. This should be largely condensed. Instead, the discussion should be used to discuss the plausibility of the findings and whether they support the conclusion or not. This crucial scientific process is missing.
- The following lines require clarifications:
- L51: What are “thresholds of leaf N:P ratios”?
- L61: It is unlikely that “all data assimilation studies” that exist were considered. A different adjective should be used, or a specific subset of data assimilation studies should be clarified.
- L65: What is meant by “Varying parameters” as a modeling approach?
- L138: What is meant by the leaf photosynthetic efficiency, and how is this connected to foliar C:N ratios? Also, formula (1) does not define LPE but SNvcmax. Are they the same? If not, what is SNvcmax? Also, what are the “estimated and defined C:N ratios of foliage”? How are they defined?
- L186: The term C exit rates are introduced, but the reader is left alone with figuring out what these are. Therefore, they should be listed right away.
- Results: Section 3.3 nicely guides the reader to the subplots in the figures to describe the results. This is missing in Section 3.2, which makes it difficult to understand.
- L229: What is the definition of a “well-constrained parameter”? There is no reference to any Figure or metric.
- L370: What is the definition of significance to assess whether the estimated model parameters were significantly different?
- L377: The sentence says that models with different structures should show similar dynamics, which is a confusing statement. Should the sentence rather state that the same model applied to the same ecosystem across different N treatments should show similar responses?
- L396: Unclear conceptualisation of N limitation. What is meant by “the decrease of the simulation”?
- L420: Detailed explanation is needed because it is not obvious how C exit rates cause a unimodal response in C pools and how a unimodal response provides information on nitrogen limitation.
- The quality of language limits readability. The final version should be proofread by a native speaker to resolve semantic issues. Sentences that were difficult to read are on lines 45 (predicting C cycle), 46 (which remain), 63 (because-clause refers to no other sentence), 138 (sentence has no verb), 145 (The use of “similarly” is inappropriate), 158 (“no” should be “not”), 308 (sentence has no verb), 362 (unclear to what “their” refers to), 364 (unclear to what “those variables” refers to)
- Formatting should be improved on lines 198, 203, 215, 218, 219, and 267.
ReferencesArora, Vivek K., Anna Katavouta, Richard G. Williams, Chris D. Jones, Victor Brovkin, Pierre Friedlingstein, Jörg Schwinger, et al. 2020. “Carbon–Concentration and Carbon–Climate Feedbacks in CMIP6 Models and Their Comparison to CMIP5 Models.” Biogeosciences 17 (16): 4173–4222. https://doi.org/10.5194/bg-17-4173-2020.
Davies-Barnard, Taraka, Johannes Meyerholt, Sönke Zaehle, Pierre Friedlingstein, Victor Brovkin, Yuanchao Fan, Rosie A. Fisher, et al. 2020. “Nitrogen Cycling in CMIP6 Land Surface Models: Progress and Limitations.” Biogeosciences 17 (20): 5129–48. https://doi.org/10.5194/bg-17-5129-2020.
IPCC. 2021. “Chapter 5: Global Carbon and Other Biogeochemical Cycles and Feedbacks.” In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, 673–816. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. 10.1017/9781009157896.007
Jiang, Mingkai, Sönke Zaehle, Martin G. De Kauwe, Anthony P. Walker, Silvia Caldararu, David S. Ellsworth, and Belinda E. Medlyn. 2019. “The Quasi-Equilibrium Framework Revisited: Analyzing Long-Term CO2 Enrichment Responses in Plant–Soil Models.” Geoscientific Model Development 12 (5): 2069–89. https://doi.org/10.5194/gmd-12-2069-2019.
Zaehle, Sönke, Belinda E. Medlyn, Martin G. De Kauwe, Anthony P. Walker, Michael C. Dietze, Thomas Hickler, Yiqi Luo, et al. 2014. “Evaluation of 11 Terrestrial Carbon–Nitrogen Cycle Models against Observations from Two Temperate Free-Air CO2 Enrichment Studies.” New Phytologist 202 (3): 803–22. https://doi.org/10.1111/nph.12697.
Wang, Song, Yiqi Luo, and Shuli Niu. “Reparameterization Required After Model Structure Changes From Carbon Only to Carbon‐Nitrogen Coupling.” Journal of Advances in Modeling Earth Systems 14, no. 4 (April 2022). https://doi.org/10.1029/2021MS002798.
