On-Farm Composting of Hop Plant Green Waste—Chemical and Biological Value of Compost
Round 1
Reviewer 1 Report
Composting is a viable option for utilization of lignocellulosic waste biomass. Authors investigtaed three composting ’protocols’ in the manuscript, ohytotoxicity, nutrients level, and, bacterial and fungal growth was used as control parameter. The auscript can provide new technical information. The novelties of the study are not given and highlighted.
Introduction summarizes the production of hop biomass (and problems related to use it for composting). The main influential factors of composting are just mentioned. Materials and methods are described clearly. The manuscript contains some useful technical information but in some cases the results are discussed superficially.
Comments:
I suggest reconsidering the key word. For example the very similar keywords: hop; Humulus lupulus; biomass after harvest.
Please define clear the novelties of the study. Many papers deal with the composting technologies, efficiency, influential parameters for different raw materials and biomass ’mix’.
Discuss the parameters affecting the efficiency of composting in more details (Introduction section).
The theoretical background and earlier experiences related the composting protocols -applied by the authors – are not discussed in the Introduction section.
Weather condition alone is not important and informative in this form (line 92-103), in my opinion
The real reasons for pH increment are not discussed (line 183-187).
How was the biochar dosage selected/determined? What dosage was used for EM™ -selected microorganisms (CFU, or toother parameters)?
Authors mentioned in line 215-217: ’ the lowest TN content in the final compost was found in pile ZUP (2.0%) (Figure 2). The reason for this can be attributed to added biochar, which reduces nitrogen losses from composting material’. Have the authors checked the nitrogen adsorption on biochar?
The quality and visibility of Figure 4 and 5 is low.
Authors used EM™ for ROZ pile (and not for the other ’protocols’). It should be conduct microbial analysis (to determine microbial diversity) to investigate the reason for the different ’efficiency’.
Conclusion section is too lengthy.
Author Response
I suggest reconsidering the key word. For example the very similar keywords: hop; Humulus lupulus; biomass after harvest.
- Keyword hop was removed.
Please define clear the novelties of the study. Many papers deal with the composting technologies, efficiency, influential parameters for different raw materials and biomass ’mix’.
- The novelty of the study is to compost hop biomass after harvest on the hop farms and based on different protocols find out how to get nutrient-rich compost for usage on these farms to enrich and improve farm soils. Hop biomass is a nutrient-rich re-source for composting and hop farmers can recycle such a huge mass of agro-waste sourced at the end of hop season into compost.
Discuss the parameters affecting the efficiency of composting in more details (Introduction section).
The theoretical background and earlier experiences related the composting protocols -applied by the authors – are not discussed in the Introduction section.
- Up to today, no study on composting of hop plant biomass was published, therefore these findings will be crucial for the composting practices on hop producing farms. Similar research has been tackled in Germany, where the possibility of composting waste hop biomass is also being studied (Lohr et al., 2021).
Weather condition alone is not important and informative in this form (line 92-103), in my opinion
- Weather conditions played a role in consideration to temperatures and moisture content inside the compost piles.
The real reasons for pH increment are not discussed (line 183-187).
How was the biochar dosage selected/determined? What dosage was used for EM™ -selected microorganisms (CFU, or toother parameters)?
Authors mentioned in line 215-217: ’ the lowest TN content in the final compost was found in pile ZUP (2.0%) (Figure 2). The reason for this can be attributed to added biochar, which reduces nitrogen losses from composting material’. Have the authors checked the nitrogen adsorption on biochar?
The quality and visibility of Figure 4 and 5 is low.
Authors used EM™ for ROZ pile (and not for the other ’protocols’). It should be conduct microbial analysis (to determine microbial diversity) to investigate the reason for the different ’efficiency’.
- Thank you for that comment. Treatment with EM™ additive and covering with foil is traditionally used by some hop growers, therefore we wanted to evaluate final compost from this treatment. If this treatment will be selected for future use, the microbial population in product will be analysed and assessed how the product effects the final compost.
