Biochar Supplementation of Recycled Manure Solids: Impact on Their Characteristics and Greenhouse Gas Emissions During Storage

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript raises an important scientific issue in the field of organic waste management. The designed experiment aimed to determine the effect of biochar addition to recycled manure solids on greenhouse gas emissions.

In my opinion, the proposed studies are too small in scope to achieve the assumed research goal. The authors proposed a 30-day experiment only for two doses of biochar, additionally not justifying their selection. The results showed ambiguity in the effect of biochar on emission levels - for a 2.5% dose, an increase in emissions was noted, and for a 10% dose, its reduction compared to the control sample, which was not satisfactorily explained and supported by the literature. Based on the obtained results, it is not possible to state a positive or negative effect of biochar addition on emissions from agricultural waste.

The introduction to the manuscript is too cursory and does not adequately justify the purpose of the experiment. The methodology does not indicate the source of biochar, lacks of justify the selection of doses, and does not explain the purpose of manual measurements and the frequency key of these measurements. In the conclusions and discussion, the authors interpret the results in a very general way. The experiment conducted is too narrow in scope, which is why the indicated conclusions are only assumptions and not statements. The manuscript lacks a summary. Additionally, in Chapter 4 there are errors in the structure of chapters and subchapters.

In accordance with the above comments, the manuscript does not represent an appropriate scientific standard for publication.

Author Response

COMMENT 1: The manuscript raises an important scientific issue in the field of organic waste management. The designed experiment aimed to determine the effect of biochar addition to recycled manure solids on greenhouse gas emissions.

RESPONSE 1: Thank you for your detailed review and for raising important concerns about the scope, clarity, and structure of the manuscript.

In response to your feedback, the manuscript has been revised with the effort to improve its scientific rigor, coherence, and clarity. To address this:

• The Introduction was rewritten to better justify the purpose of the experiment, provide a stronger policy and scientific rationale, and clarify the current research gap.
• The Materials and Methods section was expanded to include the origin of the biochar used, the reasoning behind the selected doses, and the purpose of the manual measurements.
• The Discussion was restructured to better align the interpretation of results with the data presented, acknowledge limitations, and avoid overgeneralizations.
• We addressed the ambiguity in results by adding possible explanations supported by the literature, especially regarding dose-dependent effects of biochar on GHG emissions.

Below, we address these points individually, indicating to the location in the manuscript where changes were made. We believe these changes have clarified the contribution of the study and we hope the revised version meets your expectations.

 

COMMENT 2: In my opinion, the proposed studies are too small in scope to achieve the assumed research goal. The authors proposed a 30-day experiment only for two doses of biochar, additionally not justifying their selection. The results showed ambiguity in the effect of biochar on emission levels - for a 2.5% dose, an increase in emissions was noted, and for a 10% dose, its reduction compared to the control sample, which was not satisfactorily explained and supported by the literature. Based on the obtained results, it is not possible to state a positive or negative effect of biochar addition on emissions from agricultural waste.

Response 2: Thank you for your comment. Indeed, the different effects from both doses were unexpected, however, varying results have also been observed in the literature. And there is a lack of studies evaluating the effects of adding biochar to RMS, as most studies focus on biochar addition to soils or to other processes, such as manure composting. Nevertheless, we have improved this section of the results and discussion to try to explain this outcome more clearly. Please check P14 L381-392, P15-L422-624, P17 L554,555, P18 566-575. Additionally, the reasoning behind choosing 2.5 and 10 addition doses was added to Section 2.1. Please check P3 L116-123.

 

COMMENT 3: The introduction to the manuscript is too cursory and does not adequately justify the purpose of the experiment.

Response 3: Thank you for your comment. We have improved the justification for this study, and its intended purposes. Please check P2,3 L.76-98

 

COMMENT 4: The methodology does not indicate the source of biochar, lacks of justify the selection of doses, and does not explain the purpose of manual measurements and the frequency key of these measurements.

Response 4: Thank you for your comment. This information was indeed lacking, but was now added to the respective section. Please check P3 L124-130 for the source of biochar; Please check P3  L116-123. for the justification of the chosen doses of biochar addition; Please check P5 L196-200 for the reasoning behind measuring GHG with both setup systems and the frequency of manual measurements.