Citation: https://doi.org/10.5194/bg-2023-33-RC1 -
AC1: 'Reply on RC1', Song Wang, 12 May 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2023-33/bg-2023-33-AC1-supplement.pdf
- The paper lacks a sufficient description of its methodology and requires more explicit discussion and contextualisation of the results with respect to recent publications on nitrogen limitation processes, such as Zaehle et al. 2014, Jiang et al. 2019, Arora et al. 2020, Davies-Barnard et al. 2020. The text should be improved at the following positions:
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RC2: 'Comment on bg-2023-33', Anonymous Referee #2, 08 Apr 2023
Wang et al. aim to retrieve N limitation information via data model fusion using a C-only model and coupled C-N model. They applied this approach to a field N enrichment experiment at an alpine meadow in the Qinghai-Tibet Plateau. The topic of nutrient limitation is of increasing importance in view of their role in constraining future land C sink in response to rising air CO2. I agree with most comments by Reviewer 1 and I have several additional comments that may further help to improve this work.
First, the term of nitrogen limitation needs to be clearly defined. Do you mean N limitation to plants, microbes or both? We don’t usually say “ecosystem N limitation”. Please also define “N limitation information” and clearly show how this is quantified in this study. Additionally, this manuscript seems to set up a background that N limitation occurs everywhere (L35-44). It would be helpful to provide an update of this view and mention that P instead of N is limiting in many tropical and subtropical ecosystems. The limitation by other nutrients needs to be mentioned or discussed in this manuscript. Second, there are many undefined terms and missing information in this manuscript (see specific comments). This hinders an in-depth evaluation of this work. Moreover, model structure and data assimilation are described in the method section but it is unclear how the two questions of this study were addressed (L91-93).Specific comments
L17-18 & 31: Exactly, how to provide guidance for policy making or ecosystem management?
L24: Explain “carbon exit rates”
L41-43: P limitation is also important but fully ignored.
L45-48: The best way to address N limitation issues in earth system models is better understanding and representation of various N cycling processes (especially biological N fixation).
L45-55: Uncertainties are discussed for these methods. I would expect a description of the advantage of the data model fusion approach.
L68: Explain “ecological information”
L72-74: Any advantage (e.g., more accuracy) in comparison to other approaches(45-55)?
L96-103: Provide additional information such as whether plant growth is N limited in the studied meadow and the level of N deposition.
L139: The equation is not clear.
L227: Make sure that the GECO model simulated C pools of microbes.
L231-233: Again, explain “C exit rate”
Figure 3: Does it mean C-N model overestimate LPE or C-only model underestimate LPE?
Figure 4: Please add titles for each axis
Figure 5&6: It would be good to include the field observed data in comparison to the modelled results.
Figures 3-6: Any significant differences between modeling results?
L282-293: Not clear how N limitation was quantified and compared here. Do you mean the strength of N limitation changes with different levels of N additions for the same meadow?Citation: https://doi.org/10.5194/bg-2023-33-RC2 -
AC2: 'Reply on RC2', Song Wang, 12 May 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2023-33/bg-2023-33-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Song Wang, 12 May 2023
Status: closed
-
RC1: 'Comment on bg-2023-33', Anonymous Referee #1, 31 Mar 2023
Peer Review for bg-2023-33
Summary
The paper “Nitrogen limitation information retrieved from data assimilation” by Wang et al. proposes a method to apply data from nitrogen addition experiments in combination with calibration of C-only and CN-coupled models to obtain information on nitrogen limitation. By comparing the calibrated values of either model setup and by comparing the predicted carbon pools, they claim to identify ecosystem processes that are responsible for nitrogen limitation in any given vegetation model. The paper tries to disentangle different carbon-nitrogen feedbacks to guide future model development regarding potential nitrogen limitation under elevated CO2.
However, the paper shows three major issues: (1) The methodology and results are not suitable for inferring main conclusions. (2) The paper has multiple insufficiencies regarding the quality of science and presentation (e.g., unclear use of concepts and missing definitions, imprecise wording, and hard-to-read display items). (3) The use of references is inadequate (e.g., methodology and Figure 1 are largely copied from a published but not referenced paper, referenced publications are mostly from authors, lack of references to remaining literature, and some references in the main text are not in the bibliography).
Moreover, most of the criteria of Biogeosciences are not fulfilled. Even if these major issues are addressed, the paper is unlikely to be worthy of publication. Therefore, this paper is recommended to be rejected.
Criteria of Biogeosciences Journal
Does the paper address relevant scientific questions within the scope of BG?
- Yes. It combines modelling and data assimilation to inform on relevant carbon-nitrogen feedback processes.Does the paper present novel concepts, ideas, tools, or data?
- No. The methods are largely the same as in Wang et al. 2022.Are substantial conclusions reached?
