What comes after the Sun? On the integration of soil biogeochemical pre-weathering into microplastic experiments

Büks, Frederick; Kaupenjohann, Martin

doi:https://doi.org/10.5194/soil-8-373-2022

Articles | Volume 8, issue 1

SOIL, 8, 373–380, 2022

https://doi.org/10.5194/soil-8-373-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

SOIL, 8, 373–380, 2022

https://doi.org/10.5194/soil-8-373-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 8, issue 1

Forum article

23 May 2022

Forum article | 23 May 2022

What comes after the Sun? On the integration of soil biogeochemical pre-weathering into microplastic experiments

Frederick Büks and Martin Kaupenjohann

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Final revised paper (published on 23 May 2022)
Preprint (discussion started on 06 Sep 2021)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on soil-2021-67', Anonymous Referee #1, 05 Oct 2021

General comments

The manuscript addresses an important point: the fact that weathering of plastics in the environment and, more specifically, in soils, will alter the surface (and bulk) properties of the plastic and thereby its environmental fate. As a consequence, it is expected that “aged” (weathered) plastics behave differently from “juvenile” (non-aged) plastics. This raises the question to what extent fate and effect studies conducted with juvenile plastics address the fate and effects of aged plastics. This point is valid and certainly of relevance to future microplastic research.

At the same time, the “review” has some shortcomings. First, the above point is, strictly speaking, relatively obvious (trivial) and also not novel (in fact, “ecocorona” formation has been a big topic in nanoparticle research for years; similarly, biodeterioration of plastics is well studied). Second, the first “weathering” process that the authors refer to, photochemical reactions, are extremely well studied by polymer chemists. And expert reviews exist on the topic. The discussion of photochemical reactions in this review, however, is rather rudimentary. Third, “biogeochemical aging” in soils, the second weathering process proposed by the authors, seems very poorly studied in the primary literature. As a consequence, a “review” seems premature on this topic. Instead, this section is written almost like an outlook (“what one should do”) rather than a review. This raises the question whether the entire manuscript actually qualifies as a “review”.

To the reviewer, this is a piece worth publishing. But it seems that the message can easily presented as a viewpoint on 2-3 page. This is particularly true because the level of discussion in this review is not very deep (for instance, it is not a novel insight that many processes in soils depend on the properties of particle surfaces, etc). The reviewer would therefore recommend that the manuscript is either substantially shortened and converted into a viewpoint OR that the authors spend significantly more efforts on reviewing photochemical processes of plastics. Finally, the fact that plastic biodegradation (and biodegradable plastics) are not discussed is a missed opportunity, in the eyes of the reviewer. In that line, terms like “degradation”, “decay”, “aging”, “weathering” seem to be used interchangeably. This is confusing and calls for clearer delineation and language/use of terminology.

Specific comments

Abstract

Line 10: have reported on the effects if micropastic on ...

Line 13: decreasing instead of reducing (because the latter has a chemical connotation)

Line 16: «the small sized fraction» not clear what is meant. Nanoplastics OR the small size fraction of soil particles?

Line 17: the entire plastic life cycle? Or while the plastics are in soils?

Line 20-23: chemical reactions and physical processes are not clearly delineated; also, how do soil enzymes «weather» conventional plastics? The latter typically are chemically highly inert and it’s not clear which enzymes can act on these materials

Line 31: the studies are not “widespread”, the plastic is

Line 37: maybe fragmentation is the better term than comminution?

Line 38: but fragmentation is a (physical) degradation process.

Line 55: why “however”? Where is the contradiction?

Line 80: Why were studies on biodegradable plastics excluded? Why were papers on polymer photooxidation excluded (by requiring that the term “soil” was included in the search).

Line 83-85: the authors then write a review based on one of their previous reviews?

Line 100-111: these are summary of past findings but no information is provided as to why such effects (or absence thereof) were seen. Did the authors provide explanations or were the results presented in a mere descriptive manner?

Line 114-116: And why would this be? What is the underlying cause?

