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Thank you for carefully addressing all the Reviewers' and PeerJ editorial concerns. The manuscript is now suitable for publication.
[# PeerJ Staff Note - this decision was reviewed and approved by Jörg Oehlmann, a PeerJ Section Editor covering this Section #]
Thank you for revising the manuscript. The Reviewers were mostly satisfied with the revisions made and responses provided but Reviewer 2 had some additional comments. Please address this additional concern before the manuscript can be accepted for publication.
All comments have been addressed. The manuscript quaility has been improved since last submission. New figures and new text in the discussion section improved the manuscript. It can be accepted.
No comments.
No comments.
The authors have adequately addressed all comments, except the reporting of the sodium hexametaphosphate solution pH. Even though the pH was not adjusted, the value needs to be reported, because it indicates the mechanism of stabilization of the aqueous suspension, i.e. highly negatively charged particle surfaces inducing electrostatic stabilization. Please report the pH value. No further action is required.
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Three Reviewers have provided their opinion and suggestions for improving the manuscript. Please address the questions and comments in the response letter and make respective changes in the manuscript.
The submitted article is very interesting and suitable for their publication, although I'm not sure about a multidisciplinary journal such as PeerJ. The paper has several new information and potential interest due to the topic's novelty. I like the paper!
However, the paper needs to improve its discussion with other papers because this question is missing in the present version. Ok, the authors compare their data with german fields, but comparison with other countries is scarce. See some examples: https://doi.org/10.5194/soil-6-649-2020 https://doi.org/10.1016/j.scitotenv.2020.141917 or https://doi.org/10.1016/j.scitotenv.2019.03.368 During the reviewing process, i didn't see that this manuscript is a short communication or a technical report. For these reasons, I think that a good discussion is needed. Linked to this question, the authors refer several times to the previous paper, but i think more information can be added here.
L15 (abstract). I suggest to add "Three fields without any visual plastic" We can not talk about without any plastic, considering the multiple plastic sources in the
environment.
L47. "Crop rotations and shorter plastic applications of less than three years appear" Even less time (sometimes 6 monhts). Three years is the expected life-time for plastic greenhouse not for a mulch that usually is applied for each cultivation period (around 3-6 months). I suggest to read (EIP-AGRI Focus Group Reducing the plastic footprint of agriculture) https://ec.europa.eu/eip/agriculture/sites/default/files/eip-agri_fg_plastic_footprint_minipaper_a_final.pdf
L170. "So far, only few studies have investigated macroplastics on arable land." In Germany? I suggest to see https://doi.org/10.5194/soil-6-649-2020 https://doi.org/10.1016/j.scitotenv.2020.141917 or https://doi.org/10.1016/j.scitotenv.2019.03.368 Besides, i suggest to add a comparison table with studies from all countries, and not only Germany. This section needs to be improved—a lot.
L70-79. Please, add species names as in the previous study https://doi.org/10.5194/soil-8-31-2022
L77-79. "None of the six sites has received any compost or sewage sludge applications in recent years. While this explicitly excluded some other potential sources of plastic inputs, littering and wind drift could not be controlled for." I agree with the authors on this question because plastics have been converted into an environmental contaminants such as Pb. However, can you add a bit more information about the study area? Potential contamination from industrial or urban sources?
L80. Only a minor comment: Can you add the study area's location on a Germany map?
L90-94.
- Why only these soil properties and not also pH or other properties? In the previous study, more properties were measured https://doi.org/10.5194/soil-8-31-2022
- "The soil texture on-site was determined in accordance with ASTM D422-63 (2007)." Hydrometer?
L96-100.
- Some researchers are indicating that a sieve tower should be used for soil sieving and plastic separation (5 mm, 2 mm and 1 mm). Why were separated directly by 2mm and not previously by 5 mm?
Why NaCl solution and not other solutions (H2O, CaCl2, ZnCl2, olive oil, etc)? NaCl could be good for LDPE plastics, but in an agricultural field, you could find other plastic items (PVC, PP, PET, etc) that could don't have a result with NaCl. Besides, other authors suggest the use of sequential extractions with different solutions at a different density ranges (e,g, H2O+NaCl, or NaCl+ZnCl2) (e.g. https://doi.org/10.1016/j.scitotenv.2017.10.213 or https://doi.org/10.1016/j.scitotenv.2020.141917) I agree with the authors on the use of NaCl (cheap, environmentally friendly...), but this question should be improved.
