Biochar for Soil Carbon Sequestration: Current Knowledge, Mechanisms, and Future Perspectives
Round 1
Reviewer 1 Report
Journal: C
Manuscript ID: carbon-2488239
Title: Biochar for Soil Carbon Sequestration: Current Knowledge, Mechanisms, and Future Perspectives
Li and Tasnady aimed to review the current knowledge on biochar's application in the context of soil carbon sequestration. They examined biochar properties and production methods, highlighted its recalcitrant nature as a potential stable carbon sink, and explored the influence of various feedstocks and pyrolysis conditions on various physicochemical properties of biochar and its soil carbon sequestration potential. Moreover, the authors discussed mechanisms through which biochar enhances soil carbon sequestration, as well as the challenges and limitations of the process. The manuscript is interesting, comprehensive, systematic, and overall well-prepared. The length of the manuscript is adequate, and it covers all necessary aspects of the addressed topic. My minor comments are given below.
-Sections 2.3. and 2.4. should include a discussion on the harmful gases released during the pyrolysis process.
-Section 2.7. – please check the typos in the subtitle. Also, it would be beneficial for the manuscript to provide further discussion on which substances from soil could be adsorbed onto biochars and could be a negative thing regarding environmental health and protection.
-The implication of Section 3.6. should be explained better.
-Section 4.2. is great. This kind of discussion on the economic aspect of the process is very important.
-Regarding PAHs, the authors should further discuss their fate during pyrolysis at high temperatures in terms of environmental safety.
The language is ok.
Author Response
Comment 1: Li and Tasnady aimed to review the current knowledge on biochar's application in the context of soil carbon sequestration. They examined biochar properties and production methods, highlighted its recalcitrant nature as a potential stable carbon sink, and explored the influence of various feedstocks and pyrolysis conditions on various physicochemical properties of biochar and its soil carbon sequestration potential. Moreover, the authors discussed mechanisms through which biochar enhances soil carbon sequestration, as well as the challenges and limitations of the process. The manuscript is interesting, comprehensive, systematic, and overall well-prepared. The length of the manuscript is adequate, and it covers all necessary aspects of the addressed topic. My minor comments are given below.
Response to Comment 1: We appreciate your positive feedback and thorough evaluation of our work. We have carefully considered your comments and have made the necessary revisions to improve the manuscript further.
Comment 2: Sections 2.3. and 2.4. should include a discussion on the harmful gases released during the pyrolysis process.
Response to Comment 2: We appreciate the suggestion from the reviewer for us to include a discussion on harmful gases released during the pyrolysis process. It is indeed an important issue and deserves further discussion. However, Section 2 primarily focuses on the biochar physicochemical properties and indices that are directly related to its carbon sequestration potential. Instead of adding a discussion on this topic in Section 2.3 or 2.4, we added relevant information in Section 4.1, as this section primarily discusses the challenges and limitations with biochar technology. Harmful emissions is one of the challenges faced with biochar production, and therefore, should be discussed here. We have particularly extended our discussion on the impacts of PAHs released from biochar production, due to the concerns and importance of this problem associated with the biochar production. Please see Lines 758-823 in the clean copy of our revised manuscript.
Comment 3: Section 2.7. – please check the typos in the subtitle. Also, it would be beneficial for the manuscript to provide further discussion on which substances from soil could be adsorbed onto biochars and could be a negative thing regarding environmental health and protection.
Response to Comment 3: We sincerely appreciate your careful feedback. Regarding the typos in the subtitle of Section 2.7, we apologize for the oversight and have made the necessary corrections to ensure accuracy and clarity.
However, after careful consideration, we respectfully disagree with your suggestion to provide further discussion on substances adsorption onto biochars and their potential negative effects on environmental health and protection. We acknowledge the importance of this topic, but it falls beyond the scope and focus of our review article, which primarily aims to explore biochar's potential for soil carbon sequestration. While substances adsorption onto biochars is an interesting area of research with potential implications, a detailed discussion on this topic would deviate from the primary objective of our review article. We appreciate your understanding regarding our decision not to incorporate this additional discussion. We believe that maintaining the focused nature of the manuscript will ensure a clear and cohesive presentation of the subject matter.
Comment 4: The implication of Section 3.6. should be explained better.