Line 120: has this been shown in independent studies comparing sandy and clayey soils, for instance?

Line 138-140: The authors of the review, in the intro, state that it is important to work with “weathered” plastics as many processes are surface related. Sure. However, the point that past effect studies seem to have used unrealistically high particle numbers (and hence much too large plastic surfaces) is not mentioned and criticized. Presumably unrealistically high plastic concentrations have much larger effects than the effect of plastic surface weathering at much lower, environmentally realistic plastic concentrations.

Line 140: The wording makes it sound as if the plastic is either “juvenile” or aged. However, the juvenile plastic will age when in soils. Also, it seems that the terms polymer and plastic are not clearly defined and used. They are not the same.

Line 150: a linear pattern of flaws? Not clear what is meant

Line 158: this means that aged plastics are less of an issue?

Line 165: Do these studies also use very high concentrations?

Line 179: How can we say that the fractional mass is small? We simply may not detect all micro (and nano)plastic particles?

Line 182: experiments with mixed particles sizes / polydisperse particles?

Line 193: please provide citations for the accumulation argument

Line 197: The fact that leachates also play a role, does this mean that this effect will weaken if weathered plastics are used? Because the leachates have leached out of the plastics by the time they reach the soil? Or do the authors envision that weathering increases the potential leaching-related effects? Furthermore, the authors stress the importance of “surface” properties of the particles, yet leachates relate to the bulk volume of the plastic (while surface may control the rate of leaching). Maybe this is worth mentioning/delineating?

Line 201: does soil have a “metabolism”? Organisms do.

Line 217: True, counting and massing may dominate. But once particles are identified and their sizes known, surfaces can be calculated.

Line 228-230: The reviewer is confused. Specific surface area is a function of particle size, no? So how informative is a reported surface area of 12.6 mm2/kg?

Line 229: macrofragments of what? Plastics?

Line 238: sure, there may be metal filters with small pore sizes. But is it “feasible” to sieve a soil through such meshes? Is it this approach that the authors propose?

Line 249: Not all plastics are “hydrophobic”. PET, for instance, contains ester bonds which are polar (H bond accepting)

Line 253: smoothness must depend on how the microplastic is formed/generated. Are the authors therefore sure that the microplastic is always “smooth”?

Line 257: Chromophores are no “flaws”. Also, there is indirect photolysis in which the polymers must not directly absorb light. Finally, most plastics that have exposure to sunlight are photostabilized. Photostabilizers slow down these reactions. This is not mentioned here.

Line 254: which of the conventional polymers contains NH groups?

Line 260: Carbonyls are uncharged

Line 258: “Is not the final chapter”. Presumably German speaking scientists understand this “saying”. The rest may not.

Line 270: what are “biogeochemical attacks”? And moisture is also present during the use period of the plastic.

Line 292: Any surface (i.e., leaves and other plant residues) will be colonized by microorganisms in soils … and colonizer communities differ in composition from bulk soil microbial communities. It seems that this important point is often overlooked.

Line 299-300: Which enzymes? The reviewer is not aware of any enzymes that have been shown to be active on conventional plastics.

Line 303: What is meant by “degradation”? Biodegradation? But this would warrant showing that CO2 forms from carbon in the conventional plastic.

Line 306: The provided PZC must depend on plastic material and the organisms and molecules that colonize and are adsorbed to the plastic

Line 307: Why would plastics in soil with a low pH have a positive charge? For instance, if we take PP and PE, there are no chemical groups that can be protonated. If we take PET, the surface will contain OH groups (which cannot be protonated – note that the pKa is very different from mineral OH groups) and COO-/COOH groups. The latter are negative at circumneutral pH and will become protonated at low pH to form uncharged COOH. The reviewer doubts that this statement is correct unless the authors specify which functional groups can be protonated at a circumneutral pH to give rise to a positive net charge under acidic conditions

Line 328: Is this now a reference to biodegradable polymers?