Conclusion section is too lengthy.
- Thank you, we tried to avoid unneeded information.
Reviewer 2 Report
Dear Authors,
congrtulation, this is a very, very nice work with a high potential to be really used ascompost. But take care about the microbial comunity, in order to avoid a microbial competition, or the nitrogen absortion etc.
Kind regards
Author Response
Thank you for the encouraging comment. Kind regards.
Reviewer 3 Report
General comments
-This manuscript, by athors, studied “On-farm Composting of Hop Plant Green Waste - Chemical and Biological Value of Compost”.
Overall, the topic is of interest to Applied Sciences, readers. However, following are my few concerns, addressing which can improve the quality of the article.
Following are the specific comments on the article before publication as major revision.
-Major comments and Suggestions
- Although the introduction talks clearly about the need for the study. Adding more information on how this study is different previous studies will add more scope for the paper.
- No proper references in introduction section.
- Hop (Humulus lupulus ) cones are crucial in the beer brewing industry because they contain resins, tannins and aromatic substances……Reference??? Similarly, need references for other statements as well.
- Need to revise with proper references as authors stated statements.
- First describe the observations, then, propose explanations in the light of previous work.
- Figures need to revise clearly.
- Need to clear your fertilization comparison in methodology, results and then Discussion accordingly.
-You may check following references and more related articles to strengthen your studies.
“Ammonia Volatilization and Greenhouse Gases Emissions during Vermicomposting with Animal Manures and Biochar to Enhance Sustainability”
“Nutrients Recovery during Vermicomposting of Cow Dung, Pig Manure, and Biochar for Agricultural Sustainability with Gases Emissions”
-The English needs editing.
Recheck comma, space and spellings. Grammar mistakes
Specific Comments and Suggestions
Title
Title can be improved by providing a short and ground breaking claim.
-Abstract
-Methodology (Statistical analysis), need to clear or improve in better way.
-Mention control treatment clearly.
-Introduction
-Better to explain more importance of Hop Plant worldwide.
- Although the introduction talks clearly about the need for the study. Adding more information on how this study is different previous studies will add more scope for the paper.
-Separate your statements with references clearly.
-Materials and Methods
- Not consistent covering in treatments as mentioned in Table 1? Why?
-Clearly mention your ZUP, LES and ROZ abbreviations for treatments or specific terminology.
Or change it to COMP 1, COMP 2 and control or COMP 3.
-Please add one picture of your experimental setup.
- “There were a lot of rainy days in the last week of September and in the first half of 92 October, with an average of 9.7 mm of rain per day”. Check this sentence or rewrite it.
-What is your baseline for comparison?
-How do you compare all composts?
- Stated with references in Section 2.5, 2.6, 2.7, and 2.8.
-Why there is no consistency on sampling dates. How you select it? Which baseline? As mentioned in Figure 1.
-Results and Discussion
- (can improve headings to more clear your presented results).
-This is vague, not sure exactly what it is meant to say in results and what is a discussion to support your own studies?
-Need to clear your results and then Discussion accordingly.
- “In the trials conducted by Choy et al. [12], horticultural waste composting also resulted in an increase of pH”. Please state clearly what studies they did and how it is supported with your results.
-Need to clear your fertilization comparison in methodology, results and then Discussion accordingly.
-“Compost pile ZUP, on the other hand, was noticeably cooler after each turning. This pattern can be the result of particle size, because compost pile ZUP had larger particle size than pile LES which contained additive biochar. Pile ROZ cooled down quickly after pile formation because no oxygen was supplied to the fermented biomass after one month of composting.” Please clear your treatments in methodology section, then your results with abbreviations your ZUP, LES and ROZ abbreviations will give clear meaning .
-Conclusions
-Too simple and irrerelavant. State with your specific finding and significance.
-Summarize your conclusions. One or two paragraphs enough for conclusions, you can transfer it in Results sections where appropriate.