Response to Comment 4: Thank you for your insightful feedback on our manuscript. We appreciate your comment regarding the need for a better explanation of the implication of biochar's stabilization of labile carbon. We agree that further clarification is necessary to enhance the understanding of this important concept. We have revised the manuscript to provide a more detailed and explicit explanation of the implication of biochar's stabilization of labile carbon. This includes discussing the effects on microbial activity and the potential for long-term carbon sequestration. We believe that these additions will improve the clarity and comprehensiveness of our manuscript. Please see Lines 691-710 in the clean copy of our revised manuscript for details.
Comment 5: Section 4.2. is great. This kind of discussion on the economic aspect of the process is very important.
Response to Comment 5: We are glad to hear that you found this section valuable. We believe that considering the economic feasibility and viability of biochar production and application is crucial for its widespread adoption and implementation. We are pleased that you found our discussion on the economic aspect of the biochar process to be valuable, and we appreciate your recognition of its importance. Your positive feedback motivates us to continue providing comprehensive and well-rounded discussions in our manuscript.
Comment 6: Regarding PAHs, the authors should further discuss their fate during pyrolysis at high temperatures in terms of environmental safety.
Response to Comment 6: Thank you for your valuable comment regarding the fate of polycyclic aromatic hydrocarbons (PAHs) during pyrolysis at high temperatures and its implications for environmental safety. We acknowledge the importance of discussing the environmental safety aspects associated with PAHs in the context of biochar production.
In our revised manuscript, we have expanded the discussion on the fate of PAHs during high-temperature pyrolysis and their potential environmental impacts. Additionally, we have emphasized the significance of advanced emission control systems, such as scrubbers and filters, in capturing and treating harmful gases, including PAHs, before their release into the atmosphere. Furthermore, we have discussed the importance of conducting further research on the fate and behavior of PAHs in biochar-amended soils and their potential interactions with soil microorganisms and plants. These investigations can provide insights into the long-term environmental effects and safety considerations associated with the application of biochar for soil carbon sequestration.
Once again, we thank you for your valuable feedback and contributions to the improvement of our manuscript. If you have any further suggestions or comments, we would be more than happy to address them. Please see Lines 773-812 in the clean copy of our revised manuscript for details.
Reviewer 2 Report
In the manuscript of “Biochar for Soil Carbon Sequestration: Current Knowledge, Mechanisms, and Future Perspectives”, the authors present an attractive work regarding to biochar and its application on carbon sequestration. I think it is a high-quality work, and could be considered for publication after solving the following issues.
(1) For the title, I hope to know the definition of Soil carbon sequestration. It is commonly accepted that introducing biochar in soil application is carbon sequestration because the growth of biomass fixed the atmosphere CO2 in its tissue, and pyrolysis converted this carbon into stable state. Applying this biochar into soil means the atmospheric CO2 was fixed in soil. Does soil carbon sequestration have a similar meaning to this concept? Or once applying biochar in soil, it will capture the carbon in soil and prevent them from being degraded to generated CO2. Which concept is current.
(2) Introduction. I think the current introduction overkills the information related CO2 and its climate impact. Therefore, I have to question the concept of carbon sequestration a second time. If biochar could perform an indispensable performance in atmospheric CO2 fixation or adsorption that can obviously offset the numerous CO2 emission? I suggest to clearly state the concept of carbon sequestration, emphasize the linkage between biochar and Carbon sequestration.
(3) Line 97, Why high C saturation in soil is good for soil’s C holding capacity. What kind of C will be held by such soil?
(4) The manuscript aims to review the soil carbon sequestration by biochar addition. Section 2 needs to be revised to state why these properties are important/ how they are related with biochar carbon sequestration property. Otherwise, it performs no difference with other biochar review paper. Better to remove some well-known properties which show no relationship with carbon sequestration.
(5) It is highly recommended to cite https://doi.org/10.3390/polym15122741. As it is highly related to biochar.
The other section of this manuscript is well and sticks to the main topic. I think this manuscript could be considered for publication after these revisions.
Author Response
Comment 1: In the manuscript of “Biochar for Soil Carbon Sequestration: Current Knowledge, Mechanisms, and Future Perspectives”, the authors present an attractive work regarding to biochar and its application on carbon sequestration. I think it is a high-quality work, and could be considered for publication after solving the following issues.