Line 333: these groups

Line 339: Terms “decay” and “degradation” remain poorly defined. Aren’t they describing the same overall loss of plastic integrity (either in terms of physical or chemical changes)

Line 340: “weight loss” is misleading. Because there is also Mw (molecular weight). The reviewer assumes the authors refer to mass loss?

Line 345: These polymers certainly decay. The authors mean that they don’t biodegrade?

Line 350: What is meant by “microbial decay”? Biodegradation? If so, this term ought to be used to avoid confusion.

Line 380: Would we not have expected this from the get-go? It seems to the reviewer that it is not a big surprise that “surfaces are altered” for items that enter soil environments.

Line 384: the term “decomposition” is not defined. This is a general problem as the authors do not clearly define any of the terms. It seems that “weathering”, decay, degradation, decomposition are all used interchangeably. Also, the term “biodegradation” is not defined

Line 390: Is this not rather a trivial point, that “materials age” both during use as well as post-use (eg when in the environment)

Line 395: Why would one expect similar reactions? Photochemical reactions often trigger radical chemistry and needs light absorption and electron promotion to occur. This is not the case for subsurface reactions. So it seems very unlikely that the very different reactions result in the same products (unless, of course, the chemistry is looked at in a blunt manner, eg: increase in “oxygen” content)

Line 396: The reviewer cannot understand why “photochemistry” is separated from “geochemical” reactions. Aren’t photochemical reactions also “geochemical”? For instance, according to Wikipedia (quick check, and not a scientific source, but most likely accurate here): · Photogeochemistry is the study of light-induced chemical reactions that occur or may occur among natural components of the Earth's surface.^[12]

Line 402: “early material science”. What is meant by “early”? Photochemical aging of plastics is extremely well studied but does not seem to be “the early days” of material sciences

Line 418-422: and this is representative of what happens in nature?

Line 423: “dimmed world”? Why dimmed? Is this not “dark”?

Line 451-457: But would this not be very case specific? For instance, photochemical weathering depends very much on the use of the plastic (e.g., if it is light exposed). Also, how does one account for the counteracting effects of photostabilizers? Same for the subsurface: the types of reactions that may occur presumably strongly depend on both the plastic properties as well as the conditions that prevail in a given soil. How generalizable are such aging reactions?

Line 473: Is it reasonable to develop “THE” standard aging method for plastics in soils? See previous point

Citation: https://doi.org/10.5194/soil-2021-67-RC1
- AC1: 'Reply on RC1', Frederick Büks, 02 Feb 2022
  