 

COMMENT 5: In the conclusions and discussion, the authors interpret the results in a very general way. The experiment conducted is too narrow in scope, which is why the indicated conclusions are only assumptions and not statements. The manuscript lacks a summary. Additionally, in Chapter 4 there are errors in the structure of chapters and subchapters. In accordance with the above comments, the manuscript does not represent an appropriate scientific standard for publication.

Response 5: Thank you for your comment. We believe that this experiment, although at a small scale, provides valuable insight on the impact of biochar addition to RMS and its impacts on the chemical properties and GHG emissions during storage. Indeed, some complementary analysis might be lacking, to further support our findings, but the data we presented represents an important first step in promoting more sustainable solutions to manure management, such as introducing biochar during RMS storage. Further studies should be conducted in the future to complement our findings. Nevertheless, we included more detailed discussion and a “conclusions” section. Please check P19 L605-621.

Reviewer 2 Report

Comments and Suggestions for Authors

omments to Agronomy-3504059

General comments

Dairy farming typically generates a substantial amount of manure solids, which pose significant environmental risks to waste management systems. Greenhouse gas (GHG) emissions from manure solids storage pose significant environmental risks, particularly due to their contribution to climate change. Biochar is commonly applied to mitigate GHG and ammonia emissions during manure composting. Generally, biochar derived from crop straw demonstrates higher efficacy in reducing GHG and ammonia emissions compared to biochar derived from manure. Additionally, supplementing manure solids with sulfuric acid or pyroligneous liquor (a byproduct of biochar production) to induce acidification can mitigate GHG and ammonia emissions as well. This manuscript studied the effects of biochar on reduction of GHG emission during manure solids storage. However, critical details, such as the specific types of biochar and the basic physicochemical properties of both the biochar and manure solids, was absent in the Materials and Methods section. Incorporating an analysis of ammonia emissions would significantly strengthen the scientific rigor and completeness of this manuscript. the results were poorly organized and very scattered, and also Tables and Figures.

 

Special comments

1. the Introduction should be integrated into three or four paragraphs.
2. the basic physicochemical properties of both the biochar and manure solids should be added in the Materials and Methods.
3. The inventory cycle of manure solids should be explained in the Introduction, i.e. why the incubation experiment was lasted 30 days, according to what.
4. The significant increase in heavy metal levels shown in Table 3 can likely be attributed to the biochar. However, the type of biochar used and the reasons for the elevated heavy metal concentrations should be clarified.
5. Figure 1, the string diagram was too vague.
6. In the Abstract, “Cumulative methane (CH4) emissions did not show significant differences” came up against Table 4, “p” should be italicized.
7. The GHG emissions showed the incubation experiment may conducted under anaerobic environment. Please provide more details in “2.2 Small scale Experiment”.

Author Response

COMMENT 1: Dairy farming typically generates a substantial amount of manure solids, which pose significant environmental risks to waste management systems. Greenhouse gas (GHG) emissions from manure solids storage pose significant environmental risks, particularly due to their contribution to climate change. Biochar is commonly applied to mitigate GHG and ammonia emissions during manure composting. Generally, biochar derived from crop straw demonstrates higher efficacy in reducing GHG and ammonia emissions compared to biochar derived from manure. Additionally, supplementing manure solids with sulfuric acid or pyroligneous liquor (a byproduct of biochar production) to induce acidification can mitigate GHG and ammonia emissions as well. This manuscript studied the effects of biochar on reduction of GHG emission during manure solids storage. However, critical details, such as the specific types of biochar and the basic physicochemical properties of both the biochar and manure solids, was absent in the Materials and Methods section.

Response 1: Thank you for your comment and valuable insights. Indeed, that information was lacking. The chemical properties of both RMS and biochar were in Tables 1-3, however, this might have not been made clear in the manuscript. Therefore, we added a mention to these tables on PL. Additionally, we also added the feedstock for biochar production and other characteristics we know. Please check P5 L196-200.

 

COMMENT 2: Incorporating an analysis of ammonia emissions would significantly strengthen the scientific rigor and completeness of this manuscript. the results were poorly organized and very scattered, and also Tables and Figures.

Response 2: Thank you for your comment. Indeed, measuring NH3 emissions are in future plans, but was not possible due to infrastructure issues. We improved the discussion and results sections and the reference to the tables, to improve clarity, expand methodological descriptions, and strengthen the scientific justification of our choices. We hope that the revised version better reflects the scientific merit and relevance of the study.

