Characterization of the Residue (Endocarp) of Acrocomia aculeata and Its Biochars as a Potential Source for Soilless Growing Media

Round 1

Reviewer 1 Report

This manuscript fully investigated the properties of Acrocomia aculeata residue biochar, and their application in substitution of peat substrate. This research is of wide research interest and deserved to be publication. However, current form is limited by several drawbacks. First, the novelty should be highlighted, e.g., why select the selected properties. Second, the methods and materials were not detailed enough, e.g., should introduce how to conduct the substrate experiment. Most importantly, the overall structure and figure and table quality should be strongly improved and carefully revised. Specific comments were given as below:

1. Line 17: Please modify this sentence “The pyrolysis-induced loss of organic matter increased the ash content and thus pH and electrical conductivity.”, what did the “latter” refer to?

2. Line 21: How much does the water content of biochar treatment increase compared with peat treatment? Please also specify the other parameters. Line 23: “tomato growth” should be changed to “tomato seed growth”.

3. Line 21-23: The authors should firstly introduce the experimental treatments and then described the corresponding changes and advances.

4. The keywords “Field Emission Scanning Electron Microscopy”, “specific BET surface area” and “biochar evaluation” should be deleted, “biochar substrate” should be added.

5. Line 39: “2011 about 5 t of kernel oil·ha-1”, the unit was wrong in format, and please avoid using the data which was ten years ago.

6. Methods and materials: How to conduct the experiment of soilless gardening substrates?
7. Figure 6: Please improve the Fig. 6 according to previous published papers.

8. Several parameters of current biochar were proposed in this study, the authors should carefully explain why select these parameters for biochar characterization in the introduction. Reorganize the introduction in logical demonstration. Current form make readership can not realize the novelty of this work. Herein, the authors should also highlight their novelty and research objectives.

9. Line 31-51: Please shorten this introduction and just introduction why this biomass should be adopted to produce biochar in brief. Afterwards, the authors should introduce why they analyze the corresponding properties in following manuscript.

10. After introduction and discussion of biochar properties, the authors should further tell why the biochar can be considered to be used as soilless gardening substrates based on the common feature between peat and biochar.

11. The authors should extend the introduction by referring to doi.org/10.1016/j.scitotenv.2022.157037 and doi.org/10.3390/su14137924.

Author Response

Dear Editor, Dear Reviewers:

First of all, I want to thank the reviewer for the helpful comments that helped considerably to improve the quality of the manuscript.

As you can see, we followed the suggestion when it was appropriate (which was mostly the case) and provided an explanation and justification when we didn`t do it.

In addition, we included some more detailed discussion in particular with respect to the data which were requested to be included. Concomitantly, we did improve the English and corrected typos when we found them.

We hope that after this revision, the quality of our manuscript has improved sufficiently for being considered for publication in your journal.

With kind regards

Heike Knicker

Here a detailed list of our answers to the comments of the reviewers:

Reviewer 1:

This manuscript fully investigated the properties of Acrocomia aculeata residue biochar, and their application in substitution of peat substrate. This research is of wide research interest and deserved to be publication. However, current form is limited by several drawbacks. First, the novelty should be highlighted, e.g., why select the selected properties. Second, the methods and materials were not detailed enough, e.g., should introduce how to conduct the substrate experiment. Most importantly, the overall structure and figure and table quality should be strongly improved and carefully revised. Specific comments were given as below:

1. Line 17: Please modify this sentence “The pyrolysis-induced loss of organic matter increased the ash content and thus pH and electrical conductivity.”, what did the “latter” refer to?

Answer: We removed  …and electrical conductivity…. because it didn´t increase with pyrolysis temperature. In contrast, for the finely milled samples of the feedstock it is considerably higher than for the corresponding biochar produced at 450°C. I have to admit that it took a while until we understood this behavior but considering the low pH of the vegetal feedstock, it is likely that dissolved organic acids are present that can dissociate and contribute to electrical conductivity. With increasing heating those compounds are degraded and only salts remains contribute to the electrical conductivity. This discussion is now included in section 3.5. We also removed the “latter”.

2. Line 21: How much does the water content of biochar treatment increase compared with peat treatment? Please also specify the other parameters. Line 23: “tomato growth” should be changed to “tomato seed growth”.

Answer: We included  tomato seed growth. Literature data for the approximate bulk density and volume-based water holding capacity (W_maxv) are included in the text in section 3.3.

3. Line 21-23: The authors should firstly introduce the experimental treatments and then described the corresponding changes and advances.

Answer: Considering the limited allowed words for the abstract, we preferred to combine description of the used methods directly with the obtained results.

4. The keywords “Field Emission Scanning Electron Microscopy”, “specific BET surface area” and “biochar evaluation” should be deleted, “biochar substrate” should be added.