  Dear Referees.
  Thank you very much for your mindful reviews and your very helpful comments. It has helped us to see some points which still need clarification. In the following, we want to explain how we propose to adjust our article based on the comments and also explain, why in some cases we do not agree with the proposed changes.
  First – and most important – we deeply agree with the referees, that the manuscript contains different topics, that are
  - already known (introduction, photochemical weathering of plastic)
  - already reviewed (influence of MP on the soil fauna)
  - or not widely researched (MP and soil structure, biogeochemical weathering)
  and that the focus of this work should be on the latter, but by use of a shorter, more perspective-based manuscript type.
  Our response is split into two parts: This first part contains a restructured manuscript in the format of a forum article with focus on soil biochemical weathering of microplastic surfaces and the possible need for soil-like pre-weathering of experimental microplastic. The second part (influence of microplastic on soil structure) will be re-submitted separately and is not part of this answer.
  In the following you can find a list of all your points addressed (with numbers representing the order within the old document, excluded numbers no longer are part of the revised manuscript):
  [5] Line 20-23: chemical reactions and physical processes are not clearly delineated; also, how do soil enzymes «weather» conventional plastics? The latter typically are chemically highly inert and it’s not clear which enzymes can act on these materials
  → We tried to clarify this within the abstract and discuss it later within the main text.
  [7] Line 37: maybe fragmentation is the better term than comminution?
  → done across the manuscript
  [9] Line 80: Why were studies on biodegradable plastics excluded? Why were papers on polymer photooxidation excluded (by requiring that the term “soil” was included in the search).
  → We focused on the named non-biodegradable polymers that represent …% of the plastic produced since the 1950s and their still growing legacy in soils. On the other hand, biodegradables are relatively new and only a small part of the plastic introduced into soils.
  [14] Line 140: The wording makes it sound as if the plastic is either “juvenile” or aged. However, the juvenile plastic will age when in soils. Also, it seems that the terms polymer and plastic are not clearly defined and used. They are not the same.
  → That’s correct. When juvenil plastic is added to the experimental soil ist starts aging. However, in short-term experiments it unlikely that even with the initial formation of biofilm cover there is an extended aging of surface characterists. To focus in the process, we replaced “aged” by “aging” in some cases. After shortening the text, “polymers” and “plastic, however, are now used in the correct way without any text modifications.
  [24] Line 253: smoothness must depend on how the microplastic is formed/generated. Are the authors therefore sure that the microplastic is always “smooth”?
  → We never had contrary impressions from REM/light microsopic images of juvenile commercial plastic items.
  [25] Line 257: Chromophores are no “flaws”. Also, there is indirect photolysis in which the polymers must not directly absorb light. Finally, most plastics that have exposure to sunlight are photostabilized. Photostabilizers slow down these reactions. This is not mentioned here.
  → Replaced by “weak bonds” and “indirect photolysis” added.
  [26] Line 254: which of the conventional polymers contains NH groups?
  → deleted.
  [27] Line 260: Carbonyls are uncharged
  → For soil environments, that’s incorrect. Depending on the environmental pH, carbonyl groups (e.g. -COOH) are subject to (de-)protonation, which leads to variable charges. This is a process well known for soil organic matter and the soil mineral matrix strongly controlling adsorption of molecules and interaction with other particles in soil.
  [29] Line 270: what are “biogeochemical attacks”? And moisture is also present during the use period of the plastic.
  → Replaced: “The plastic is now faced to new mechanical stresses such as (bio)turbation, largely moist conditions and exposed to a variety of biogeochemical processes.”
  [35] Line 339: Terms “decay” and “degradation” remain poorly defined. Aren’t they describing the same overall loss of plastic integrity (either in terms of physical or chemical changes) / Line 384: the term “decomposition” is not defined. This is a general problem as the authors do not clearly define any of the terms. It seems that “weathering”, decay, degradation, decomposition are all used interchangeably. Also, the term “biodegradation” is not defined
  → Both, decay and degradation describe the breakdown of organic matter by the soil (micro-)biome, while in other parts of the text the word was replaced by “aging”, “depletion” or otherwise clearyfied.
  [32] Line 340: “weight loss” is misleading. Because there is also Mw (molecular weight). The reviewer assumes the authors refer to mass loss?
  → Yes, thank you very much.
  [37] Line 345: These polymers certainly decay. The authors mean that they don’t biodegrade?
  → We instead used the term “biodegradation”.
  [38] Line 395: Why would one expect similar reactions? Photochemical reactions often trigger radical chemistry and needs light absorption and electron promotion to occur. This is not the case for subsurface reactions. So it seems very unlikely that the very different reactions result in the same products (unless, of course, the chemistry is looked at in a blunt manner, eg: increase in “oxygen” content)
  → That’s exactly our point: Pre-weathering of plastic for laboratory experiments is mainly conducted by use of climate chambers with photooxidation, but do not the outcome of aging in soil (that we do not know). We now have emphasized that point.
  [39] Line 396: The reviewer cannot understand why “photochemistry” is separated from “geochemical” reactions. Aren’t photochemical reactions also “geochemical”? For instance, according to Wikipedia (quick check, and not a scientific source, but most likely accurate here):·Photogeochemistry is the study of light-induced chemical reactions that occur or may occur among natural components of the Earth's surface.
  → For clarification we added “soil (bio)geochemical” throughout the document.
  [40] Line 402: “early material science”. What is meant by “early”? Photochemical aging of plastics is extremely well studied but does not seem to be “the early days” of material sciences
  → Deleted.
  [41] Line 423: “dimmed world”? Why dimmed? Is this not “dark”?
  → Not necessarily. There can be faint light within the upper centimeters. And you can dim something until its dark.
  [42] Line 473: Is it reasonable to develop “THE” standard aging method for plastics in soils? See previous point
  → A standard approach, that includes influcenes by plastic type and additives but also so respective soil environment (e.g. arid/humid, active soil fauna, Corg).
  
