The Influence of Biochar and Solid Digestate on Rose-Scented Geranium (Pelargonium graveolens L’Hér.) Productivity and Essential Oil Quality

Round 1

Reviewer 1 Report

The study describes the general characteristics of a biochar made from digestate solids, as well as a plant-soil interaction example for a locally relevant crop using industry-relevant metrics (oil yield/quality) beyond observing soil property changes. This kind of study is useful for understanding the feasibility of biochar and pyrolysis in a real system.

While generally well organized and written, there are a few sentences that read awkwardly or that contain typos. SPAD needs to be defined in the text, especially given the contrast between improvements from biochar (essential oils/total biomass) and potential negative effects (chlorosis). A picture of the chrolosis between the three treatments would be helpful for showing severity.

More context about the levels of Fe and Mn relative to recommended fertilizer levels and toxicity in geranium would be helpful. How "high" were the values seen in the leaves and media with biochar? Was this within a "normal" range? Is elevated pH a potential cause for chlorosis in geranium, or might the mild level of stress enhance the yield of essential oil/metabolism of geraniol to cintronellol? It was not clear if the observed effects were going to be a serious trade off for the feasibility of using solid digestate biochar or if the effects were simply being noted in the case that other future growers observe the same.

The explanation of the C:G ratio's relevance to the industry would have been more helpful in the introduction of the study, perhaps as part of the study objectives/description of "success" metrics.

The authors are encouraged to focus more on the feedstock used to produce the biochar than the pyrolysis process in terms of levels of heavy metals. The current phrasing could be read as pyrolysis creating a heavy metal problem regularly for all feedstocks, rather than not mitigating a feedstock high in heavy metals. Context on heavy metal soils relative to standards for potting mixes/soil amendments used in the area would be helpful, such as the limiting factors currently used for determining acceptable solid digestate application rates and times. Were any of the amendments or the resulting media above any of the limits? Were any tests done to estimate the fraction of available rather than total nutrients/metals?

What size of reactor was used to create the biochar? How was heat provided? How did the heating rate of the biomass compare to the heating rate of the reactor?

Author Response

Comments and Suggestions for Authors

The study describes the general characteristics of a biochar made from digestate solids, as well as a plant-soil interaction example for a locally relevant crop using industry-relevant metrics (oil yield/quality) beyond observing soil property changes. This kind of study is useful for understanding the feasibility of biochar and pyrolysis in a real system.

While generally well organized and written, there are a few sentences that read awkwardly or that contain typos. SPAD needs to be defined in the text, especially given the contrast between improvements from biochar (essential oils/total biomass) and potential negative effects (chlorosis).

We thank the reviewer for the comment. We have now defined SPAD

A picture of the chlorosis between the three treatments would be helpful for showing severity. 

Done (L:317)

More context about the levels of Fe and Mn relative to recommended fertilizer levels and toxicity in geranium would be helpful.

We thank the reviewer for the comment. Very few information on Pelargonium graveolens fertilization have been found. Therefore, we retain that our study could be one of the first informative papers on this aspect.

How "high" were the values seen in the leaves and media with biochar? Was this within a "normal" range?

We thank the reviewer for the comment. The sentence has been partly rewritten (L:321-323)

Is elevated pH a potential cause for chlorosis in geranium, or might the mild level of stress enhance the yield of essential oil/metabolism of geraniol to cintronellol?

We thank the reviewer for the comment. Unfortunately, we don't have sufficient information to confirm or deny this hypothesis. Therefore, we preferred do not address this aspect in the text

It was not clear if the observed effects were going to be a serious trade-off for the feasibility of using solid digestate biochar or if the effects were simply being noted in the case that other future growers observe the same.

We thank the reviewer for the comment. In the current literature none chlorosis cases linked with the use of biochar in pelargonium have been found. Therefore, we preferred just describe the observations avoiding speculations that we are not sure to address.

The explanation of the C:G ratio's relevance to the industry would have been more helpful in the introduction of the study, perhaps as part of the study objectives/description of "success" metrics.

We thank the reviewer for the comment. We preferred to maintain the explanation of the C:G ratio's relevance in the discussion to maintain the fluently of the introduction section for the readers.

The authors are encouraged to focus more on the feedstock used to produce the biochar than the pyrolysis process in terms of levels of heavy metals. The current phrasing could be read as pyrolysis creating a heavy metal problem regularly for all feedstocks, rather than not mitigating a feedstock high in heavy metals.