- No. The two main conclusions (the potential of global upscaling and that current ESMs overestimate the effect of nitrogen limitation) cannot be derived from the presented results.Are the scientific methods and assumptions valid and clearly outlined?
- Insufficient. The applied GECO model is described only at a high-level, and relevant carbon-nitrogen dynamics of the model are not explained (response to N-deposition, changes in carbon-to-nitrogen ratios of plant tissues, changes in allocation, nitrogen uptake). Additionally, parameters from structurally different models (C-only and CN-coupled models) cannot be compared directly, which is a basic assumption made in the paper.Are the results sufficient to support the interpretations and conclusions?
- No. As argued above, the results do not support the main conclusions.Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?
- No. The methodology is insufficiently explained: Some references are not listed in the list of references (e.g., Shi et al. 2016), and there is no reference to access the code to reproduce the results (data is accessible, but the figshare-link is broken).Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
- No. Methodology and Figure 1 are largely the same as in Wang et al. 2022 without reference.Does the title clearly reflect the contents of the paper?
- Partially. The title could be improved to indicate that the paper compares C- and CN-Models.Does the abstract provide a concise and complete summary?
- Partially. The abstract misses a connection from stating multiple results to the final conclusion.Is the overall presentation well-structured and clear?
- No. Part of the methodology is only introduced in the results section. Subsections in the Discussion are not separated as indicated by their headings. The discussion holds copied parts from the introduction.Is the language fluent and precise?
- No. The text holds multiple unclear formulations and multiple redundant sentences.Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?
- Yes, with the exception of formula (1).Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
- Yes. Large parts should be revisited due to redundancy and lack of argumentation (see recommendations below).Are the number and quality of references appropriate?
- Partially. References are often from co-authors, and there is little connection to the remaining literature.Is the amount and quality of supplementary material appropriate?
- Partially. The supplementary material gives insight into some model dynamics, but a detailed model description should have been provided.Major Issues
- The paper lacks a sufficient description of its methodology and requires more explicit discussion and contextualisation of the results with respect to recent publications on nitrogen limitation processes, such as Zaehle et al. 2014, Jiang et al. 2019, Arora et al. 2020, Davies-Barnard et al. 2020. The text should be improved at the following positions:
- L94: For readers not familiar with the GECO model, it would be useful if its processes were explained in more detail. More importantly, to derive any information on nitrogen limitation, the model’s carbon-nitrogen feedback must be explained clearly (e.g., response to N-deposition, changes in carbon-to-nitrogen ratios of plant tissues, changes in allocation, nitrogen uptake). Also, without a process-based explanation of GECO’s carbon-nitrogen dynamics and without comparison to representations in other Earth System Models, one cannot draw conclusions on whether current Earth System Models simulate N limitation realistically or not.
- L157: The definition of the N-limitation matrix is presumably smaller than 1 (not clearly defined in the methodology). This implies that the calibrated parameters of the CN-model will be larger than the parameters of the C-only model. This behaviour is visible in the results, and therefore the discussed difference in model parameters is a methodological consequence and holds no information on N-limitation. A strong argument needs to be provided why calibrated parameters from models with different structures should be directly comparable.
- L325: The proposed upscaling approach is not convincing because it lacks a more detailed explanation. The method seems not to be generalizable across space, which weakens any inference on global nitrogen limitation effects (as done in the abstract and conclusion). The authors should clarify the following questions: What gives the confidence that other data assimilation studies do not need N addition data? How is a “nitrogen limitation degree” quantifiable? What are “basic C and N conditions” and “global data products”, and how can the latter hold information on the former? How can results from different models be inter-comparable to achieve “global nutrient limitation distributions”?
- L338: It is not apparent how the results can inform ecosystem services and management. This requires a more detailed explanation.
- L369: The authors state that some parameters are significantly different across different N addition treatments, whilst other parameters are not. This requires more detail and should address the following questions: Which are the exact parameters that are referred to here? Is it reasonable that the respective parameters are (not) influenced by nitrogen limitation? Does this agree with findings from other studies?
- L404: The final sentence of the paragraph makes a strong claim that current ESMs overestimate N constraints. This requires more argumentation than stating it is a consequence of not re-calibrating CN-models. What are the parameters that need to be re-calibrated most urgently? Are other studies also hinting towards too stringent N limitation in current ESMs?
- Quality of presentation (Figure listed here, specific sections in text are listed in under "Minor Issues" below):
- Figure 1: It is unclear what the light-blue “Regulation” arrow denotes.