  Citation: https://doi.org/10.5194/soil-2021-67-AC1
RC2:
'Comment on soil-2021-67', Anonymous Referee #2, 18 Oct 2021
This manuscript includes many aspects regarding microplastics effects in soil and comes up with suggestions to take the factor of plastic weathering processes into account in the design of future experiments. This is an important point in microplastics research in soil environments that merits a closer consideration. However, the structure and content of the manuscript has some shortcomings. To my opinion there is a – much shorter - perspectives paper within this manuscript that should be extracted, with a clearer focus on the weathering aspect and the consequential proposed future research direction / study design.

The manuscript doesn’t appear to be a review paper. From the title I had expected a synthesis of the scientific literature available on impact studies of microplastics in soils that consider weathered microplastic particles. Expectedly, there are very few studies available, probably too few to fill a complete review paper.

However, the manuscript includes lengthy paragraphs that partly have no direct connection with weathering, or that are written rather like a technical report lacking new knowledge on the impact of weathering on soil – microplastic interaction as stated in the title. Specifically:

Section 1: There is nothing new that has not been stated by other reviews

Section 3: The reviewed studies did not use weathered plastics – this section seems to be out of scope

Section 4: Mostly citing their own work, not new information

Section 5: This is an interesting suggestion to future research – could be included in a perspectives paper

Section 6: This section reads like a technical report

Section 7: This section contains a review of weathering methods. It is not sufficient for a review on its own, and it doesn’t fit the title’s scope

Section 8: The title of this section in misleading, it is rather a conclusions section.

In essence, some parts focussing on future research merit publication, but should be condensed in a much shorter and focussed perspectives article.
Citation: https://doi.org/10.5194/soil-2021-67-RC2
- AC2: 'Reply on RC2', Frederick Büks, 02 Feb 2022
  