We thank the reviewer for the comment. We think that this is not the case. Heavy metals are introduced in the process through the feedstock, and be retained in the char, unless the metals of the reactor release it during the process (which is not the case of our study). The possible presence of heavy metals in digestate clearly depends on the feedstock that is sent to the anaerobic digestion process, and in that case it goes through the reactor and is found at the outlet in the digestate. However, if carbonised, biochar properties are such that these metals are kept in the highly porous product. We better specified in the text (L:36-43).

Context on heavy metal soils relative to standards for potting mixes/soil amendments used in the area would be helpful, such as the limiting factors currently used for determining acceptable solid digestate application rates and times. Were any of the amendments or the resulting media above any of the limits?

We thank the reviewer for the comment.  We now compared our values with the threshold suggested by the European Biochar Certificate (EBC) (L:277-278) and the Italian Ministry of Agriculture (L:280-281).

Were any tests done to estimate the fraction of available rather than total nutrients/metals?

We thank the reviewer for the comment. Unfortunately, we did not have estimate the fraction of available nutrients/metals

What size of reactor was used to create the biochar?

We thank the reviewer for the comment. We better specified in the text (L:76-81) The pyrolysis reactor is a pilot slow and intermediate pyrolysis unit, able to process the biomass up to 550 °C max. For the specific work the plant has been operated batch-wise, i.e. loaded with biomass, heated and then cooled down to ambient temperature, and biochar extracted. A total of approximately 8 kg of dry matter carbonized material was produced.

How was heat provided? How did the heating rate of the biomass compare to the heating rate of the reactor?

We thank the reviewer for the comment. The Heating Rate applied to the biomass during the slow pyrolysis process is stated in the article at page 2 line 93 (25 °C/min). The amount of biomass (few kg) well follow the temperature increase of the metal reactor, being a small mass compared to the whole. More details of the reactor can be found in: “Buffi M, Rizzo AMR, Pari L, Chiaramonti D Intermediate pyrolysis campaign to assess products yield and quality varying biomass particles size, temperatures and composition. Int. J. Oil, Gas and Coal Technology, Vol. 17, No. 3, pp 355-373, 2018” and other publications by our group. A scheme of the system is given below (from the IJOGCT publication).

Author Response File: Author Response.pdf

Reviewer 2 Report

General comment

This manuscript reports the effects of different substrate-based growing media on geranium cultivation based essential oil production and quality at a greenhouse-based study. The topic matches the aim and scope of the journal, and presents an interesting dataset demonstrating differential impacts of amendments on the measured variables. The manuscript can be improved by 1) formulating some experimental hypothesis in relation the objective, and relating experimental results back to hypotheses, this may help the authors better address observed differences in different growth substrates; and 2) improving the integration/correlation of the digestate/biochar characterization data with crop growth performance and essential oil yield/quality within the context of the experiment and the cited literatures.

In addition, this manuscript needs fine English language editing from the native speaker before it can be accepted for publication into Agronomy as it contains several grammatical errors, also lacks adequate flow & smoothness in sentence formulation and restructuring in its present form.

Specific comments

Line 11: Replace ‘generated large’ with “has created a global”

Line 15: Add ‘growth’ after crop

Line 27: Remove the word ‘slow’ as biochar can be produced under different modes of pyrolysis (slow/fast, dry/wet, high/low temperature etc..)

Line 32: Replace ‘pH’ with “potential”

Line 34: Delete ‘o’

Line 41: ‘Charcoal...’ Do authors mean to say biochar here?

Line 49: Replace ‘raw’ with “nutrient-poor”

Lines 51–52: Rephrase the sentence.

Line 62: What do authors mean by right weather conditions?

Line 63: “higher CEC…” Did authors calculate this important parameter in their study as well?

Lines 65–70: The objective of this research study is not clearly illustrated in its present form. It would be nice if authors could formulate some experimental hypothesis that could be tested and verified through the experimental results.

Line 90: Replace ‘mineral fraction content’ with “elemental composition”

Lines 96–97: Describe in brief how each physical parameter was determined for better understanding for the readers.

Lines 110–111: How did the authors verify that the substrates had 60% moisture content?

Line 117: Define SPAD.

Line 118: What criteria were taken into consideration while deciding three representative leaves?

Line 119: It is reported that drying of biomass at 65 °C can lead to loss of some volatile elements (compounds) including nitrogen resulting in an underestimation of some elements. Authors need to convince the reviewer on this aspect.

Line 137: Log & square root transformations of original data not fulfilling ANOVA assumption are the most widely used methods in statistics? Could authors justify for particularly using “Arcsin” instead of log & square root transformation?   