Answer: We followed the suggestion and removed the requested keywords and included biochar-based substrate

5. Line 39: “2011 about 5 t of kernel oil·ha-1”, the unit was wrong in format, and please avoid using the data which was ten years ago.

Answer: We removed this citation and included new data from the FAO: Now it reads:

Worldwide, palm oil production has grown rapidly during the last 50 years due to the large demand for vegetable oil. It increased approx. 35 times from 2 million tons per year in the 1970th to 75 million tons in 2019 [3]

6. Methods and materials: How to conduct the experiment of soilless gardening substrates?

Answer: it is not clear what the reviewer tries to suggest. We provided a detailed description how the biochars were produced and subsequently analyzed. However, we did not perform a pot experiment.

7. 
   Figure 6: Please improve the Fig. 6 according to previous published papers.

Answer: I don`t know which previous published papers are meant, but we changed the colors and increased the size of letters.

8. Several parameters of current biochar were proposed in this study, the authors should carefully explain why select these parameters for biochar characterization in the introduction. Reorganize the introduction in logical demonstration. Current form make readership can not realize the novelty of this work. Herein, the authors should also highlight their novelty and research objectives.

Answer: The goal of our study was an in depth characterization of the biochar in order to reveal if physical and chemical characteristics of this material allow its application as additive to substrates or even as substrate in soilless horticulture. We did not focus on specific parameters in order not to limit ourselves. The respective methods we used were those which we considered best suitable and available. Characterization of biochar for its suitability as a certain application is not really a new scientific idea but to the best knowledge of the authors the biochar of endocarp of A. aculeate has not been characterized sufficiently.

9. Line 31-51: Please shorten this introduction and just introduction why this biomass should be adopted to produce biochar in brief. Afterwards, the authors should introduce why they analyze the corresponding properties in following manuscript.

Answer: We described the problematic that palm oil production creates a huge amount of biowaste that is presently mostly used for combustion. The novelty of our approach is its conversion into biochar and subsequently into an additive to substrates that can reduce the use of peat. We included the aspect of peat consume and the associate reduction of an important carbon sink as well as the problematic of high salt or P concentration in biochars for plant growth.

10. After introduction and discussion of biochar properties, the authors should further tell why the biochar can be considered to be used as soilless gardening substrates based on the common feature between peat and biochar.

Answer: We introduced some sentences in the result and discussion part explaining how the measured parameter may affect the quality of the plant growing media. This is in particular the case, when discussing pH, the newly added data about EC and exchangeable sodium.

11. The authors should extend the introduction by referring to doi.org/10.1016/j.scitotenv.2022.157037 and doi.org/10.3390/su14137924.

Answer: We had a closer look on the suggested references. The first is very interesting but is dealing with P leaching in soils and a special treatment of biochars that can modify this.  To be honest it is not really related to our work presented here and a detailed discussion of the P cycling in biochar amended soils is outside of the scope of the present work. The second is also focusing on the P cycle in soils as it is affected by biochar. However, we included the two references where we felt that it is appropriate.

Reviewer 2 Report

The title could be improved e.g Characterization of the residue (endocarp) of Acrocomia aculeata and its biochars as potential source for soilless growing substrate( or media.In the Introduction  sentences like this could be added.  There has been increasing interest in biochars, an organic carbon rich solid by-product over the last decade or so, particularly its use in soil, not only as a soil improver but also as its ability to sequester carbon. However, since about 2014 there has been increasing interest in biochar as a peat replacement or as a component of growing media. Publication in this area has seen an exponential growth especially in the last two or three years. At least six review articles have been published (Kaudal et al., 2016 Kern et al., 2017; Alvarez et al., 2018; Zulfiqar et al., 2019; Huang and Gu, 2019, Prasad & Tzortzakis 2021)Line 69, green cuts garden prunings. L 76 or substrate in hydroponic horticulture or growing substrate  for container grown plants and hydroponic horticulture L 122 The authors must provide bulk density(BD) of the material i.e grams per liter.Plants are grown in volume basis not weight basis in soilless culture e.g 1 liter or 2 liter container. In addition all results in at international level and certainly in Europe in soilless culture are presented in volume basis.If the reader knows the BD he can convert the EC to volume basis. L131 Olsen-P is not normally used in growing media , there are comprehensive  CEN( The European Committee for Standardization) methods available for most plant nutrients.L 404  The high pH of biochars  may not be related to Ca levels See Prasad and Tzortzakis 2021 