-Make sure you are presenting only new theoretical findings that originate firstly from your work and are not deducible from other literature, clearly indicate whether your research hypothesis tends to be confirmed or not?
-Explain in your results with a discussion how you have drawn this conclusion?
Author Response
-You may check following references and more related articles to strengthen your studies.
“Ammonia Volatilization and Greenhouse Gases Emissions during Vermicomposting with Animal Manures and Biochar to Enhance Sustainability”
“Nutrients Recovery during Vermicomposting of Cow Dung, Pig Manure, and Biochar for Agricultural Sustainability with Gases Emissions”
The English needs editing.
Recheck comma, space and spellings. Grammar mistakes
- The paper was checked by native speaker.
Specific Comments and Suggestions
- Title
Title can be improved by providing a short and ground breaking claim.
- Abstract
-Methodology (Statistical analysis), need to clear or improve in better way.
-OK
-Mention control treatment clearly.
- Compost without additives was observed as a control (CON), despite that the composts differed in other parameters.
- Introduction
-Better to explain more importance of Hop Plant worldwide.
- Hop plant is produced for cones that are used in brewing industry. The acreage of hop production and the amount of hop plant biomass that is left at the end of the hop harvest is enormous. 100 000 tonnes of hop ‘waste’ is produced every year.
- Although the introduction talks clearly about the need for the study. Adding more information on how this study is different previous studies will add more scope for the paper.
- Up to today, no study on composting of hop plant biomass was published, therefore these findings will be crucial for the composting practices on hop producing farms. Similar research has been tackled in Germany, where the possibility of composting waste hop biomass is also being studied (Lohr et al., 2021).
-Separate your statements with references clearly.
- Added.
- Materials and Methods
- Not consistent covering in treatments as mentioned in Table 1? Why?
- Compost pile EM was hermetically covered with black foil one month after it had been built as it simulated fermentation. Other piles were not covered.
-Clearly mention your ZUP, LES and ROZ abbreviations for treatments or specific terminology. Or change it to COMP 1, COMP 2 and control or COMP 3.
- Changed treatments name into BC, EM, and CON.
-Please add one picture of your experimental setup.
- Added.
- “There were a lot of rainy days in the last week of September and in the first half of 92 October, with an average of 9.7 mm of rain per day”. Check this sentence or rewrite it.
- Checked.
-What is your baseline for comparison? How do you compare all composts?
- All three composts were compared with each other to figure out which of existable compost treatment is optimal for composting hop biomass on hop farms. Compost without additives was observed as a control, despite that the composts differed in other parameters.
- Stated with references in Section 2.5, 2.6, 2.7, and 2.8.
- Added.
-Why there is no consistency on sampling dates. How you select it? Which baseline? As mentioned in Figure 1.
- The piles were turned when the temperature reached 65 °C.
- Results and Discussion
- (can improve headings to more clear your presented results).
-This is vague, not sure exactly what it is meant to say in results and what is a discussion to support your own studies?
-Need to clear your results and then Discussion accordingly.
- We wanted to discuss our results simultaneously.
- “In the trials conducted by Choy et al. [12], horticultural waste composting also resulted in an increase of pH”. Please state clearly what studies they did and how it is supported with your results.
- We removed this citation due to the difference in study research background. Thank you for comment.
-Need to clear your fertilization comparison in methodology, results and then Discussion accordingly.
- This was only recalculation of results, given in Table 4 and compared to manure composition, cited in Slovenian Guidelines for Fertilisation (reference given in text).
-“Compost pile ZUP, on the other hand, was noticeably cooler after each turning. This pattern can be the result of particle size, because compost pile ZUP had larger particle size than pile LES which contained additive biochar. Pile ROZ cooled down quickly after pile formation because no oxygen was supplied to the fermented biomass after one month of composting.” Please clear your treatments in methodology section, then your results with abbreviations your ZUP, LES and ROZ abbreviations will give clear meaning .