- Figure 2b-g: Plots are generally too small. Axes are not legible. It is difficult to judge whether the predicted interannual cycle matched observations.
- Figures 4 and 5: Increase font size, respectively, size of the entire figure.
- Figure 7: Description of upscaling procedure should be more exhaustive because it is not evident from the concept figure alone.
- Regarding references
- Intransparent re-use of published methods: Large parts (e.g. Figure 1) of the methodology are identical to Wang et al. 2022, which requires clear referencing.
- The bibliography does not include all references from the main text (e.g. Shi et al. 2016, Koven et al., 2013; Sokolov et al., 2008; Zaehle and Friend, 2010)
- Friedlingstein et al. 2022 (Global Carbon Budget 2022) is a carbon accounting paper and is not a suitable reference for the uncertain feedback of nitrogen limitation to climate change across Earth System Models. For example, Chapter 5 of IPCC (2021) and the references therein are more suitable.
- The bibliography relies strongly on references from co-authors. A broader literature base is necessary to reflect critically on the results and to support conclusions.
Minor Issues
- The paper often mixes different types of statements that should be separated:
- L186: At the end of the model description, a key part of the method is described. This should appear towards the beginning of the section so that the reader knows why the model description is needed at all.
- L282: Here, a new methodology is introduced. This belongs to the methods and not to the results.
- L331: This is an exact repetition of the relevance statement from the introduction. This should either be removed or shortened and moved to the beginning of the discussion as a short reminder for the reader.
- The discussion holds multiple lines at which the same information (the finding that information on N-limitation is extractable from comparing calibrated parameters) is repeated. This should be largely condensed. Instead, the discussion should be used to discuss the plausibility of the findings and whether they support the conclusion or not. This crucial scientific process is missing.
- The following lines require clarifications:
- L51: What are “thresholds of leaf N:P ratios”?
- L61: It is unlikely that “all data assimilation studies” that exist were considered. A different adjective should be used, or a specific subset of data assimilation studies should be clarified.
- L65: What is meant by “Varying parameters” as a modeling approach?
- L138: What is meant by the leaf photosynthetic efficiency, and how is this connected to foliar C:N ratios? Also, formula (1) does not define LPE but SNvcmax. Are they the same? If not, what is SNvcmax? Also, what are the “estimated and defined C:N ratios of foliage”? How are they defined?
- L186: The term C exit rates are introduced, but the reader is left alone with figuring out what these are. Therefore, they should be listed right away.
- Results: Section 3.3 nicely guides the reader to the subplots in the figures to describe the results. This is missing in Section 3.2, which makes it difficult to understand.
- L229: What is the definition of a “well-constrained parameter”? There is no reference to any Figure or metric.
- L370: What is the definition of significance to assess whether the estimated model parameters were significantly different?
- L377: The sentence says that models with different structures should show similar dynamics, which is a confusing statement. Should the sentence rather state that the same model applied to the same ecosystem across different N treatments should show similar responses?
- L396: Unclear conceptualisation of N limitation. What is meant by “the decrease of the simulation”?
- L420: Detailed explanation is needed because it is not obvious how C exit rates cause a unimodal response in C pools and how a unimodal response provides information on nitrogen limitation.
- The quality of language limits readability. The final version should be proofread by a native speaker to resolve semantic issues. Sentences that were difficult to read are on lines 45 (predicting C cycle), 46 (which remain), 63 (because-clause refers to no other sentence), 138 (sentence has no verb), 145 (The use of “similarly” is inappropriate), 158 (“no” should be “not”), 308 (sentence has no verb), 362 (unclear to what “their” refers to), 364 (unclear to what “those variables” refers to)
- Formatting should be improved on lines 198, 203, 215, 218, 219, and 267.
ReferencesArora, Vivek K., Anna Katavouta, Richard G. Williams, Chris D. Jones, Victor Brovkin, Pierre Friedlingstein, Jörg Schwinger, et al. 2020. “Carbon–Concentration and Carbon–Climate Feedbacks in CMIP6 Models and Their Comparison to CMIP5 Models.” Biogeosciences 17 (16): 4173–4222. https://doi.org/10.5194/bg-17-4173-2020.
Davies-Barnard, Taraka, Johannes Meyerholt, Sönke Zaehle, Pierre Friedlingstein, Victor Brovkin, Yuanchao Fan, Rosie A. Fisher, et al. 2020. “Nitrogen Cycling in CMIP6 Land Surface Models: Progress and Limitations.” Biogeosciences 17 (20): 5129–48. https://doi.org/10.5194/bg-17-5129-2020.