What about Na? It can have a major effect on pH and EC Did they not measure Na? In Coir Na levels are very high. I do not know much about this crop but why no data on Na is given. High K can decrease uptake of Mg and  Ca more so than N (L412).As the biochar component increases in a peat/biochar-mixtures, this is affecting nutrient levels in plants; for example, the levels of Mg content in a leaf dropped due to the antagonistic effect of K on Mg. This is primarily due to the excessive level of K, which is a feature of most but not all biochars. Additionally, in our recent study, K, P and Cu accumulation and Mg deficiency in cabbage leaves were related to the biochar presence and feedstock  In that study, the biochar’s feedstock, rate and the addition of fertilizers could affect the cabbage seedling performance. (Prasad et al.,Plant Nutrient Availability and pH of Biochars and Their Fractions, with the Possible Use as a Component in a Growing Media. Agronomy 2020, 10, 10; doi:10.3390/agronomy10010010). In Table 5 they have shown the effect of temperature and particle size on specific surface area.Recent work has shown that at low rates biochar with high specific area can reduce EC.(Prasad and Tzortzakis 2021.Acta Hort., reference details above)These results are very significant as most non-peat growing media usually have high EC and it is often the limiting factor in their use in peat dilution (Nieto et al., 2016). Perhaps this should be discussed.Although Electrical Conductivity was measured no results are provided as far as I can see. Why not?EC is often the most likely limiting factor when biomass is used in growing media. 

The reference of Sanju et al is not very relevant as it applies to tomatoes growing outdoor in mineral soil. Yields of tomatoes under protected conditions are often 4 or 5 times higher than under outdoor field conditions. No doubt the extra available P in the biochar is relevant when nutrient management of the crop growing in biochar is considered but to say if it is adequate is not supported.  Perhaps a reference Prasad et al., 1981, Soil testing of horticultural substrates, iv Desirable phosphorus values for 1:1.5 water extract andOlsen's extract. Comm. in Soil Science and Plant Analysis  12:855-865

Author Response

Dear Editor, Dear Reviewers:

First of all, I want to thank the reviewer for the helpful comments that helped considerably to improve the quality of the manuscript.

As you can see, we followed the suggestion when it was appropriate (which was mostly the case) and provided an explanation and justification when we didn`t do it.

In addition, we included some more detailed discussion in particular with respect to the data which were requested to be included. Concomitantly, we did improve the English and corrected typos when we found them.

We hope that after this revision, the quality of our manuscript has improved sufficiently for being considered for publication in your journal.

With kind regards

Heike Knicker

Here a detailed list of our answers to the comments of the reviewers:

Reviewer 2:

The title could be improved e.g Characterization of the residue (endocarp) of Acrocomia aculeata and its biochars as potential source for soilless growing substrate( or media.

Answer: Has been done as suggested

In the Introduction  sentences like this could be added.  There has been increasing interest in biochars, an organic carbon rich solid by-product over the last decade or so, particularly its use in soil, not only as a soil improver but also as its ability to sequester carbon. However, since about 2014 there has been increasing interest in biochar as a peat replacement or as a component of growing media. Publication in this area has seen an exponential growth especially in the last two or three years. At least six review articles have been published (Kaudal et al., 2016 Kern et al., 2017; Alvarez et al., 2018; Zulfiqar et al., 2019; Huang and Gu, 2019, Prasad & Tzortzakis 2021)

Answer: The respective sentence and literature has been included

Line 69, green cuts garden prunings.

Answer: The change has been made

L 76 or substrate in hydroponic horticulture or growing substrate  for container grown plants and hydroponic horticulture

Answer:The requested change has been made.

L 122 The authors must provide bulk density(BD) of the material i.e grams per liter. Plants are grown in volume basis not weight basis in soilless culture e.g 1 liter or 2 liter container. In addition all results in at international level and certainly in Europe in soilless culture are presented in volume basis.If the reader knows the BD he can convert the EC to volume basis.

Answer: Thanks for this helpful recommendation: We measured the bulk density of most of our samples, however, of some only very small amounts are left. In several cases it was not enough to obtain meaningful and reliable data. Nevertheless, for the most important samples we were able to provide the data. A interpretation of those data is also included in section 3.3.

L131 Olsen-P is not normally used in growing media , there are comprehensive  CEN( The European Committee for Standardization) methods available for most plant nutrients.L 404  The high pH of biochars  may not be related to Ca levels See Prasad and Tzortzakis 2021 

Answer: Olsen P is the most commonly used methods for soils and represents a reliable methods used in the analytical service unit of the university of Seville (Which we used)

With respect to the Ca, we agree in particular since our data confirm that Ca is not the main player (We included the reference)

What about Na? It can have a major effect on pH and EC Did they not measure Na? In Coir Na levels are very high. I do not know much about this crop but why no data on Na is given.