L101-112. Usually, organic matter digestion is an issue in agricultural soils, and for these reasons is, used Fenton or H202 extraction to remove organic matter https://doi.org/10.1021/acs.est.8b01517. However, why this approach was not carried out for microplastics? The sodium hexametaphosphate solution can be used as a substitute for Fenton/H2O2 reagent? I'm interested too because I have similar samples.
L107-112. The solutions and reagents were used for which interest? Sample purification? Py-GC/MS analysis? Please, add more information. Although this information could be previously explained (https://doi.org/10.5194/soil-8-31-2022), i suggest improving this information also here. All readers want to read the information in the paper and not read the previous paper ;)
L113. Please, add information about quality controls and measures to avoid plastic contamination in the lab. Steel or glass equipment? A positive control was carried out? This information was indicated in the previous work but not here https://doi.org/10.5194/soil-8-31-2022
L136. No information about statistical analysis.
Please, try to compare your data with previous data from other countries (not only Germany).
The manuscript reports a well-defined objective, that is to determine the presence of plastic micro-, meso-, and macroparticles in plastic-mulched soils from agricultural lands. A major shortcoming in the reporting of data is the definitions used for these three types of particles. Throughout the literature there are various ranges used to define macro-, meso-, micro-, and nanoplastics based on the physical dimensions of the particles, which creates a major barrier for comparing results from different studies. A publication by Hartmann et al. in 2019 (Are We Speaking the Same Language? Recommendations for a Definition and Categorization Framework for Plastic Debris https://doi.org/10.1021/acs.est.8b05297) had pointed out the need to use a common terminology in order to address this issue. I believe it is imperative to start using a common framework and suggest that the authors re-group their results using the limits suggested in this publication, i.e. microplastics: 1 - 1,000 µm, mesoplastics: 1 - 10 mm, and macroplastics: larger than 1 cm.
In addition, the concentration of macroplastics is reported as number of particles per area of field, mesoplastics as number of particles per mass of soil, and microplastics as mass of particles per mass of soil. Although it is perfectly understandable that the choice of units derives from the quantification methods used, I suggest that the authors go one step further and make appropriate assumptions in order to provide a qualitative comparison between the three types of particles, using the same units; preferably as number of particles per volume of soil. If the authors feel that other units are more appropriate, please feel free to use them.
The experimental design is very well planned and executed. The choice of reference fields provides the means to take into account the effect of factors other than mulching on the presence of plastics in soil. I would like to see a discussion about the possibilities of sample contamination during handling in order to further strengthen the quality of the experimental work presented.
Line 82: please elaborate on the four-eye principle, either in the main text or as supporting information.
Line 102: please report the concentration and pH of the sodium hexametaphosphate solution used.
no comment
This is an excellent field study on a subject with scientific and socioeconomic interest, which I recommend to be published once the issues mentioned above are addressed.
I would like to thank the authors for developing comprehensive but not easy to apply (because of the necessity of a sophisticated instrument) methods for microplastic in soil matrices. The chemical used in the analysis is not also environmentally friendly.
The methodology was already applied in another previously published study by the same authors. However, I have a few concerns about the density separation and the chemicals that they used.
- NaCl based density separation is not suitable for PET or other high-density plastics. They should mention this limitation somewhere in the discussion section.
- Xylene has melting effects on especially polystyrene plastics. How did they tackle this issue? This part needs clarification. Also similar concerns for 1,2,4-trichlorobenzene.
- Please be consistence about units. fragments or particles or pieces or MPs.
- Where do plastic size categories come from? Please be consistent with the literature about size classification.
- As they mentioned that only PE and PP type mulches are used for mulching and then why did they consider PS? This part needs to be clarified.
- The term soil-associated debris is not actually the correct term for soil plastic pollution. If they used this term then we need to talk about non-soil-associated debris as well? Since the authors work on soil plastic pollution then it is not necessary to mention soil-associated debris in the text. Please fix this issue.
- How did they measure the contamination issue? Any blank?
- The plastic analysis part of the paper is not reader-friendly. Please provide a flow chart for explaining each step.
A couple of issues need to be clarified.
1) The corrosive effects issues of chemicals used for py-GC/MS have to be mentioned.
2) The limitation of the NaCl separation method needs to be mentioned?
3) The blank procedure is missing.
4) What about contamination control?
Already mentioned in previous section.
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