IPCC. 2021. “Chapter 5: Global Carbon and Other Biogeochemical Cycles and Feedbacks.” In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, 673–816. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. 10.1017/9781009157896.007
Jiang, Mingkai, Sönke Zaehle, Martin G. De Kauwe, Anthony P. Walker, Silvia Caldararu, David S. Ellsworth, and Belinda E. Medlyn. 2019. “The Quasi-Equilibrium Framework Revisited: Analyzing Long-Term CO2 Enrichment Responses in Plant–Soil Models.” Geoscientific Model Development 12 (5): 2069–89. https://doi.org/10.5194/gmd-12-2069-2019.
Zaehle, Sönke, Belinda E. Medlyn, Martin G. De Kauwe, Anthony P. Walker, Michael C. Dietze, Thomas Hickler, Yiqi Luo, et al. 2014. “Evaluation of 11 Terrestrial Carbon–Nitrogen Cycle Models against Observations from Two Temperate Free-Air CO2 Enrichment Studies.” New Phytologist 202 (3): 803–22. https://doi.org/10.1111/nph.12697.
Wang, Song, Yiqi Luo, and Shuli Niu. “Reparameterization Required After Model Structure Changes From Carbon Only to Carbon‐Nitrogen Coupling.” Journal of Advances in Modeling Earth Systems 14, no. 4 (April 2022). https://doi.org/10.1029/2021MS002798.
Citation: https://doi.org/10.5194/bg-2023-33-RC1 -
AC1: 'Reply on RC1', Song Wang, 12 May 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2023-33/bg-2023-33-AC1-supplement.pdf
- The paper lacks a sufficient description of its methodology and requires more explicit discussion and contextualisation of the results with respect to recent publications on nitrogen limitation processes, such as Zaehle et al. 2014, Jiang et al. 2019, Arora et al. 2020, Davies-Barnard et al. 2020. The text should be improved at the following positions:
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RC2: 'Comment on bg-2023-33', Anonymous Referee #2, 08 Apr 2023
Wang et al. aim to retrieve N limitation information via data model fusion using a C-only model and coupled C-N model. They applied this approach to a field N enrichment experiment at an alpine meadow in the Qinghai-Tibet Plateau. The topic of nutrient limitation is of increasing importance in view of their role in constraining future land C sink in response to rising air CO2. I agree with most comments by Reviewer 1 and I have several additional comments that may further help to improve this work.
First, the term of nitrogen limitation needs to be clearly defined. Do you mean N limitation to plants, microbes or both? We don’t usually say “ecosystem N limitation”. Please also define “N limitation information” and clearly show how this is quantified in this study. Additionally, this manuscript seems to set up a background that N limitation occurs everywhere (L35-44). It would be helpful to provide an update of this view and mention that P instead of N is limiting in many tropical and subtropical ecosystems. The limitation by other nutrients needs to be mentioned or discussed in this manuscript. Second, there are many undefined terms and missing information in this manuscript (see specific comments). This hinders an in-depth evaluation of this work. Moreover, model structure and data assimilation are described in the method section but it is unclear how the two questions of this study were addressed (L91-93).Specific comments
L17-18 & 31: Exactly, how to provide guidance for policy making or ecosystem management?
L24: Explain “carbon exit rates”
L41-43: P limitation is also important but fully ignored.
L45-48: The best way to address N limitation issues in earth system models is better understanding and representation of various N cycling processes (especially biological N fixation).
L45-55: Uncertainties are discussed for these methods. I would expect a description of the advantage of the data model fusion approach.
L68: Explain “ecological information”
L72-74: Any advantage (e.g., more accuracy) in comparison to other approaches(45-55)?
L96-103: Provide additional information such as whether plant growth is N limited in the studied meadow and the level of N deposition.
L139: The equation is not clear.
L227: Make sure that the GECO model simulated C pools of microbes.
L231-233: Again, explain “C exit rate”
Figure 3: Does it mean C-N model overestimate LPE or C-only model underestimate LPE?
Figure 4: Please add titles for each axis
Figure 5&6: It would be good to include the field observed data in comparison to the modelled results.
Figures 3-6: Any significant differences between modeling results?
L282-293: Not clear how N limitation was quantified and compared here. Do you mean the strength of N limitation changes with different levels of N additions for the same meadow?Citation: https://doi.org/10.5194/bg-2023-33-RC2 -
AC2: 'Reply on RC2', Song Wang, 12 May 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2023-33/bg-2023-33-AC2-supplement.pdf
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AC2: 'Reply on RC2', Song Wang, 12 May 2023
Song Wang et al.
Song Wang et al.
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