Root-Zone Amendments of Biochar-Based Fertilizers: Yield Increases of White Cabbage in Temperate Climate

Round 1

Reviewer 1 Report

This is an interesting research. The authors are focusing on two different root-zone amendments of biochar (concentrated hotspot biochar root-zone and biochar-soil-mix) at low dosage (1.3 t/ha) and different mineral N compounds (NH₄NO₃ and Urea) on plant productivity and root growth response. The results of research indicated that the low-dose root-zone amendments of biochar could become an economic benefit for the farmer. It is creative and novelty, especially, they founded the biochar root-zone amendments and the N fertilization method can influence the root architecture in a way.

There are several parts of the manuscript need to be improved.

1. As we know, it’s a good way to test BC by field-spreading. Unfortunately, there is no a treatment of BC-field-spreading. Could you please explain the reason, Why? In my opinion, the treatment of BC-field-spreading is more suitable/reasonable than the treatment of no-BC as control.
2. Line 221-245 and Figure 2. For the NH₄NO₃, the yield of soil-surface fertilization treatment increased more than N-enhanced biochar treatment for both BC -hotspot and BC -soil -mix. However, for the urea, the result was opposite, please explain why.
3. Why do you design two different mineral N? It needs to be told in the background clearly.
4. To calculate the N utilization efficiency of different treatments may be helpful to the interpretation of the results, it is worth to take a try.
5. Table 1: Why did the last treatment have to be dismissed?
6. In B and C of Figure 4: It feels a bit difficult to know which bar is the control. The histogram of the control should be added clearly.
7. L391-394: The text said that “...the type of biochar root-zone amendment significantly influenced root areas”, however, the Figure 4B shows “a”. Could you please tell why that is?
8. Line 56 and Line 67: There are five or six literatures followed in a short sentences, is it necessary? It’s too many, I think.

Author Response

We thank the reviewer for examining our manuscript in detail and the helfpul comments. Please find our detailed responses in red font below:

(1) As we know, it’s a good way to test BC by field-spreading. Unfortunately, there is no a treatment of BC-field-spreading. Could you please explain the reason, Why? In my opinion, the treatment of BC-field-spreading is more suitable/reasonable than the treatment of no-BC as control.

An additional treatment that represents the field-spreading use case would certainly have been complementary and appropriate. Nevertheless, we focused our study on the root zone application of biochar, which is why the field-spreading case was not tested. In our opinion, a fertilized control without biochar amendment is absolutely necessary to evaluate the effect of biochar on plant growth as it is the current farmer practice.

(2) Line 221-245 and Figure 2. For the NH₄NO₃, the yield of soil-surface fertilization treatment increased more than N-enhanced biochar treatment for both BC -hotspot and BC -soil -mix. However, for the urea, the result was opposite, please explain why.

We were not able to work out the reason for this, and our data do not allow us to make a firm statement in this regard (which is now added in line 290). However, we appreciate your detailed examination of our manuscript. Our attempts to explain this remained highly speculative and have therefore not been included in the manuscript. When comparing the amendment of initially pure biochar combined with soil surface fertilization, the differences between Urea and NH₄NO₃ could be as follows: NH₄⁺ ions from NH₄NO3 may bind to the negatively charged sites of the pure biochar when applied to the pot. Thus, less cations (K+, Na+,..) from the soil were immobilized after fertilization and thus, the nutrient availability in these pots was less affected by biochar-induced nutrient immobilization. The interaction between urea and the biochar may be less intensive (only hydrogen bonds are present between urea and biochar, which are weaker) and thus, more positively charged nutrients from the soil may have been immobilized by biochar. However, once applied to the soil, urea is rapidly transformed in to NH₄⁺, too, and actually more NH₄⁺ could be present in the urea treatment. Another effect of the urea treatment (soil-surface fertilization and the pure hotspot biochar) compared to the complementary NH₄NO₃ fertilized treatment is, that hydrolysis of urea to NH₄⁺ causes an increase in soil pH. This may increase N losses through ammonia volatilization, which is likely when considering the initial soil pH of 7.1. Maybe, when urea was a priori loaded onto the biochar (N-enhanced biochar) the release of urea from biochar was slowed down, which also slowed down urea hydrolysis and thus resulted in more timely provision of nitrogen to the plant. This could explain the better performance of the hotspot amendment of enhanced biochar compared to the hotspot amendment with soil surface fertilization within the urea treatments.

(3) Why do you design two different mineral N? It needs to be told in the background clearly.

This was done to highlight potential differences between the two N species and to research a combination of biochar application and N fertilization that could be expected to increase yields for both different N variants. We added a short explanation on this aspect in the Introduction (line 75).