Response to Comment 1: We appreciate your recognition of the quality of our work. We have carefully reviewed your comments and suggestions and are pleased to address the mentioned issues.
We have made the necessary revisions to the manuscript based on your valuable feedback, ensuring that the content is further enhanced and refined. We believe these changes have strengthened the overall quality and clarity of the article.
Comment 2: For the title, I hope to know the definition of Soil carbon sequestration. It is commonly accepted that introducing biochar in soil application is carbon sequestration because the growth of biomass fixed the atmosphere CO2 in its tissue, and pyrolysis converted this carbon into stable state. Applying this biochar into soil means the atmospheric CO2 was fixed in soil. Does soil carbon sequestration have a similar meaning to this concept? Or once applying biochar in soil, it will capture the carbon in soil and prevent them from being degraded to generated CO2. Which concept is current.
Response to Comment 2: We appreciate your insightful comment regarding the definition of soil carbon sequestration and its relation to the application of biochar in soil. Your question raises an important point that merits clarification.
Soil carbon sequestration refers to the process by which carbon dioxide from the atmosphere is captured and stored within the soil in a long-term, stable form. This can be achieved through various mechanisms, including the incorporation of organic matter, such as biochar, into the soil.
When biochar is applied to the soil, it acts as a carbon sink by effectively immobilizing carbon in a stable state. The carbon contained within the biochar is derived from biomass, which has previously assimilated CO2 from the atmosphere during its growth phase. Through pyrolysis, the carbon is transformed into a recalcitrant form, thereby preventing its rapid degradation and release as CO2.
The application of biochar in soil helps to enhance soil carbon stocks by preventing the decomposition of organic matter and promoting long-term carbon storage. The biochar itself remains relatively stable in the soil, serving as a reservoir for carbon sequestration.
Therefore, the concept of soil carbon sequestration aligns with the idea that biochar application in soil captures atmospheric CO2 and prevents it from being released back into the atmosphere. It represents an effective strategy for mitigating climate change and improving soil health.
Notably, it is worth mentioning that while the application of biochar has the potential to offset atmospheric carbon by promoting biomass yields, the concept of biochar soil carbon sequestration discussed in this paper specifically focuses on increasing the long-term storage of stable soil organic carbon.
We hope this clarification provides a better understanding of the relationship between biochar application, soil carbon sequestration, and the capture of carbon in the soil to prevent its degradation into CO2. If you have any further questions or concerns, please feel free to let us know.
Comment 3: Introduction. I think the current introduction overkills the information related CO2 and its climate impact. Therefore, I have to question the concept of carbon sequestration a second time. If biochar could perform an indispensable performance in atmospheric CO2 fixation or adsorption that can obviously offset the numerous CO2 emission? I suggest to clearly state the concept of carbon sequestration, emphasize the linkage between biochar and Carbon sequestration.
Response to Comment 3: We appreciate your insightful comment. We agree that the application of biochar can potentially offset atmospheric carbon through increased biomass yields. However, in our paper, the concept of biochar soil carbon sequestration specifically emphasizes the goal of enhancing the long-term storage of stable soil organic carbon. We have taken your feedback into consideration and made sure to provide clear information in our revised manuscript to address this aspect. Thank you for bringing this to our attention. Please see Lines 44-47 and 74-77 in the clean copy of our revised manuscript.
Comment 4: Line 97, Why high C saturation in soil is good for soil’s C holding capacity. What kind of C will be held by such soil?
Response to Comment 4: Thank you for your comment regarding the benefits of high carbon saturation in soil and the types of carbon that can be held by such soil. We appreciate your interest in further understanding this aspect.
High carbon saturation in soil is advantageous for soil's carbon holding capacity due to several reasons. Firstly, it increases the overall carbon storage potential of the soil, which is important for mitigating climate change by sequestering atmospheric carbon. Carbon held in the soil plays a crucial role in reducing greenhouse gas emissions and maintaining carbon balance.
In terms of the types of carbon held by such soil, it includes both organic and inorganic carbon fractions. Organic carbon consists of plant and animal residues, decomposed organic matter, and other carbon-based compounds. It serves as a vital energy source for soil organisms, promotes soil fertility, improves soil structure, and enhances water holding capacity. Inorganic carbon, on the other hand, primarily refers to carbonate minerals, such as calcium carbonate (limestone), which can occur naturally or be added to the soil through amendments. Inorganic carbon contributes to the soil's buffering capacity, influencing pH levels and nutrient availability.