Answer: As recommended we included the contents of exchangeable Na of our samples. It is interesting that compared to other crops, those concentrations are relatively low. Thus, it can be assumed that this element will not create problems for plant growth

 High K can decrease uptake of Mg and  Ca more so than N (L412).As the biochar component increases in a peat/biochar-mixtures, this is affecting nutrient levels in plants; for example, the levels of Mg content in a leaf dropped due to the antagonistic effect of K on Mg. This is primarily due to the excessive level of K, which is a feature of most but not all biochars. Additionally, in our recent study, K, P and Cu accumulation and Mg deficiency in cabbage leaves were related to the biochar presence and feedstock  In that study, the biochar’s feedstock, rate and the addition of fertilizers could affect the cabbage seedling performance. (Prasad et al.,Plant Nutrient Availability and pH of Biochars and Their Fractions, with the Possible Use as a Component in a Growing Media. Agronomy 2020, 10, 10;

doi:10.3390/agronomy10010010). In Table 5 they have shown the effect of temperature and particle size on specific surface area.Recent work has shown that at low rates biochar with high specific area can reduce EC.(Prasad and Tzortzakis 2021.Acta Hort., reference details above)These results are very significant as most non-peat growing media usually have high EC and it is often the limiting factor in their use in peat dilution (Nieto et al., 2016). Perhaps this should be discussed.Although Electrical Conductivity was measured no results are provided as far as I can see. Why not?EC is often the most likely limiting factor when biomass is used in growing media. 

Answer: we included the EC, in particular we had the data. As it is demonstrated in the newly included discussion, our samples reveal a very low EC, most likely because the feedstock has a very small ash content. Thus, EC should not be a problem for plant growth.

The reference of Sanju et al is not very relevant as it applies to tomatoes growing outdoor in mineral soil. Yields of tomatoes under protected conditions are often 4 or 5 times higher than under outdoor field conditions. No doubt the extra available P in the biochar is relevant when nutrient management of the crop growing in biochar is considered but to say if it is adequate is not supported.  Perhaps a reference Prasad et al., 1981, Soil testing of horticultural substrates, iv Desirable phosphorus values for 1:1.5 water extract and Olsen's extract. Comm. in Soil Science and Plant Analysis  12:855-865

Answer: we changed the text and included the suggested reference.

Round 2

Reviewer 1 Report

The authors have strongly improved the manuscript quality, some issues still need to be addressed given as below:

(1) Figure 5: The quality need further improvement, e.g., SEM figure size were not the same. 

(2) Table 6: The AC320 need to be added in biochar types.

(3) Line 510: "reported [54]" should be changed to "reported [54].".

(4) Line 511: The total P content was also influenced by pyrolysis temperature, the authors should not omit this factor. The P will be gasified at c.a. 600~700 ℃.

(5) Line 546-550: These discussions should not be presented in the conclusion part.

Author Response

Dear Editor:

I want to thank again the first reviewer again. We have followed the suggestions and improved Figure 5.

With kind regards

Heike Knicker:

Comments of Reviewer 21

The authors have strongly improved the manuscript quality; some issues still need to be addressed given as below:

(1) Figure 5: The quality need further improvement, e.g., SEM figure size were not the same.

- Corrected as requested

(2) Table 6: The AC320 need to be added in biochar types.

- AC325 is now included in the table

(3) Line 510: "reported [54]" should be changed to "reported [54].".

- Corrected as requested

(4) Line 511: The total P content was also influenced by pyrolysis temperature, the authors should not omit this factor. The P will be gasified at c.a. 600~700 ℃.

- we included a more detailed discussion about the P: Now it reads_

 - For our samples, P_t shows a relative enrichment after pyrolysis which is in line with previous publications [9,19,27]. This can be related to the concomitant losses of carbon (C), nitrogen (N) and hydrogen (H) observed during the heat treatment (Table 2) and by heat-induced immobilization reactions leading to its relative accumulation. However, at higher temperature > 600°C, P can be gasified. The underlying mechanisms explaining this immobilization of P during pyrolysis, however, are not yet completely understood. Complexation of P with Al, Ca and Fe has been observed and suggested to play a key role [13,20,30]. Note that solubility of P in soils or plant growing substrates depends on the pH and has its maximum in the near neutral pH region which coincide with the lowest degree of P fixation by Ca, Al, and Fe minerals [57]. However, in our experiment the relative contribution of Olsen P to P_t in the biochars increased with increasing pyrolysis temperature. Possibly their low Ca contents lowered the efficiency of P fixation.

In this context, it is also worth to mention that in recent pot experiments with biochar amended soils, it was demonstrated that P immobilized during pyrolysis is partially remobilized during plant growth [58].

(5) Line 546-550: These discussions should not be presented in the conclusion part.

- Answer: We removed the respective sentence … on the other hand