 

COMMENT 3: the Introduction should be integrated into three or four paragraphs.

Response 3: We improved this section, to provide more insights on the purpose of this study. Please check P2 and 3 L.76-98. We tried to limit to introduction to fewer paragraphs, however, considering the new information added, this was difficult to accomplish. We hope the introduction is within your standards now.  

 

COMMENT 4: the basic physicochemical properties of both the biochar and manure solids should be added in the Materials and Methods.

Response 4: The basic physicochemical properties of the recycled manure solids were already presented in Tables 1–3. As for the biochar, while some analytical values were determined and included (e.g., dry matter, TOC, nutrient content), we did not have access to all physicochemical characteristics (such as porosity or surface area), as these were not disclosed by the supplier. This limitation is now clearly acknowledged in the Materials and Methods, and its potential impact is discussed when interpreting the results. Please check P14 L381-392, P19 L612-814.

 

COMMENT 5: The inventory cycle of manure solids should be explained in the Introduction, i.e. why the incubation experiment was lasted 30 days, according to what.

Response 5: We have clarified the rationale for the 30-day incubation period in Section 2.2 (Small Scale Experiment). This duration was selected to simulate a typical storage period of recycled manure solids in dairy farms prior to reuse as bedding or field application. It also aligns with incubation durations used in other short-term GHG monitoring studies, as noted in the revised text. Please check P4 L139-141.

 

COMMENT 6: The significant increase in heavy metal levels shown in Table 3 can likely be attributed to the biochar. However, the type of biochar used and the reasons for the elevated heavy metal concentrations should be clarified.

Response 6: The biochar used in this study was a commercial pine-derived product sourced from Agrodrone (Portugal), as now stated in Section 2.1. While we observed an increase in heavy metal concentrations in the biochar-supplemented treatments (particularly 10B), we attribute this to the intrinsic composition of the biochar itself. As the specific production parameters and detailed physicochemical profile (e.g., ash content, metal speciation) were not disclosed by the supplier, this limitation is now acknowledged in the manuscript. We also discuss this possible contribution in Section 3.1, where the nutrient and metal content results are interpreted. Please check P14 L381-392

 

COMMENT 7: Figure 1, the string diagram was too vague.

Response 7: We apologize, but we do not understand this comment.

 

COMMENT 8: In the Abstract, “Cumulative methane (CH4) emissions did not show significant differences” came up against Table 4, “p” should be italicized.

Response 8: The abstract was revised and p italicized. Please check P1 L25-28 and L32 - 35.

 

COMMENT 9: The GHG emissions showed the incubation experiment may conducted under anaerobic environment. Please provide more details in “2.2 Small scale Experiment”.

Response 9: In Section 2.2, we now clarify that the experiment was designed to maintain aerobic conditions, mimicking typical RMS storage environments with passive aeration. However, due to the high moisture content and material density, we acknowledge the likely formation of localized anaerobic microzones, particularly in the lower layers of the substrate. This scenario is common in real-world RMS piles and was considered when interpreting GHG emissions. This clarification is now explicitly stated in the revised manuscript. Please check P4 L144-150

 

Reviewer 3 Report

Comments and Suggestions for Authors

Congratulations to the authors on developing this valuable article. The study assesses the influence of biochar addition to recycled manure solids at two doses, compared to acidified and no-treatment control, on GHG emissions over a one-month storage period under controlled conditions, using both continuous and intermittent measurement methods.

Lines 85 – 87: It would be useful to mention the type of feedstock and the origin of biochar.

Lines 93 – 94: To clarify the rationale, consider providing a justification or reference for the chosen biochar doses (2.5% and 10%), considering the assumptions of the control as 0% and equidistant levels (e.g., 0%, 5%, and 10%).

Lines 212 – 215: It is suggested to reference Tables 1, 2, and 3, which contain the biochar characteristics.

Lines 223, 229, 232, 275, 300: A minor formatting suggestion to improve readability. Keep only horizontal gridlines in tables.

Lines 271, 295, 296: For better visualization of the graphs, consider either angling the x-axis labels or removing the hour notation since measurements are daily-based.

Lines 275, 300: Instead of tables, consider presenting Tables 4 and 5 as bar charts.

Line 308: The phrase “…, C- showed lower DM and TOC content than C-.” Should it be C+ instead of the first C-? Please verify.