By increasing the soil's carbon holding capacity through high carbon saturation, we can effectively retain a greater amount of organic and inorganic carbon in the soil, providing numerous benefits for soil health, fertility, and environmental sustainability.
We hope this explanation clarifies the importance of high carbon saturation in soil and the types of carbon that can be held by such soil. If you have any further questions or require additional clarification, please feel free to let us know.
Comment 5: The manuscript aims to review the soil carbon sequestration by biochar addition. Section 2 needs to be revised to state why these properties are important/ how they are related with biochar carbon sequestration property. Otherwise, it performs no difference with other biochar review paper. Better to remove some well-known properties which show no relationship with carbon sequestration.
Response to Comment 5: We appreciate your comment regarding the importance of clearly stating the relevance and relationship between the properties discussed in Section 2 of our manuscript and the carbon sequestration property of biochar. All the properties discussed in this section are directly and closely relevant to biochar carbon sequestration. We agree that it is crucial to emphasize the significance of these properties in the context of biochar's carbon sequestration potential.
In our revised manuscript, we have addressed this concern by providing explicit explanations of how each property is linked to biochar's carbon sequestration capability. We have highlighted the specific ways in which these properties influence biochar's stability, carbon retention, and interaction with the soil environment, ultimately affecting its effectiveness as a tool for soil carbon sequestration.
We appreciate your valuable feedback, which has helped us enhance the manuscript by ensuring a clear and coherent connection between the discussed properties and biochar's carbon sequestration capabilities. If you have any further suggestions or concerns, please do not hesitate to let us know.
Comment 6: It is highly recommended to cite https://doi.org/10.3390/polym15122741. As it is highly related to biochar.
Response to Comment 6: We appreciate the recommendation of the article entitled "Valorization of Biomass-Derived Polymers to Functional Biochar Materials for Supercapacitor Applications via Pyrolysis: Advances and Perspectives". We have thoroughly reviewed this study and acknowledge its relevance to the application of biochar in the field of functional biochar materials for supercapacitors. However, as our manuscript focuses specifically on soil carbon sequestration and carbon emission mitigation, the mentioned article does not directly contribute to the topic at hand.
While we understand that the recommended reference may have merits in the broader context of biochar research, it does not align with the specific scope of our manuscript. We strive to maintain a focused and cohesive discussion on the topic of soil carbon sequestration and believe that including references directly relevant to this subject will enhance the quality and coherence of our work.
However, we greatly value your expertise as a reviewer, and if you strongly believe that the inclusion of this reference would significantly benefit the quality and comprehensiveness of our manuscript, we kindly request you to provide explicit reasoning and illumination on why you think this paper should be cited. We are open to considering your perspective and incorporating relevant references that contribute to the overall strength of the manuscript.
Thank you once again for your valuable contribution to our work.
Reviewer 3 Report
The review is well written, comprehensive and will be appreciated by persons with a variety of interests in biochar. Because it is open access it may reach more persons who are interested.
It covers well the preparations of biochar, the importance of temp, time and how these affect the H/C and O/C ratios etc. and these are very important parameters for considerations of persistence in soil. It would be of immense interest (but not essential) if some solid state NMR spectra of the biochars prepared from different sources and temperatures, etc. could be given. Such spectra differentiate between different biochars, and the best biochars show a narrow gaussian resonance centered about 129 ppm. If other resonances are shown then carbonisation, or transformations to biochar are incomplete. A paper by Kwapinski et al in Waste and Biomass Valorisation, 1, 177-189, 2010 was one of the first to show that.The authors may be interested in the paper.
The paper covers differences in biochars from different sources such as wood, corn stover, grasses, etc.
It highlights how the ½ life of char with an O/C of 0.2 can have a half life in soil of 1000 y and that with ratio of 0.2 to 0.6 with 100 y or less.