- Changed treatments name into BC (added biochar), EM (added Effective microorganisms), and CON (without additives).
- Conclusions
-Too simple and irrerelavant. State with your specific finding and significance.
-Summarize your conclusions. One or two paragraphs enough for conclusions, you can transfer it in Results sections where appropriate.
- Thank you, we tried to avoid unneeded information.
-Make sure you are presenting only new theoretical findings that originate firstly from your work and are not deducible from other literature, clearly indicate whether your research hypothesis tends to be confirmed or not?
- Up to today, no study on composting of hop plant biomass was published; therefore, these findings will be crucial for the composting practices on hop producing farms.
-Explain in your results with a discussion how you have drawn this conclusion?
- ok
Reviewer 4 Report
In this study, the authors evaluated the composting process of Hop Plant Green Waste under different conditions such as natural conditions (as single substrate), the use of additive or microorganisms and cover the pile with black foil. Moreover, they carried out chemical, microbiological, respiratory, germination and growth tests at the beginning and at the end of composting time. This research study seems to be interesting from a technical point of view; however, the information, structure and results of the manuscript reveal important shortages, which need significant revision prior to be published in Applied Sciences.
GENERAL COMMENTS
1) The English of the paper should be corrected and checked thoroughly. Possibly, under the supervision of a native speaker.
2) Abstract should be revised.
3) The introduction section needs to be improved.
4) The main objective of this study needs to be highlighted.
5) Material and methods section needs to be clarified in some points.
5) The results and discussion section needs to be clarified in some points.
4) Conclusions could be shortened highlighting the most relevant results.
SPECIFIC COMMENTS
_ Lines 10-12: This amount is specific to a particular area, and it would be convenient to do it generically. It's good for the introduction.
_ Line 12: Protocol? The authors evaluated the composting process under different conditions and if they want to stablish a protocol, operational conditions throughout the process need to be evaluated. I suggest changing protocol for treatment throughout the manuscript.
_ Lines 17: DM means dry matter? It needs to be defined for the better understanding of the readership.
_ Lines 19-20: The particle size does not imply a higher nutrient content. It could favor the retention of nutrients. Please revise it.
_ Lines 21-23: Revise this sentence.
_ Keywords: hop; Humulus lupulus. One of the two is enough.
_ Lines 46-48: A references is required.
_ Line 57-60: What about the lignin and cellulose content? This needs to be explained and how it affects the composting process. There are previous studies that explain the influence of lignocellulosic materials in the composting process. https://doi.org/10.1016/j.jenvman.2019.06.076
_ Lines 64-69: The main objective of this study needs to be better highlighted.
_ Lines 92-103: Moisture is another important variable to be monitored during composting process. Was it evaluated?? I wonder if ROZ pile increased significantly the moisture content and how it affected to microorganisms and the transfer of oxygen through the pile? Please, explain it.
_ Line 103 and 105: the word filtrate or water extraction is not adequate. I suggest “aqueous extract”.
_ Lines 105-106: So the sampling was not only in April??? Check the sentence.
_ Lines 145-149: Is it a standardized method? Reference??
_ Lines 167-168: And this conclusion? It's not understood.
_ Figura 1: The intermittent temperature drops in ZUP are due to the turning process?? Explain.
And the temperature drops in LES??? Is it the same?
_ In the case of ROZ, the pile was covered after one month. What is the reason to cover it? It needs to be clarified. There are previous studies that have covered the composting piles to minimize the odor impact, even increasing the temperature during the hydrolytic stage. Compare with this study.
https://doi.org/10.1016/j.envres.2019.108624
_ Table 3: Why the initial pH was not measured in ZUP?
_ Table 4: Ammoniacal nitrogen showed a significant increase at the end of the composting time. There was no loss at the beginning of the experiment by leaching? Was this evaluated?.
_ Lines 246-247: According to figure 3a this is not the case. Please, explain it.