Line 336: There seems to be a missing comma in “…, BCr, …”, which should be written as “…, B, Cr, …”.

Line 419: If not redundant, consider clarifying that the reduction in GWP (for the biochar-amended treatments) compared with C-…

Lines 381 – 382, 422 – 423: Since 10% biochar was the most effective, it might be relevant to recommend further investigation of intermediate levels (e.g., 5%, 7.5%, 10%) to optimize its application.

Lines 461 – 464: If relevant, consider discussing the cost balance or offset for biochar addition, in the context of C sequestration incentives under current or possible EU/national policies. Additionally, consider mentioning how biochar feedstock type (e.g., residues, wood, etc.) could influence the balance.

Lines 466 – 477: Optionally, consider adding a "Conclusions" heading before these paragraphs to highlight the study's key findings.

Author Response

COMMENT 1: Congratulations to the authors on developing this valuable article. The study assesses the influence of biochar addition to recycled manure solids at two doses, compared to acidified and no-treatment control, on GHG emissions over a one-month storage period under controlled conditions, using both continuous and intermittent measurement methods.

Response 1: Thank you very much for your kind words and for recognizing the value of our study. Your constructive feedback has helped us improve the manuscript, and we are grateful for your support in refining both the content and clarity of our work.

 

COMMENT 2: Lines 85 – 87: It would be useful to mention the type of feedstock and the origin of biochar.

Response 2: The biochar used in this study was a commercial pine-derived product sourced from Agrodrone (Portugal), as now stated in Section 2.1. However the specific production parameters and detailed physicochemical profile (e.g., ash content, metal speciation) were not disclosed by the supplier, this limitation is now acknowledged in the manuscript. Please check P3 L124-130.

 

COMMENT 3: Lines 93 – 94: To clarify the rationale, consider providing a justification or reference for the chosen biochar doses (2.5% and 10%), considering the assumptions of the control as 0% and equidistant levels (e.g., 0%, 5%, and 10%).

Response 3: Thank you for your comment. In the revised Section 2.1, we now clarify that the 2.5% and 10% biochar doses were selected based on previous microbiological studies using the same biochar and RMS, where these concentrations showed measurable impacts on microbial dynamics. The inclusion of a 5% intermediate dose was initially planned, but due to technical limitations in the continuous gas sampling system (the multipoint sampler is only equipped with 12 channels), we were restricted to a maximum of four treatments, each with three replications. This limitation is now acknowledged, and we have added a recommendation in the Discussion for future studies to test intermediate doses for better dose–response resolution. Please check P3 L116-123.

 

COMMENT 4: Lines 212 – 215: It is suggested to reference Tables 1, 2, and 3, which contain the biochar characteristics.

Response 4: Thank you for your comment. Tables are now referenced accordingly in the results section. Please check P6 L237,239,240,242,244,245,250,251,257-259 .

 

COMMENT 5: Lines 223, 229, 232, 275, 300: A minor formatting suggestion to improve readability. Keep only horizontal gridlines in tables.

Response 5: Thank you for your comment. Tables are now formated with only horizontal gridlines. Please review the edited tables in the Results section.

 

COMMENT 6: Lines 271, 295, 296: For better visualization of the graphs, consider either angling the x-axis labels or removing the hour notation since measurements are daily-based.

Response 6: Thank you for your comment. We changed the axis and removed the hours to improve graphics vizualization

 

COMMENT 7: Lines 275, 300: Instead of tables, consider presenting Tables 4 and 5 as bar charts.

Response 7: Thank you for your comment. Although we understand your point-of-view, we believe that presenting this data via tables is clearer and enables a better understanding of the results for the reader. We hope you understand our preference.

 

COMMENT 8: Line 308: The phrase “…, C- showed lower DM and TOC content than C-.” Should it be C+ instead of the first C-? Please verify.

Response 8: Thank you for catching this error. You are absolutely correct—this was a typographical mistake. The sentence has been corrected in the revised manuscript. Please check P6 L243.

 

COMMENT 9: Line 336: There seems to be a missing comma in “…, BCr, …”, which should be written as “…, B, Cr, …”.  

Response 9: Thank you for your careful Reading of the manuscript. We have made the suggested correction. Please check P14 L381.