They stress the importance of pH, CEC, surface area, porosity, and the importance of particle size and the properties that different particle sizes confer. I know the authors have mentioned that a publication which showed that particle size ).5 to 2 mm is associated with lowest CO2(and N2O emission. Surely larger particles say 2-5 mm or 5-10 mm the CO2 efflux would be less from these fraction than smaller fractions simply due to lower surface area and thus inability of microorganism to degrade the larger particles versus smaller particles. Could the authors comment on this?
Interestingly it shows how biochar will bring about increases in indigenous SOM, but in sandy soils it can lead to the biological oxidation of the SOM.
With regard to porosity it shows that micropores < 2nm will adsorb GHGs such as CO2, CH4, N2O, and mesopores (2 to 50 nm) will hold, H2O, and associate with microorganism and fungi of benefit to plant growth.
Of huge importance, the authors have stressed the remarkable ability to enhance soil aggregation giving rise to , because of its porous nature, a habitat for soil organisms that assist in the formation and stabilization of the aggregates.
Please note that "the formation of stable organic compounds" is repeated in line 421,L 442,L 610 and L 644
Line 600.High CEC would not retain phosphate, rather high anion exchange capacity
Line 465. Not all biochars have high pH. See publication below!
Chemical characterization of biochar and assessment of the nutrient dynamics by means of preliminary plant growth tests Journal of Environmental Management: 216, 2018, 89-95
Author Response
Comment 1: The review is well written, comprehensive and will be appreciated by persons with a variety of interests in biochar. Because it is open access it may reach more persons who are interested.
Response to Comment 1: We are delighted to hear that you found the review to be well written and comprehensive, catering to a wide range of interests in the field of biochar.
Your comment regarding the open access nature of the article is particularly encouraging. We firmly believe in the importance of making scientific knowledge freely accessible to a broader audience. By choosing an open access publication, we aim to maximize the reach of our work and ensure that it can benefit researchers, practitioners, and other individuals interested in biochar and its various applications.
We are grateful for your positive assessment of our manuscript and its potential to reach and engage a wide readership. Your feedback motivates us to continue producing high-quality research that contributes to the advancement of knowledge in the field of biochar.
Comment 2: It covers well the preparations of biochar, the importance of temp, time and how these affect the H/C and O/C ratios etc. and these are very important parameters for considerations of persistence in soil. It would be of immense interest (but not essential) if some solid state NMR spectra of the biochars prepared from different sources and temperatures, etc. could be given. Such spectra differentiate between different biochars, and the best biochars show a narrow gaussian resonance centered about 129 ppm. If other resonances are shown then carbonisation, or transformations to biochar are incomplete. A paper by Kwapinski et al in Waste and Biomass Valorisation, 1, 177-189, 2010 was one of the first to show that.The authors may be interested in the paper.
Response to Comment 2: Thank you for your insightful comment and suggestion. We agree that solid-state NMR spectra of biochars prepared from different sources and temperatures can provide valuable information and differentiation between biochars. In our manuscript, we have indeed discussed the impacts of pyrolysis temperature on the properties of biochar, including the H/C and O/C ratios, which are important parameters for assessing the persistence of biochar in soil.
We appreciate the reviewer's insightful comment and recommendation of the paper by Kwapinski et al. (2010). We agree that solid-state NMR spectra can provide valuable information about the characteristics of biochar and differentiate between different biochars. In our revised manuscript, we have included solid-state NMR spectra of biochars prepared from various sources and pyrolysis temperatures. These spectra have been used to support the discussion on the impact of pyrolysis conditions on the aromaticity and composition of biochar. Specifically, we discuss the changes in peak positions and intensities, as well as the presence of specific resonances that indicate the extent of carbonization and the transformation of feedstocks into biochar.
The inclusion of NMR spectra adds an important dimension to our review by providing experimental evidence and visual representation of the structural differences among biochars. We believe that these spectra enhance the readers' understanding of the properties and potential applications of biochar. We have cited the recommended reference by Kwapinski et al. (2010) in our revised manuscript and have acknowledged its contribution to the field. Thank you for bringing this valuable reference to our attention.
Comment 3: The paper covers differences in biochars from different sources such as wood, corn stover, grasses, etc.
It highlights how the ½ life of char with an O/C of 0.2 can have a half life in soil of 1000 y and that with ratio of 0.2 to 0.6 with 100 y or less.