(4) To calculate the N utilization efficiency of different treatments may be helpful to the interpretation of the results, it is worth to take a try.

This would definitely be an interesting parameter to calculate in future research. We did not quantify N contents in the biomass as this was beyond the scope of our study. Therefore, we are not able to calculate the N use efficiency of the white cabbage plants. Please keep in mind that all treatments, despite different N species, received equal amount of nitrogen per pot.

(5) Table 1: Why did the last treatment have to be dismissed?

Unfortunately, an error occurred in the amount of fertilized N in this treatment, which is why it had to be dismissed.

(6) In B and C of Figure 4: It feels a bit difficult to know which bar is the control. The histogram of the control should be added clearly.

Actually, there was no "no-BC Control" plotted in these panels as only the interaction of biochar root-zone amendments and N fertilization method is presented. However, you are absolutely right that a bar with the no-BC control would be helpful in these panels and it was added accordingly in the revised manuscript. Please note however, that the statistical analysis in these panels does not include the no-BC control as the intention for this plot is to elaborate the interactions between the type of BC root zone amendment and method of N fertilization (N fertilization method "N-enhanced BC" is not applicable for the no-BC control).

(7) L391-394: The text said that “...the type of biochar root-zone amendment significantly influenced root areas”, however, the Figure 4B shows “a”. Could you please tell why that is?

It is possible that you calculate a p-value with the Anova that is below 0.05 but the post-hoc test does not provide different ranks for the different factor levels/treatments. This can occur especially with the Tukey post-hoc test, as it is not the most sensitive - but one of the most widely used. For further explanations we refer to this discussion board on ResearchGate where exactly the same question was dealt with: https://www.researchgate.net/post/Is-it-possible-to-get-non-significant-results-in-post-hoc-test-when-we-got-the-significant-result-in-ANOVA

(8) Line 56 and Line 67: There are five or six literatures followed in a short sentences, is it necessary? It’s too many, I think.

Since we would like to express in this sentence that there are now several studies on the formulation of biochar-based fertilizers, we consider it appropriate to also cite a number of references at this point.

Reviewer 2 Report

The manuscript presented results of N-fertilizer-based biochar application on white cabbage. It shows detailed data on both above and below-ground plant biomass properties and also growing media. This is a well-prepared manuscript; however, the experimental treatments presented are not clearly distinguishable.

In Table 1, the description of the treatment could be presented as three factorial arrangements, and the treatment described in the column seems unnecessary.

In Section 2.3, Line 102: the total number of treatments was indicated as 11, but in Table 2, it was presented as 10.

It is suggested to check this issue throughout the manuscript during revision.

Author Response

We thank the reviewer for his helpful comments on our manuscript and give some detailed responses in red font below: 

In Table 1, the description of the treatment could be presented as three factorial arrangements, and the treatment described in the column seems unnecessary.

You are right, the treatment description could be dismissed. We have now assigned an ID to the different treatments so that you can still easily refer to a treatment in the text.

In Section 2.3, Line 102: the total number of treatments was indicated as 11, but in Table 2, it was presented as 10.

Thank you for examining our manuscript in detail. As indicated in line 106 and Table 1, the treatment with ID 11 had to be dismissed during the experiment. Therefore, Table 2 only presents results for 10 treatments in total.

Reviewer 3 Report

The article is original and it is an interesting paper, which represents a notable contribution to the knowledge about the application of biochar to agricultural soils.

The incorporation of biochar into the soil is a mitigation tool due to its ability to fix carbon; supposes a sequestration of C that will help minimize the effects of climate change and reduce the atmospheric concentration of CO₂.

Materials and methods are well described, the experimental design is correct and is nicely shown in fig. 1. The analytical procedure seems appropriate to obtain reliable data. In addition, the supplementary material contributes to a good understanding of the experiment.

Nevertheless, some more information would be necessary. How has the biochar and N addition per pot been calculated from the proportion per hectare? What soil density and what depth has been considered? The amount of biochar and N applied seems higher than that indicated per ha.

The results are clearly presented in different figures and tables and the supplementary material improve the understanding of the article, although some complementary data should be included.

To complete the results some analysis data from the cabbage plants should be included. This would inform whether the amendment really improves the nutrition of the plants and would complement the results related to their production. What is the reason that no analysis beyond the Spad index and vitamin C is included? If any plant analysis data are available, these should be included.

Perhaps the reduction in the number of root hairs and therefore in root density when biochar amendment was localized at hotspot in the root zone, could be a problem in field conditions by reducing the soil area explored by the roots, reducing their ability to capture nutrients and water.

This should be considered for the extrapolation of the results to real field conditions.

The conclusions respond sufficiently to the aim of the work.

