Effects of Biochar-Based Fertilizers on Energy Characteristics and Growth of Black Locust Seedlings

Round 1

Reviewer 1 Report

This paper investigates the use of different biochar-based fertilisers for growing plants on karst area soil which has limited nutrients content. The main objective is to verify whether the energy characteristics of these plants are affected by different soil treatment with biochar. Three plant species were analysed under five different soil treatments in terms of content of biochar (0, 2 and 4%) and biochar source (wood and rice-husk). The results are compared to one control experiment and appropriately discussed using statistical analysis. The paper is well-organized and well-written. I believe the results are a good start point for implementation of energy crops in karst areas, although it should not be considered as a conclusive study since the tested treatment were quite limited. Moreover, the methods section must be improved to allow for experiments reproduction and comparison under other experimental conditions. Thus, I would recommend it for publication in Sustainability after major revisions are conducted.

 

Major comments:

Line 75 – I understand that the soil sample was taken at a specific location of a given karst area, which, in reality, covers a wide area and potentially has heterogeneous distribution of nutrients and soil bulk properties. Could you describe how this soil sample was proved to be representative of the whole karst area, in terms of physical and chemical properties? Would you expect different results in case the soil sample was taken at a different location?

Line 95 – Could you explain why the maximum content of biochar on the soil was set to only 4%? In my opinion, additional contents (higher %) are necessary to proper understand the effect of biochar content on the energy characteristics of such plant species, especially for recommending the optimal values for real implementations in karst areas.

Line 103 – The pulverisation method along with the separation process (sieving) must be described with details in the methods section. How relevant was the percentage of powders over 0.25 mm compared to the powder that was analysed for energy characteristics? Why not analyse the complete sample?  

Line 370 – Authors propose the use of 4% rice husk biochar-based fertiliser for the cultivation of Hong-sen locust in karst areas. I believe this statement might be misleading as higher % of biochar in the soil might give even better results, which was not investigated in this study.

Line 291 – It is important to mention in the paper that this correlation is strictly valid for the tested conditions only. I would also recommend adding a sentence commenting on how more general correlations – and useful for field applications - can be obtained.

 

Minor comments:

Line 77 - Is it the loose bulk density of the soil or it was measured at tapped or compaction conditions? Please add this information in the text.

Line 87 – Why choose 60 °C for drying in this case? How many hours were used for the drying process?

Line 95 – Please specify whether the % refers to the total mixture (soil+fertiliser+biochar) or it is only the ratio to the soil mass.

Lines 99 - In this case, does the 9 kg of soil stands for the mixture of soil+biochar+fertiliser?

Line 103 – Was the drying process performed at 100 or 85 °C? Was it performed with an oven without convection for 24h?

Line 285 – Y1, Y2 and Y3 in the equations are a function of all 8 parameters (X1 to X8) which might be an overfitting. I would suggest showing in the paper also other equations that depend on less parameters (e.g., the most important ones, X1 to X4 or others) and compare it with equations 2 to 4.

Author Response

Please check the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

First of all, I would like to say that the Results section is described well. Unfortunately, the other parts of the article are too superficial (including the discussion). In addition, this is only a one-year pot experiment. Some suggestions for improvement are in the comments in the attached file. The goals of the work and hypotheses should be clearly formulated and specified. The discussion and the conclusion should pay more attention to them. To sum it up, even though these are only one-year results, they can contribute to understanding the issues. However, it is necessary to work on the article and increase its academic level.

Comments for author File: Comments.pdf

Author Response

Please check the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors presented an improved version of the manuscript, thus I would recommend it for publication after some minor revisions:

 

1) I appreciate the explanations you have given in the Major Queries number 2, 3 and 4 (this are the questions of the first revision starting with the names "Line 95" , "Line 103" and "Line 370"). I consider very important that this information is included in the paper, even in a shorter form. Could you please add this answers to your paper, for example, in the methods or results sections? 

 

2) The explanation for bulk density is confusing, specially the ratio to the water content that would give a dimensionless number. Please review this explantation. The bulk density of a particulate material is a function of the applied pressure, hence it can be measured in loose or tapped conditions. Please indicate which one you have measured (loose or tapped) and how was the exact procedure to do it. If the tapped bulk density was measured, it is necessary to indicate what was the applied pressure or number of taps used.

 

 

Please could you include in the manuscript 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Dear author,
thanks for improving the article. It looks much better. In my opinion, however, there are a few important little things that need to be addressed. There are four comments in the attached file. In addition, I am not completely satisfied with your answer regarding pH:

Question: There should also be described information on how the soil pH changed at the end of the experiment:

Answer: The study of this text mainly focuses on the influence of biochar-based fertilizers on the calorific value characteristics of plants, so we neglected the determination of soil pH in the experiment, and now the soil samples have been lost. However, we determined a series of soil nutrient indexes except pH, which have been published in the graduation thesis of the second author (Zhu Qian). The results are as follows (Table S2).

 

I understand that you can no longer determine the pH. But mention the effect of biochar on soil pH in the Introduction or Discussion sections. The impact of biochar application on pH can be significant (especially in acidic soils).

 

Best regards.

Comments for author File: Comments.pdf

Author Response

Please check the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Dear author,

Thank you for the explanation. You cite (in your answer) articles that have studied the effect of biochar on pH. Just because the addition of biochar or biochar-based fertilizer had a significant effect on the pH of karst soils, this fact should be mentioned in your article. It would be sufficient to enter the following sentence in the Introduction section:

It was found that the addition of biochar may affect the pH of karst soils [Zhidong et al; Jiaman et al.].

Zhidong, Z .; Taotao, Y .; Qian, Z .; Xiaoli, B .; Bin, C .; Jianhui, X .; Yongbo, W. Bacterial community structure shifts induced by biochar amendment to karst calcareous soil in southwestern areas of China. J Soil Sediment 2019, 19. 2.

Jiaman, S ,; Xiaoli, B .; Yongbo, W .; Jianhui, X. Effects of biochar application on the growth of Ｒobinia pseudoacacia L. seedlings and soil properties in limestone soil in a karst mountain site. Chinese Journal of Ecology 2016, 35 (12): 3250-3257.

 

Best regards.

Author Response

Dear reviewer,

Thanks for your patient review of my manuscript.

I have added the influence of biochar addition on soil pH in the introduction section. References have been added and reordered.

Thanks for your consideration.

Sincerely.