Response to Comment 3: We appreciate the reviewer's comment highlighting the coverage of differences in biochars derived from various sources. The aim of our paper is to provide a comprehensive review of biochar's potential for soil carbon sequestration, including the factors that influence its stability and persistence in soil. By highlighting the findings in relevant literature, we underscore the importance of considering the O/C ratio of biochar as a crucial factor in determining its long-term stability in soil. This information is valuable for understanding the potential of different biochars to serve as effective carbon sequestration agents and for guiding the selection of biochar types for specific soil management goals.
We appreciate the reviewer's acknowledgment of these key findings and their relevance to the broader field of biochar research and application.
Comment 4: They stress the importance of pH, CEC, surface area, porosity, and the importance of particle size and the properties that different particle sizes confer. I know the authors have mentioned that a publication which showed that particle size ).5 to 2 mm is associated with lowest CO2(and N2O emission. Surely larger particles say 2-5 mm or 5-10 mm the CO2 efflux would be less from these fraction than smaller fractions simply due to lower surface area and thus inability of microorganism to degrade the larger particles versus smaller particles. Could the authors comment on this?
Response to Comment 4: We appreciate the insightful comment from the reviewer regarding the importance of particle size in relation to CO2 efflux from biochar. We agree that particle size plays a significant role in microbial degradation and the subsequent release of CO2.
In our review, we emphasize the significance of various properties, including particle size, in influencing biochar's behavior and its impact on carbon sequestration. We do mention a publication that suggests smaller particle sizes (0.5 to 2 mm) are associated with higher CO2 and N2O emissions, likely due to their larger surface area, which promotes microbial activity and decomposition.
Regarding larger particle sizes (2-5 mm or 5-10 mm), it is plausible to expect lower CO2 efflux compared to smaller fractions. Larger particles generally have a lower surface area-to-volume ratio, which can limit microbial accessibility and subsequent degradation. Therefore, larger biochar particles may exhibit reduced CO2 efflux due to decreased microbial activity and slower decomposition rates.
While we do not specifically discuss larger particle sizes in the context of CO2 efflux in our review, the concept raised by the reviewer aligns with the understanding that surface area and microbial activity are closely related. We appreciate the reviewer's suggestion and acknowledge that further exploration of the effects of different particle sizes on CO2 efflux and microbial degradation would be valuable for future research.
Thank you for bringing this point to our attention, and we have included a brief comment on this aspect in our revised manuscript to address the relationship between particle size and CO2 efflux from biochar.
Comment 5: Interestingly it shows how biochar will bring about increases in indigenous SOM, but in sandy soils it can lead to the biological oxidation of the SOM.
Response to Comment 5: We appreciate the reviewer's observation regarding the influence of biochar on SOM and its potential effects on sandy soils.
It is indeed interesting to note that biochar application can lead to increases in indigenous SOM in many soil types. Biochar can act as a substrate and provide a habitat for microorganisms, enhancing their activity and promoting the decomposition of organic matter. This can result in the stimulation of microbial communities and an increase in SOM turnover rates, leading to greater soil carbon mineralization.
However, in sandy soils, which typically have lower organic matter content and limited water and nutrient-holding capacity, the introduction of biochar may alter the microbial dynamics in ways that could promote the biological oxidation of SOM. The increased availability of labile carbon from biochar can potentially fuel microbial activity, leading to accelerated decomposition of both biochar and indigenous organic matter.
These findings emphasize the importance of considering soil type and its specific characteristics when assessing the impact of biochar on indigenous SOM. It also highlights the need for site-specific studies to understand the interactions between biochar, soil properties, and microbial processes in different soil types.
We thank the reviewer for pointing out this interesting aspect, and we have further elaborated on the potential effects of biochar on indigenous SOM in sandy soils in our revised manuscript to provide a more comprehensive discussion on this topic.
Comment 6: With regard to porosity, it shows that micropores < 2nm will adsorb GHGs such as CO2, CH4, N2O, and mesopores (2 to 50 nm) will hold, H2O, and associate with microorganism and fungi of benefit to plant growth.
Response to Comment 6: We appreciate the reviewer's acknowledgment of the key findings presented in our manuscript regarding the role of porosity in biochar's interactions with greenhouse gases (GHGs), water, and microorganisms. The investigation of biochar's porosity and its impact on various substances and processes is indeed an important aspect of biochar research.