Author Response

We appreciate the detailed examination of our manuscript by the reviewer and give responses to the comments in red font below:

Nevertheless, some more information would be necessary. How has the biochar and N addition per pot been calculated from the proportion per hectare? What soil density and what depth has been considered? The amount of biochar and N applied seems higher than that indicated per ha.

Since we looket at root zone application, the conversions between field and pot were based on the number of plants (40.000 cabbage plants per hectare) rather than the area. The biochar amendment and N fertilization amounts per hectare have been calculated as follows: We applied 32g of biochar and 2.5 g of N per pot = per plant. This results in 0.032 kg BC x 40.000 plants ha^-1=1280 kg ha^-1 for biochar and 0.0025 g N x 40.000 plants ha^-1=100 kg N ha^-1, as indicated in line 115-118 of the revised manuscript.

To complete the results some analysis data from the cabbage plants should be included. This would inform whether the amendment really improves the nutrition of the plants and would complement the results related to their production. What is the reason that no analysis beyond the Spad index and vitamin C is included? If any plant analysis data are available, these should be included.

We have no further plant analyses to present, as this was beyond the scope of our study. You are right, further analysis of the plant material e.g., the nutrient content would provide additional insights. However, we focused in this study on the biomass production and crop yield. The influence of this method of biochar-based fertilization on e.g. the nutrient uptake should be studied in future research. We added a short sentence in line 323 of the revised manuscript. 

Perhaps the reduction in the number of root hairs and therefore in root density when biochar amendment was localized at hotspot in the root zone, could be a problem in field conditions by reducing the soil area explored by the roots, reducing their ability to capture nutrients and water. This should be considered for the extrapolation of the results to real field conditions.

We fully agree, this is an important point. This would have to be studied in field trials conducted under real conditions or in additional pot trials e.g. under simulated drought. We added this aspect in line 460 of the revised manuscript.

Reviewer 4 Report

In the line 227 the sentence is not completed: "While the application of the N-fertilizer on to..." Please complete the sentence.

In lines 352 to 354: Assumption of about 20 % yield increase upon the use of biochar-based fertilizers does not have to reflect to the field conditions since your experimental plants probably suffered from the scarcity of nutrients in the limited soil volume in pots. Please add this comment in a line 364.

Author Response

Dear Reviewer,

we appreciate your examination of our manuscript and your helpful comments to improve it. Our replies to your comments are given in red font below:

(1) In the line 227 the sentence is not completed: "While the application of the N-fertilizer on to..." Please complete the sentence.

Thank you for this reference, we completed the sentence (line 233), which was by mistake interrupted by the presentation of Figure 2 and Table 2.

(2) In lines 352 to 354: Assumption of about 20 % yield increase upon the use of biochar-based fertilizers does not have to reflect to the field conditions since your experimental plants probably suffered from the scarcity of nutrients in the limited soil volume in pots. Please add this comment in a line 364.

We completely agree with your comment, that was exactly the aspect we wanted to clarify in the concerning paragraph. As suggested, we have expanded the relevant paragraph (line 387) to include the aspect of limited plant growth due to a smaller soil volume in the pot than in the field.

Reviewer 5 Report

Biochar is the solid product of biomass pyrolysis, which not only plays an important role in greenhouse gas emissions, but also improves crop yield. However, it needs to increase a large amount of dose to play a role, and the cost is high, so it is difficult to be applied in practice. In order to make up for the low nitrogen content in biochar, the author prepared nitrogen enhanced biochar by mixing with nitrogen fertilizer.

In addition, the addition of N enhanced biochar in the root zone is innovative, which can effectively improve the fertilizer efficiency, reduce the dose of biochar and reduce the cost. The results are helpful to the practical application of biochar. On the whole, the design and data analysis of the paper are reasonable, and the deficiency is only the results of pot experiment without field data support. Here are a few small problems to pay attention to.

Line 20 The nitrogen is wrong font.

Line 227 Line 227 “While the application of the N-fertilizer loaded onto” is incomplete.

Table 2 “Urea-Fertilizer”  align left. The font is too large, the format needs to be adjusted, and it is not perfect.

Author Response

We appreciate the comments of the reviewer which helped to further improve our manuscript. Please finde our detailed responses to the reviewer comments in red font below:

(1) Line 20 The nitrogen is wrong font.

We changed the font size accordingly.

Line 227 Line 227 “While the application of the N-fertilizer loaded onto” is incomplete.

The sentence was interrupted by Figure 2 and Table 2. The sentence is now displayed without interrruption (line 233).

Table 2 “Urea-Fertilizer” align left. The font is too large, the format needs to be adjusted, and it is not perfect.

We changed the local site orientation to horizontal which allows a better format of the Table.