Review Reports - Carbon Sequestration in Support of the “4 per 1000” Initiative Using Compost and Stable Biochar from Hazelnut Shells and Sunflower Husks

Round 1

Reviewer 1 Report

Review Report 1

The manuscript titled “Carbon sequestration in support of the “4 per 1000” initiative using stable biochar from hazelnut shells and sunflower husks” is very interesting research and has the virtue of stating the underlying the fundamental aspects of producing biochar in a straight forward manner, in this case, fixed bed and rotary kiln. At the same time, the research recognized in the literature review section about the dilemma that revolves around the use of biochar as a soil amendment; it’s recalcitrant nature that often discourages microbial growth.

Abstract

Well written and concise. Please, provide the scientific names of hazelnut and sunflower in this section.

Introduction

Please, elaborate the last paragraph of this section. Include what are the quality parameters, what processes were compared etc.

Materials and Methods

Please, elaborate on the van-Krevelen diagram for the readers. Please, state what was the statistical tools or software that was used to analyze the generated data?

Results

This section is very well described. Under the “3.2 Biochar” section, please, move the texts before Table 1. Please, make a subsection for the van-Krevelen diagram in this section and state the results.

Discussion

The discussion part is very written well but lacks coherency, especially, it was difficult for this reviewer to relate the results to the discussion. The authors may consider a more concise discussion, especially, from Line 250 to Line 278.

Conclusions

“Longer retention times resulted in higher gas fractions”. Please, consider omitting this sentence.

Specific Comments

Line 268. Instead of writing “own results” write demonstrated in the result section.

Line 186. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Line 225. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Line 299. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Line 300. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Author Response

Thank you for the encouraging comment.

Abstract

Well written and concise. Please, provide the scientific names of hazelnut and sunflower in this section.

Latin names have been amended.

Introduction

Please, elaborate the last paragraph of this section. Include what are the quality parameters, what processes were compared etc.

The planned analyses have been detailed.

Materials and Methods

Please, elaborate on the van-Krevelen diagram for the readers. Please, state what was the statistical tools or software that was used to analyze the generated data?

A more comprehensive overview about the meaning and use of the van-Krevelen diagram has been added.

The SigmaPlot-software, used for statistics and graphical presentations, was listed in sub-section 2.2.6.

Results

The structure of the section Results has been modified, sub-section titles have been elaborated, the distribution of text and tables has been re-arranged. The second sub-section (3.2) starts now with a text paragraph. A new sub-section for describing the results with regard to the van-Krevelen diagram has been created (3.3).

Discussion

The first paragraphs of the Discussion were re-written to show a clearer coherence between the results presented in section 3 and the derived calculations and conclusions in section 4.

Conclusions

“Longer retention times resulted in higher gas fractions”. Please, consider omitting this sentence.

The sentence has been omitted to avoid confusion and the Conclusions generally have been revised.

Specific Comments

Line 268. Instead of writing “own results” write demonstrated in the result section.

This term has been modified as suggested.

Line 186. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Fig. 3 has been re-drawn

Line 225. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Fig. 4 has been re-drawn

Line 299. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Line 300. Please, remove the background lines from the graph and this reviewer suggests the authors use a graph making software other than Microsoft excel.

Fig 5 has been re-drawn (there should be just one graph for Fig. 5).

Reviewer 2 Report

Authors had a very value project idea and reading this manuscript I was really looking forward to see the results of this work. Unfortunately I feel little disappointment after reading this manuscript. Authors have try to answer the question how much biochar and compost should we put in to the soil to reach carbon sequestration goal but there are many inconsistence in results description and discussion. The biggest objection is that Authors focused on two types of biochar and there is defiantly more about hazelnut shell biochar compared to sunflower husk biochar. Table 1 describes elemental composition of HNS biochar, what about sunflower husk biochar? Why the sets of data that were compared are different (Table 1 and 2)? Figure 3 presenting effect of the process again only HNS is described. Discussion part Authors again focused on HNS biochar, what about SFH biochar? Which one could be more efficient? Please make a comment on that and rewrite results and discussion part consequently. Table 3. the title is too long and some futures like column 2 and 3 are doubled? It’s really interesting but hard to follow the idea and calculations of the Authors. Discussion is cut, there is no end, no final sentence and I’m not really sure that all calculations from Table 3. and Figure 5 should be in this part. It would be better to move this data to results part discussing it in discussion part only. An finally please think over the title of the Article. The previous one is very promising, but in the manuscript there is an discussion about biochar, mix of compost and biochar (not presented in the Aims of study) and Authors negate the profits of BC use for carbon sequestration.