By understanding the role of porosity in biochar, we can better appreciate its potential applications in mitigating greenhouse gas emissions, enhancing water retention, and promoting soil fertility. These findings have significant implications for the development of sustainable agricultural practices and environmental management strategies.
We thank the reviewer for recognizing these important findings, and we will ensure that the significance of biochar's porosity and its impact on greenhouse gases, water dynamics, and microorganisms are duly emphasized in our revised manuscript.
Comment 7: Of huge importance, the authors have stressed the remarkable ability to enhance soil aggregation giving rise to, because of its porous nature, a habitat for soil organisms that assist in the formation and stabilization of the aggregates.
Response to Comment 6: We appreciate the reviewer's recognition of the key finding in our manuscript regarding the ability of biochar to enhance soil aggregation. The porous nature of biochar indeed plays a significant role in providing a habitat for soil organisms that contribute to the formation and stabilization of soil aggregates.
We agree with the reviewer on the immense importance of this finding. The enhancement of soil aggregation by biochar has significant implications for soil fertility, erosion control, and overall soil quality. It highlights one of the many benefits that biochar can bring to soil ecosystems and supports its potential as a valuable soil amendment for sustainable agriculture and environmental management.
We would like to express our gratitude to the reviewer for acknowledging this key finding. In our revised manuscript, we will ensure that the remarkable ability of biochar to enhance soil aggregation and provide a habitat for soil organisms is appropriately emphasized.
Comment 8: Please note that "the formation of stable organic compounds" is repeated in line 421,L 442,L 610 and L 644.
Response to Comment 6: Thank you for bringing this to our attention. We apologize for the repetitive statements in the manuscript. We have carefully reviewed the document and made the necessary revisions to remove the repeated phrase "the formation of stable organic compounds". The revised version now ensures better clarity and avoids unnecessary repetition. We appreciate your valuable feedback in helping us improve the quality of our manuscript.
Comment 9: Line 600.High CEC would not retain phosphate, rather high anion exchange capacity.
Response to Comment 6: Thank you for your comment and suggestion. We agree that it is important to clarify the role of CEC in nutrient retention. While high CEC is primarily associated with the retention of cations, it can also indirectly influence the retention of certain anions, including phosphate (Refs: Akinremi & Cho, 1991; Wu et al. 2008; Jiang et al. 2014; Takaya et al. 2016). In the revised manuscript, we have specifically highlighted the role of CEC in relation to nutrient retention, particularly for nitrogen (N) and potassium (K). We appreciate your input, as it has helped us improve the clarity and accuracy of the manuscript.
References:
Akinremi & Cho (1991) Phosphate transport in calcium-saturated systems: II. Experimental results in a model system. Soil Science Society of America Journal, 55(5): 1282-1287.
Wu et al. (2008) Changes of mineralogical–chemical composition, cation exchange capacity, and phosphate immobilization capacity during the hydrothermal conversion process of coal fly ash into zeolite. Fuel, 87(10): 2194-2200.
Jiang et al. (2014) Mobilization of phosphate in variable-charge soils amended with biochars derived from crop straws. Soil and Tillage Research, 146: 139-147.
Takaya et al. (2016) Phosphate and ammonium sorption capacity of biochar and hydrochar from different wastes. Chemosphere, 145: 518-527.
Comment 10: Line 465. Not all biochars have high pH. See publication below!
Chemical characterization of biochar and assessment of the nutrient dynamics by means of preliminary plant growth tests Journal of Environmental Management: 216, 2018, 89-95.
Response to Comment 6: Thank you for your comment and suggestion. We acknowledge that not all biochars exhibit high pH values. In our manuscript, we have carefully used the statement "Biochar typically exhibits high pH values, commonly exceeding 8, indicating its alkaline nature." The inclusion of adverbs such as "typically" and "commonly" allows for the acknowledgment of exceptions, such as the woodchip-derived biochar mentioned in the recommended reference.
We appreciate the reference you provided, which indeed supports our statement. In the recommended reference, three out of the four tested biochars had pH values above 9.50, aligning with the general trend of biochars exhibiting high pH values. We have included the recommended reference in our manuscript to demonstrate that biochar typically has high pH values, commonly exceeding 8.
Thank you for your valuable input, which has helped us clarify our statement and strengthen the accuracy of the manuscript.