_Lines 274-275: So, what is the reason for the significant increase in ammoniacal nitrogen??? Explain it.
_ Conclusions could be shortened highlighting the most relevant results.
_Further research throughout the composting time would help to identify the biodegradation mechanisms under the different conditions evaluated.
Comments for author File: 
Comments.docx
Author Response
GENERAL COMMENTS
1) The English of the paper should be corrected and checked thoroughly. Possibly, under the supervision of a native speaker.
The paper was checked by a native speaker.
2) Abstract should be revised. OK
3) The introduction section needs to be improved. OK
4) The main objective of this study needs to be highlighted. OK
- Up to today, no study on composting of hop plant biomas was published, therefore these findings will be crucial for the composting practices on hop producing farms. Established protocols were followed in order to evaluate final composts. While there are many studies done in scope of industrial composting, fewer are considering on-farm composting.
5) Material and methods section needs to be clarified in some points. OK
5) The results and discussion section needs to be clarified in some points. OK
4) Conclusions could be shortened highlighting the most relevant results. OK
SPECIFIC COMMENTS
_ Lines 10-12: This amount is specific to a particular area, and it would be convenient to do it generically. It's good for the introduction.
- Thank you for your suggestion. We’ve transferred it to introduction section. Unfortunately, there is no data on hop biomass waste in other countries.
_ Line 12: Protocol? The authors evaluated the composting process under different conditions and if they want to stablish a protocol, operational conditions throughout the process need to be evaluated. I suggest changing protocol for treatment throughout the manuscript.
- That is correct; we followed three different treatments that has already been established on three hop-growing farms.
_ Lines 17: DM means dry matter? It needs to be defined for the better understanding of the readership.
- Corrected into dry matter.
_ Lines 19-20: The particle size does not imply a higher nutrient content. It could favor the retention of nutrients. Please revise it.
- Mature compost had the highest nutrient content due to added biochar (11 kg/t fresh biomass), while a small start particle size (˂5 cm) could favour the retention of nutrients. However, the final compost had the least bacteria and fungi due to very high temperatures in the thermophilic phase of this pile.
_ Lines 21-23: Revise this sentence.
- Thank you, corrected.
_ Keywords: hop; Humulus lupulus. One of the two is enough.
- Keyword hop was removed.
_ Lines 46-48: A references is required.
- More about project on web site: www.life-biothop.eu/
_ Line 57-60: What about the lignin and cellulose content? This needs to be explained and how it affects the composting process. There are previous studies that explain the influence of lignocellulosic materials in the composting process. https://doi.org/10.1016/j.jenvman.2019.06.076
- Lignin and cellulose parameters were not included in this pioneering hop-composting article. It is our plant to check that in future studies.
_ Lines 64-69: The main objective of this study needs to be better highlighted.
_ Lines 92-103: Moisture is another important variable to be monitored during composting process. Was it evaluated?? I wonder if ROZ pile increased significantly the moisture content and how it affected to microorganisms and the transfer of oxygen through the pile? Please, explain it.
- During the process, the moisture was measured in composted biomass, however moisture content results are not included in this article as this one is focusing on final quality of the compost. The moisture content is presented as opposite parameter -> DM -> dry matter. Pile ROZ or EM with new labels was the driest through the whole process. The most abundant microbes in this pile were yeasts that are facultative anaerobes and can survive in lack of oxygen.
_ Line 103 and 105: the word filtrate or water extraction is not adequate. I suggest “aqueous extract”.
_ Lines 105-106: So the sampling was not only in April??? Check the sentence.
- In September 2020, sampling was done at the experiment set-up (at the construction of each pile) and after 7 months of composting, in April 2021.
_ Lines 145-149: Is it a standardized method? Reference??
- This two media are standard for classic simple counting of microorganisms -> colony forming units. Similar was done in article Milinković, M., Lalević, B., Jovičić-Petrović, J., Golubović-Ćurguz, V., Kljujev, I., & Raičević, V. (2019). Biopotential of compost and compost products derived from horticultural waste—effect on plant growth and plant pathogens' suppression. Process Safety and Environmental Protection, 121, 299-306.