Lines 142–145: It would also be interesting and informative if authors could have measured CEC and surface area of digestate & biochar samples to see the pyrolysis induced changes. This may help to discuss the possible changes on crop growth performances.  

Lines 148–149: Are the concentrations of heavy metals (As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in biochar used this study within the limits set by different biochar certification standards (e.g. EBC, IBI etc.)? Did authors also consider this aspect?

Line 150: Skip the term “globally”

Lines 226–228: …negative impact not only to plant growth but also in soil biology including adverse effect to soil organisms (both micro and macro).

Lines 246–252: N content in biochar generally decreases with increase in pyrolysis temperature due to volatilization N at higher temperatures. E.g. Enders et al. cited in this study.

Line 252: Synchronize the font size….

Lines 266–270: This may be likely due to the fact that the other biochar properties such as nutrient availability, CEC, porosity, BD, WHC were good enough to mask the possible negative effect on crop growth parameters, which may otherwise occur, due to high pH of BCS in this study.   

Lines 302–307: See the comments for Lines 148–149.

Lines 297–312: Despite significant leaf discoloration, likely due to high Fe & Mn contents, in BCS treatments, the total dry matter content and essential oil content were not affected, compared to SDS treatment. Perhaps, other biochar properties were able to compensate with this negative effect on crop.  

Lines 324–326: Good observation

Lines 328–329: authors need to be cautious while stating such statement as results of biochar effect on crop growth and GHG reduction are not consistent, still lacking long-term field trials.  

Line 337: …likely due to high concentrations of Fe & Mn in BCS?

Line 342: …rates…

Lines 341–343: Also, …in relation to different soil/growing media conditions (pH & texture) for commercial production of geranium?

Author Response

Reviewer 2

General comment

This manuscript reports the effects of different substrate-based growing media on geranium cultivation based essential oil production and quality at a greenhouse-based study. The topic matches the aim and scope of the journal, and presents an interesting dataset demonstrating differential impacts of amendments on the measured variables. The manuscript can be improved by 1) formulating some experimental hypothesis in relation the objective, and relating experimental results back to hypotheses, this may help the authors better address observed differences in different growth substrates; and 2) improving the integration/correlation of the digestate/biochar characterization data with crop growth performance and essential oil yield/quality within the context of the experiment and the cited literatures.

In addition, this manuscript needs fine English language editing from the native speaker before it can be accepted for publication into Agronomy as it contains several grammatical errors, also lacks adequate flow & smoothness in sentence formulation and restructuring in its present form.

Specific comments

Line 11: Replace ‘generated large’ with “has created a global

Done

Line 15: Add ‘growth’ after crop

Done

Line 27: Remove the word ‘slow’ as biochar can be produced under different modes of pyrolysis (slow/fast, dry/wet, high/low temperature etc..)

Done

Line 32: Replace ‘pH’ with “potential”

Done

Line 34: Delete ‘o’

Done

Line 41: ‘Charcoal...’ Do authors mean to say biochar here?

We thank the reviewer for the comment. Yes, we meant biochar. The word has been changed in the text.

Line 49: Replace ‘raw’ with “nutrient-poor”

Done

Lines 51–52: Rephrase the sentence.

We thank the reviewer for the comment. We deleted the sentence that was redundant respect with the previous sentence.

Line 62: What do authors mean by right weather conditions? 

We thank the reviewer for the comment. We better specified in the text (L:60-62)

Line 63: “higher CEC…” Did authors calculate this important parameter in their study as well?

We thank the reviewer for the comment. We didn’t calculate CEC but in future studies it will certainly be taken into account.

Lines 65–70: The objective of this research study is not clearly illustrated in its present form. It would be nice if authors could formulate some experimental hypothesis that could be tested and verified through the experimental results.

We improved the description in the text (L:65-68).

Line 90: Replace ‘mineral fraction content’ with “elemental composition”

Done

Lines 96–97: Describe in brief how each physical parameter was determined for better understanding for the readers.

We improved the description in the text (L:103-111).

Lines 110–111: How did the authors verify that the substrates had 60% moisture content?

We improved the description by revising the text (L:123-126).

Line 117: Define SPAD.

Done

Line 118: What criteria were taken into consideration while deciding three representative leaves?

We thank the reviewer for the comment. We improved the description in the text (L:134-137)

Line 119: It is reported that drying of biomass at 65 °C can lead to loss of some volatile elements (compounds) including nitrogen resulting in an underestimation of some elements. Authors need to convince the reviewer on this aspect.