 

COMMENT 10: Line 419: If not redundant, consider clarifying that the reduction in GWP (for the biochar-amended treatments) compared with C-…

Response 10: Thank you for your comment. We have revised the sentence to explicitly clarify that the reduction in GWP observed in the biochar-amended treatments is in comparison to C-, the untreated RMS control. This clarification has been made to ensure that the comparison is not ambiguous for the reader. Please check P11 L329-339.

 

COMMENT 11: Lines 381 – 382, 422 – 423: Since 10% biochar was the most effective, it might be relevant to recommend further investigation of intermediate levels (e.g., 5%, 7.5%, 10%) to optimize its application.

Response 11: thank you for your valuable comment. We agree that further investigation of intermediate biochar concentrations would be valuable to determine the optimal dose. In response, we have included this recommendation in the Conclusion section, noting that future studies should explore a range of doses (e.g., 5%, 7.5%) to better understand the dose–response relationship and balance environmental benefits with practical feasibility. Please check P19 L614-616

 

COMMENT 12: Lines 461 – 464: If relevant, consider discussing the cost balance or offset for biochar addition, in the context of C sequestration incentives under current or possible EU/national policies. Additionally, consider mentioning how biochar feedstock type (e.g., residues, wood, etc.) could influence the balance.

Response 12: Thank you for your comment. In response, we have added a discussion in Section 4.3 addressing the potential for biochar to offset its production emissions through long-term carbon sequestration and reduced GHG emissions from RMS storage. We now reference the role of emerging EU frameworks and voluntary schemes (e.g., European Biochar Certificate) that may incentivize biochar use. Additionally, we briefly note that the environmental and economic performance of biochar is influenced by its feedstock type and pyrolysis conditions, both of which can affect carbon stability and the presence of residual compounds. These additions help place the results into a broader sustainability and policy context. Please check P18,19 L576-603.

 

COMMENT 13: Lines 466 – 477: Optionally, consider adding a "Conclusions" heading before these paragraphs to highlight the study's key findings.

Response 13: Thank you. A conclusion section was added to better highlight the keyfindings as well as study limitations and future directions. Please check P19 L606-621

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I appreciate the adjustment to the comments I indicated in the review form. The manuscript still has limitations resulting from, among others, the small amount of biochar doses used in the experiment. For this reason, it is impossible to draw specific conclusions from the conducted research.

However, the authors have enriched the content with a more literature-based discussion of the results. The extended and systematic introduction to the article allows the reader to better understand the purpose of the study.

Despite the (impossible to eliminate) limitations, the manuscript presents interesting results that require further experiments. In my opinion, the manuscript can be published provided that additional changes are introduced:

• the main limitations of the experiments conducted were indicated in the introduction; in my opinion, it is necessary to clearly emphasize and connect them in the analysis of the obtained results
• the summary should better correspond to the assumptions of the manuscript presented in the introduction; at the moment the summary covers only some of the indicated assumptions

Author Response

COMMENT 1: I appreciate the adjustment to the comments I indicated in the review form. The manuscript still has limitations resulting from, among others, the small amount of biochar doses used in the experiment. For this reason, it is impossible to draw specific conclusions from the conducted research.

However, the authors have enriched the content with a more literature-based discussion of the results. The extended and systematic introduction to the article allows the reader to better understand the purpose of the study.

Despite the (impossible to eliminate) limitations, the manuscript presents interesting results that require further experiments. In my opinion, the manuscript can be published provided that additional changes are introduced:

• the main limitations of the experiments conducted were indicated in the introduction; in my opinion, it is necessary to clearly emphasize and connect them in the analysis of the obtained results
• the summary should better correspond to the assumptions of the manuscript presented in the introduction; at the moment the summary covers only some of the indicated assumptions

 

RESPONSE 1:  Thank you for your additional observations. In response, we have revised:

• the Discussion to more clearly emphasize how the main limitations—such as the restricted number of biochar concentrations. P1 L622-631
• the Abstract was updated to better reflect the initial assumptions and the broader objective of the study, including a recommendation for continued assessments at different biochar doses and scales. Please see P1 L36–38

Reviewer 2 Report

Comments and Suggestions for Authors

I believe the revised version can be accepted with minor improvements in English language. 

Author Response

COMMENT 1: I believe the revised version can be accepted with minor improvements in English language.

RESPONSE 1:  Thank you for your supportive feedback. We have carefully reviewed and revised the manuscript to improve the English language and overall clarity, ensuring consistency and precision in scientific terminology throughout the text.