_ Lines 167-168: And this conclusion? It's not understood.
- Slovenian standard for hygienisation of the compost require that pile reaches 55 °C for 14 days. If there is less than biomass from one hectare, meaning that there is not enough plant material, the composting process won’t start.
_ Figura 1: The intermittent temperature drops in ZUP are due to the turning process?? Explain.
- Yes, temperature drops after compost is turned and then it is reheated again as composting process continues. The ambient temperature is cooler than the compost therefore the temperature drop is observed.
And the temperature drops in LES??? Is it the same? Yes.
_ In the case of ROZ, the pile was covered after one month. What is the reason to cover it? It needs to be clarified. There are previous studies that have covered the composting piles to minimize the odor impact, even increasing the temperature during the hydrolytic stage. Compare with this study. https://doi.org/10.1016/j.envres.2019.108624
- One compost pile – EM was hermetically covered with black foil one month after it had been built as it simulated fermentation. Research to minimize the odor impact was not included in this pioneering hop-composting article.
_ Table 3: Why the initial pH was not measured in ZUP?
- The result were lost but are not even so important for this study. We have observed hop biomass after composting in previous studies and the pH of such biomass is always around 6.
_ Table 4: Ammoniacal nitrogen showed a significant increase at the end of the composting time. There was no loss at the beginning of the experiment by leaching? Was this evaluated?.
- Ammoniacal nitrogen increased at the end of composting, but this is minimally influencing the TN content. There was loss of N due to leaching, but there was also a loss of weight, therefore the percentage of TN was increased, as carbon was lost. The leachate study will be performed this year.
_ Lines 246-247: According to figure 3a this is not the case. Please, explain it. Corrected.
_Lines 274-275: So, what is the reason for the significant increase in ammoniacal nitrogen??? Explain it.
- We have checked the literature and most of other authors explained the rise of ammoniacal nitrogen by moisture or elevated temperature but that was not the case in our study. To understand this pattern, samples should be analysed in more time stamps of composting. Other composting experiments are done on industrial scale where time of composting is much shorter than in our case where composting took place outside, exposed to environmental conditions (rainfall, wind, temperature, radiation…) and lasted for 7.5 month. It is also not related to treatment. More research is planned on that topic in the future.
_ Conclusions could be shortened highlighting the most relevant results. Corrected.
_Further research throughout the composting time would help to identify the biodegradation mechanisms under the different conditions evaluated.
- Thank you for all your suggestions, we will continue with more detailed analyses and research on on-farm hop biomass composting.
Round 2
Reviewer 1 Report
The manuscript has an interesting topic. The authors have revised the manuscript thoroughly according to reviewers’ comments and suggestions. Rephrasing of the Abstract and Conclusion, amendment of Introduction, more detailed information provided in Materials and methods, and more detailed discussion of the results made the manuscript more complete and clear. The scientific quality of the manuscript has been improved significantly due to the revision. I accept all answers and modifications made by the authors.
Author Response
Thank you very much for your efford! Best, Barbara Čeh
Reviewer 3 Report
The authors did good efforts to revise the manuscript. But, still, it can be more improved as commented before for readers.
Check reference style again. For example, Reference no: 9 and 14. Be consistent with the full name of the journal or use their abbreviations.
Author Response
The authors did good efforts to revise the manuscript. But, still, it can be more improved as commented before for readers.
- Thank you, we’ve changed the order of presenting results and then discussion in each result section.
Check reference style again. For example, Reference no: 9 and 14. Be consistent with the full name of the journal or use their abbreviations.
- Thank you for the note. Corrected.
Reviewer 4 Report
The authors have corrected the manuscript and it is ready to be published.
Author Response
Thank you for your efford! Best, Barbara Čeh