We thank the reviewer for the comment. Based on the bibliographic database and references, studies similar to ours conducted on Pelargonium graveolens use temperatures to dry the biomass within the range from 60 to 80°C. For this reason, we have adopted these temperatures.

The following are other studies that adopt the usual temperature range:

·         Morvant J., Dole J.M., Cole J.C., 2001. Fertilizer Source and Irrigation System Affect Geranium Growth and Nitrogen Retention. HortScience: a publication of the American Society for Horticultural Science 36(6)

·         Mahdieh M, Yazdani M, Mahdieh S., 2013. The high potential of Pelargonium roseum plant for phytoremediation of heavy metals. Environ Monit. Assess.185(9):7877–81. 

·         Chand S., Singh G., Rajkumari Patra D.D., 2016. Performance of rose scented geranium (Pelargonium graveolens) in heavy metal polluted soil vis-à-vis phytoaccumulation of metals. Int. J. Phytoremediation, 18, pp. 754-760

Line 137: Log & square root transformations of original data not fulfilling ANOVA assumption are the most widely used methods in statistics? Could authors justify for particularly using “Arcsin” instead of log & square root transformation?   

We agreed with the referee suggestion that Log & square root transformations are the widely used methods. We used the arcsin square root transformation because of we retained this method more appropriate to manage percentage data.

·         Ahrens, W., Cox, D., & Budhwar, G. (1990). Use of the Arcsine and Square Root Transformations for Subjectively Determined Percentage Data. Weed Science, 38(4-5), 452-458. doi:10.1017/S0043174500056824

Lines 142–145: It would also be interesting and informative if authors could have measured CEC and surface area of digestate & biochar samples to see the pyrolysis induced changes. This may help to discuss the possible changes on crop growth performances.  

We thank the reviewer for the comment. Unfortunately, in this study we did not consider these parameters. Therefore, in the conclusions we suggested the need to consider these parameters in future studies (L363-365)

Lines 148–149: Are the concent rations of heavy metals (As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in biochar used this study within the limits set by different biochar certification standards (e.g. EBC, IBI etc.)? Did authors also consider this aspect?

We thank the reviewer for the comment.  We now compared our values with the threshold suggested by the European Biochar Certificate (EBC) (L:277-278) and the Italian Ministry of Agriculture (L:280-281).

Line 150: Skip the term “globally”

Done

Lines 226–228: …negative impact not only to plant growth but also in soil biology including adverse effect to soil organisms (both micro and macro).

Thanks for the suggestion. The sentence has been improved.

Lines 246–252: N content in biochar generally decreases with increase in pyrolysis temperature due to volatilization N at higher temperatures. E.g. Enders et al. cited in this study.

We agree with the reviewer. The words “as expected” were delated. Indeed, as reported in L 267-269 we confirmed the reviewer comment.

Line 252: Synchronize the font size….

Done

Lines 266–270: This may be likely due to the fact that the other biochar properties such as nutrient availability, CEC, porosity, BD, WHC were good enough to mask the possible negative effect on crop growth parameters, which may otherwise occur, due to high pH of BCS in this study.   

We agree with the reviewer. We consider this aspect in the L:289-291.

Lines 302–307: See the comments for Lines 148–149.

We thank the reviewer for the comment.  We now compared our values with the threshold suggested by the European Biochar Certificate (EBC) (L:277-278) and the Italian Ministry of Agriculture (L:280-281).

Lines 297–312: Despite significant leaf discoloration, likely due to high Fe & Mn contents, in BCS treatments, the total dry matter content and essential oil content were not affected, compared to SDS treatment. Perhaps, other biochar properties were able to compensate with this negative effect on crop.  

We agree with the reviewer. We consider this aspect in the L:289-291.

Lines 324–326: Good observation

We thank the reviewer for the comment.

Lines 328–329: authors need to be cautious while stating such statement as results of biochar effect on crop growth and GHG reduction are not consistent, still lacking long-term field trials.  

We thank the reviewer for the comment. The sentence has been partly rewritten (L:351-352)

Line 337: …likely due to high concentrations of Fe & Mn in BCS?

We thank the reviewer for the comment. The sentence has been partly rewritten (L:360-361)

Line 342: …rates…

Done

Lines 341–343: Also, …in relation to different soil/growing media conditions (pH & texture) for commercial production of geranium?

We thank the reviewer for the comment. The sentence has been partly rewritten (L:365-367)