Author Response

Thanks to the reviewer for the valuable comments. We tried to include the suggestions as far as possible into the manuscript:

Additional explanations for the selection of hazelnut shells and sunflower husks have been included in the introduction. There are more data available about hazelnut shells because the study was part of a Chilean-Austrian cooperation project and for Chile hazelnut shells is the more important and easily available agricultural waste material. Therefore, more experimental runs and analyses could be made with this material in Chile. In Austria only supplementary production runs and analyses with sunflower husks could be made and the application of the results to the carbon sequestration objective was the focus.
In Table 1, data from sunflower husk biochar analysis have been added.
For Figure 3 the comparisons of reactors and temperatures are the focus.
The legend of Table 3 has been shortened; a part of the information has been shifted to section 2.2.5. The third column in this table is not redundant because it distinguishes between annual an perennial crops.
The title of the paper has been modified for a better description of the content of the paper
The aims of the study have been described more comprehensively.
The discussion has been modified; on the one hand to connect it better to the results section, on the other hand to provide a more satisfying end of the discussion section.
The benefits of biochar for carbon sequestration should not be neglected; we put them forward to show that biochar would be a support for the insufficient compost C sequestration.
The Conclusion section, too, has been supplemented to better summarize the derivations from the discussion.

Reviewer 3 Report

Why choose hazelnut shells (HNS) and sunflower husks (SFH) to evaluate the C sequestration potential? Please explain the important and significance in Austria.
Why use three carbonize methodology, fixed bed reactor, rotary kiln and screw reactor? In current text, the description of three methods are in detail.
The analysis of two studied biochars are few, and seems focus on elemental analysis in order to investigate the biochar stability by O/C and H/C atomic ratio and suggestions of Spokas (2010). The literatures for discussing biochar stability by O/C and H/C atomic ratio or examining biochar characteristics including O/C and H/C atomic ratio are a lot. To collect those published biochars with O/C and H/C atomic ratio and show in the same diagram (van-Krevelen diagram) will be more attractive.
From the "Discussion", two biochar and three carbonization reactors seems irrelevant. (only found HNS in P 266) The current text can be separated into two parts, before and after Discussion, and almost has no relevant between two parts. If the examination of two biochars and their production process with three reactors are not important or attractive, please correct or substituted with literature review. If possible, Please establish the relationship between the two parts.
How Figure 5 is drawn？
P270-271 "actual price for commercial biochar of 600-750 €/t "How about the price of biochar produced in current study?

Author Response

Why choose hazelnut shells (HNS) and sunflower husks (SFH) to evaluate the C sequestration potential? Please explain the important and significance in Austria.

The reasons to study hazelnut shells and sunflower husks have been explained more explicitly in the introduction now. Shortly, the study was a Chilean-Austrian cooperation. Chile is the major exporting country for hazelnuts in the Southern hemisphere, so hazelnut shells are an abundant resource there. For Austria, sunflower husks are an easily available agricultural residue as the major sunflower producing countries are located in Eastern Europe, not far away from Austria.

Why use three carbonize methodology, fixed bed reactor, rotary kiln and screw reactor? In current text, the description of three methods are in detail.

The comparison of the three pyrolysis processes was one of the aims of the cooperation project. It should be clarified if the different reactor types produce significant differences concerning the recalcitrance of the biochars. As shown in Figure 4, residence time and pyrolysis temperature are the more important process parameters.

The analysis of two studied biochars are few, and seems focus on elemental analysis in order to investigate the biochar stability by O/C and H/C atomic ratio and suggestions of Spokas (2010). The literatures for discussing biochar stability by O/C and H/C atomic ratio or examining biochar characteristics including O/C and H/C atomic ratio are a lot. To collect those published biochars with O/C and H/C atomic ratio and show in the same diagram (van-Krevelen diagram) will be more attractive.

We agree with the reviewer that a more extensive dataset would have been desirable but even with the reduced dataset we could show the hypothesized shifts of the O/C- and H/C-ratios. For the sake of clarity we have not graphically included other literature data in our van-Krevelen diagram but we have extended the discussion with additional references that had not been included before.

From the "Discussion", two biochar and three carbonization reactors seems irrelevant. (only found HNS in P 266) The current text can be separated into two parts, before and after Discussion, and almost has no relevant between two parts. If the examination of two biochars and their production process with three reactors are not important or attractive, please correct or substituted with literature review. If possible, Please establish the relationship between the two parts.

Both the Results and the Discussion section have been partly re-written, especially for showing the connection between the end of “Results”, the begin and end of “Discussion”. Also the “Conclusions” were re-written to give a clearer picture of the story we want to tell.

How Figure 5 is drawn？

Fig 5. shows two informations:

the amount of biochar necessary for achieving the “4 per 1000” objective if different amounts of compost are added to a compost-biochar mixture.
The costs for the farmer if compost is used at a fixed price and biochar price varies between 600-800 €/t.

P270-271 "actual price for commercial biochar of 600-750 €/t "How about the price of biochar produced in current study?

The price of € 600-800 is for commercial biochar. When we were using our experimental reactors, partly at lab scale (the screw reactor), this price would have been higher. But for practical implementation of the suggested strategy one rather would use biochar from commercial pyrolysis reactors and not from experimental reactors.

Round 2

Reviewer 2 Report

Authors improved manuscript according to my suggestion and it can be recommended for publication in a present form.

Reviewer 3 Report

No more comments and suggestions.