the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 May 2024
Biochar reduces early-stage mineralization rates of plant residues more in coarse than fine-texture soils – an artificial soil approach
Abstract. Quantifying the impact of biochar on carbon persistence across soil textures is complex, owing to the variability in soil conditions. Using artificial soils with precise textural and mineral composition, we could disentangle the effects of biochar from the effects of soil particle size. We can show that biochar application significantly reduces early-stage carbon mineralization rates of plant residues in various soil textures (from 5 to 41 % clay) but more significantly in sandy soils. This finding suggests that biochar can compensate for the lack of clay in promoting C persistence in soil systems. This short report significantly contributes to understanding soil texture and biochar application interactions.
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RC1: 'Comment on egusphere-2024-1143', Marie-Liesse Aubertin, 11 Jun 2024
General comments (specific comments and technical corrections are in the pdf file):
Soil texture is believed to have a strong impact on the response of biochar amendment on C sequestration, although little is known about the mechanisms involved. This short communication sheds light on potential mechanisms involved in the role of particle size and mineral composition on the early-stage decomposition of soil organic matter. The use of an artificial soil where particle size and mineral composition is fully controlled is, to my point of view, a relevant approach to focus on these specific mechanisms. Comparing the effect of biochar with soils from different textures is very interesting.
However, all parts could be improved since there are still some missing points and lack of precisions. In the results, it is not clear if the effect of biochar addition on the plant mineralization is due to the composition of biochar or to its particle size. Therefore, I think necessary to precise the size of biochar particles added and maybe to discuss this point.
To me, this study doesn’t evidence a clear effect of the texture on plant respiration in the control soil (without biochar). It is written that there is a statistic effect, but (1) results from statistics made on very small samples (n=3) should be treated with caution, (2) I am not convinced by the choice of the statistic test and (3) the curve does not show a clear trend. Hence, the results and discussion should not emphasize too much on this small tendency and not consider it as a clear result.
In the discussion, only physical explanations are presented to explain the effect of pH on plant mineralization. Additional explanations should be proposed regarding the effect of pH changes on the microbial communities and on the nutrient availabilities from the plant for the microorganisms.
The discussion should include more comparison with other studies using artificial soils. Indeed, the results from studies using artificial soils can be very different from studies using field soils (e.g. Gross et al. 2021). Although the approach of using artificial soil is relevant, the study should more emphasize on its limits to extend these results to real field soils.
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AC1: 'Reply on RC1', Franziska Bucka, 22 Jul 2024
We thank the editor and the reviewers for taking the time to evaluate the manuscript and for considering our manuscript suitable for publication. We also thank the reviewer for providing thoughtful comments and constructive feedback that helped us to improve the manuscript.
General comments (specific comments and technical corrections are in the pdf file):
Soil texture is believed to have a strong impact on the response of biochar amendment on C sequestration, although little is known about the mechanisms involved. This short communication sheds light on potential mechanisms involved in the role of particle size and mineral composition on the early-stage decomposition of soil organic matter. The use of an artificial soil where particle size and mineral composition is fully controlled is, to my point of view, a relevant approach to focus on these specific mechanisms. Comparing the effect of biochar with soils from different textures is very interesting.
Author's response: Thank you for your kind and constructive comments. We appreciate your acknowledgment of this study's innovations and suggestions for improvement. We did our best to address all the reviewers' comments. We believe that the manuscript is now clearer and more informative.
However, all parts could be improved since there are still some missing points and lack of precisions. In the results, it is not clear if the effect of biochar addition on the plant mineralization is due to the composition of biochar or to its particle size. Therefore, I think necessary to precise the size of biochar particles added and maybe to discuss this point.
Author’s response: Thank you for your observation. The biochar added in the experiment was between 2 mm and 0.063 mm. Therefore, we believe the changes in mineralization patterns are mainly due to the composition of the biochar rather than changes in particle size in the different textures. We have now added in the Material and Methods “Section 2.2 Incubation Experiment and Treatments” the details about the biochar size added. We have also added to the Discussion Section “4.1 Interactions of clay + silt content and biochar application in reducing early mineralization of plant residues” (second paragraph) an explanation regarding the effects of biochar particle size vs. composition. We believe these two distinct effects are more clearly differentiated in the manuscript.
To me, this study doesn’t evidence a clear effect of the texture on plant respiration in the control soil (without biochar). It is written that there is a statistic effect, but (1) results from statistics made on very small samples (n=3) should be treated with caution, (2) I am not convinced by the choice of the statistic test and (3) the curve does not show a clear trend. Hence, the results and discussion should not emphasize too much on this small tendency and not consider it as a clear result.
Authors' response: Thank you for your observation. Indeed, we agree that despite the significant correlations, the influence of clay and silt particles is quite minimal in our experiment, given the low slope of the curve. Therefore, as suggested, we have changed the discussion of the manuscript by minimizing the interpretations of the textural influence on plant mineralization (Discussion section 4.1, third paragraph). More details are given in the response to the specific comments.
In the discussion, only physical explanations are presented to explain the effect of pH on plant mineralization. Additional explanations should be proposed regarding the effect of pH changes on the microbial communities and on the nutrient availabilities from the plant for the microorganisms.
Author's response: Thanks for the suggestion. We have elaborated our discussion of the pH effect and added a section on the biological effects of pH changes on OC decomposability by soil, the soil microbial community, and the potential availability of nutrients. In addition, we supported our discussion of the biological effects of a changed soil pH by adding more literature references (Lines 166 – 171).
The discussion should include more comparison with other studies using artificial soils. Indeed, the results from studies using artificial soils can be very different from studies using field soils (e.g. Gross et al. 2021). Although the approach of using artificial soil is relevant, the study should more emphasize on its limits to extend these results to real field soils.
Authors’ response: Thank you for this critical observation. Indeed, the interpretations made here mainly focus on understanding the intrinsic relationships between soil particle size and biochar and how they affect C persistence. Nonetheless, such mineralization rates should not be used, e.g., quantifying C emissions from determined soil types or land uses, since this is not the purpose of this study. We now emphasize that in the meta-analysis conducted by Gross et al. 2021, the trend opposite to our findings was exclusively for field experiments, while the authors found the same trend for non-field experiments. These differences are now more comprehensively discussed in the discussion section (Lines 144-157) and the introduction (Lines 26 – 33). We also emphasize better now to what situations this study can be applied in the discussion section (Lines 139 – 149) (non-field setup) so we do not overinterpret the outcomes of this experiment. We have added more references in the discussion section by comparing our findings with other studies in natural conditions and emphasizing how the use of artificial soils can help to go beyond the intrinsic relationships between specific factors, such as biochar and soil texture, the topic of our study. We have also clarified this aspect in Section 4.2, “Interactions between clay and biochar in enhancing soil pH, and the consequences of early mineralization of plant residues” (last paragraph), by indicating other experiments done in artificial soils and emphasizing the scope of this experiment. We believe that the central message of this study is now more precise, and the limitations are more emphasized. We also mention more specific factors in this question in the answers to specific comments.
Please find our response to the specific comments and technical corrections in the PDF file.
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AC1: 'Reply on RC1', Franziska Bucka, 22 Jul 2024
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RC2: 'Comment on egusphere-2024-1143', Anonymous Referee #2, 26 Jun 2024
The authors present a study investigating the effect of biochar on carbon mineralization in soils of different texture. Although technically sound and well presented, in my opinion, the study as written is insufficient to conclusively corroborate the assertions and conclusions made by the authors. I strongly feel the authors could rewrite and present a stronger case for their study with more experimentation (e.g. utilizing more biochars from different feedstocks and comparing or using the same feedstock but varying temperature of pyrolysis or investigating effect of pH change alone without biochar. Increased pH or liming alone has been well documented to impact C mineralization with increased pH encouraging aggregation of clays and thus increased protection of carbon within the aggregates and binding to Ca2+ and clay surfaces.
Citation: https://doi.org/10.5194/egusphere-2024-1143-RC2 -
AC2: 'Reply on RC2', Franziska Bucka, 22 Jul 2024
Authors' response: We acknowledge the reviewer's observation regarding the scope of analyses and treatments but believe the novelty of our findings and their potential to promote further investigation in biochar research outweigh the need for a more extensive initial study, making a Short Communication the ideal format for presenting these concise, high-impact results. In the revised manuscript, we have made significant improvements to better illustrate this study's main innovations and how they help to understand questions regarding using biochar in soils with different textures. Despite the variety of studies evaluating the effects of biochar on C storage and liming, much information on how soil texture affects these impacts remains unknown, given that many conflicting results exist in the literature. Besides the lack of studies under a broader range of conditions, a significant reason for these uncertainties is the heterogeneous nature of soils, which often hampers interpretations regarding the exact factors responsible for the observed changes. In this sense, we showcase in this short experiment that using artificial soils to investigate the intrinsic relationships between biochar and soil particle size using soils with pre-made exact compositions can be an excellent and underutilized tool. To our knowledge, no other studies have systematically investigated using biochar under a gradient of soil textures with the same mineralogy and C amount. Also, no other studies have used artificial soils to study these interactions. We understand the desire for more comprehensive analyses. Nonetheless, we argue that even incremental advancements, such as those presented in our manuscript, play a crucial role in the progression of biochar research. Publishing these findings as a short communication allows us to promptly share valuable information with the scientific community, fostering a more dynamic exchange of ideas and accelerating overall progress.
Citation: https://doi.org/10.5194/egusphere-2024-1143-AC2
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AC2: 'Reply on RC2', Franziska Bucka, 22 Jul 2024
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