  Dear Referees.
  Thank you very much for your mindful reviews and your very helpful comments. It has helped us to see some points which still need clarification. In the following, we want to explain how we propose to adjust our article based on the comments and also explain, why in some cases we do not agree with the proposed changes.
  First – and most important – we deeply agree with the referees, that the manuscript contains different topics, that are
  - already known (introduction, photochemical weathering of plastic)
  - already reviewed (influence of MP on the soil fauna)
  - or not widely researched (MP and soil structure, biogeochemical weathering)
  and that the focus of this work should be on the latter, but by use of a shorter, more perspective-based manuscript type.
  Our response is split into two parts: This first part contains a restructured manuscript in the format of a forum article with focus on soil biochemical weathering of microplastic surfaces and the possible need for soil-like pre-weathering of experimental microplastic. The second part (influence of microplastic on soil structure) will be re-submitted separately and is not part of this answer.
  In the following you can find a list of all your points addressed (with numbers representing the order within the old document, excluded numbers no longer are part of the revised manuscript):
  [5] Line 20-23: chemical reactions and physical processes are not clearly delineated; also, how do soil enzymes «weather» conventional plastics? The latter typically are chemically highly inert and it’s not clear which enzymes can act on these materials
  → We tried to clarify this within the abstract and discuss it later within the main text.
  [7] Line 37: maybe fragmentation is the better term than comminution?
  → done across the manuscript
  [9] Line 80: Why were studies on biodegradable plastics excluded? Why were papers on polymer photooxidation excluded (by requiring that the term “soil” was included in the search).
  → We focused on the named non-biodegradable polymers that represent …% of the plastic produced since the 1950s and their still growing legacy in soils. On the other hand, biodegradables are relatively new and only a small part of the plastic introduced into soils.
  [14] Line 140: The wording makes it sound as if the plastic is either “juvenile” or aged. However, the juvenile plastic will age when in soils. Also, it seems that the terms polymer and plastic are not clearly defined and used. They are not the same.
  → That’s correct. When juvenil plastic is added to the experimental soil ist starts aging. However, in short-term experiments it unlikely that even with the initial formation of biofilm cover there is an extended aging of surface characterists. To focus in the process, we replaced “aged” by “aging” in some cases. After shortening the text, “polymers” and “plastic, however, are now used in the correct way without any text modifications.
  [24] Line 253: smoothness must depend on how the microplastic is formed/generated. Are the authors therefore sure that the microplastic is always “smooth”?
  → We never had contrary impressions from REM/light microsopic images of juvenile commercial plastic items.
  [25] Line 257: Chromophores are no “flaws”. Also, there is indirect photolysis in which the polymers must not directly absorb light. Finally, most plastics that have exposure to sunlight are photostabilized. Photostabilizers slow down these reactions. This is not mentioned here.
  → Replaced by “weak bonds” and “indirect photolysis” added.
  [26] Line 254: which of the conventional polymers contains NH groups?
  → deleted.
  [27] Line 260: Carbonyls are uncharged
  → For soil environments, that’s incorrect. Depending on the environmental pH, carbonyl groups (e.g. -COOH) are subject to (de-)protonation, which leads to variable charges. This is a process well known for soil organic matter and the soil mineral matrix strongly controlling adsorption of molecules and interaction with other particles in soil.
  [29] Line 270: what are “biogeochemical attacks”? And moisture is also present during the use period of the plastic.
  → Replaced: “The plastic is now faced to new mechanical stresses such as (bio)turbation, largely moist conditions and exposed to a variety of biogeochemical processes.”
  [35] Line 339: Terms “decay” and “degradation” remain poorly defined. Aren’t they describing the same overall loss of plastic integrity (either in terms of physical or chemical changes) / Line 384: the term “decomposition” is not defined. This is a general problem as the authors do not clearly define any of the terms. It seems that “weathering”, decay, degradation, decomposition are all used interchangeably. Also, the term “biodegradation” is not defined
  → Both, decay and degradation describe the breakdown of organic matter by the soil (micro-)biome, while in other parts of the text the word was replaced by “aging”, “depletion” or otherwise clearyfied.
  [32] Line 340: “weight loss” is misleading. Because there is also Mw (molecular weight). The reviewer assumes the authors refer to mass loss?
  → Yes, thank you very much.
  [37] Line 345: These polymers certainly decay. The authors mean that they don’t biodegrade?
  → We instead used the term “biodegradation”.
  [38] Line 395: Why would one expect similar reactions? Photochemical reactions often trigger radical chemistry and needs light absorption and electron promotion to occur. This is not the case for subsurface reactions. So it seems very unlikely that the very different reactions result in the same products (unless, of course, the chemistry is looked at in a blunt manner, eg: increase in “oxygen” content)
  → That’s exactly our point: Pre-weathering of plastic for laboratory experiments is mainly conducted by use of climate chambers with photooxidation, but do not the outcome of aging in soil (that we do not know). We now have emphasized that point.
  [39] Line 396: The reviewer cannot understand why “photochemistry” is separated from “geochemical” reactions. Aren’t photochemical reactions also “geochemical”? For instance, according to Wikipedia (quick check, and not a scientific source, but most likely accurate here):·Photogeochemistry is the study of light-induced chemical reactions that occur or may occur among natural components of the Earth's surface.
  → For clarification we added “soil (bio)geochemical” throughout the document.
  [40] Line 402: “early material science”. What is meant by “early”? Photochemical aging of plastics is extremely well studied but does not seem to be “the early days” of material sciences
  → Deleted.
  [41] Line 423: “dimmed world”? Why dimmed? Is this not “dark”?
  → Not necessarily. There can be faint light within the upper centimeters. And you can dim something until its dark.
  [42] Line 473: Is it reasonable to develop “THE” standard aging method for plastics in soils? See previous point
  → A standard approach, that includes influcenes by plastic type and additives but also so respective soil environment (e.g. arid/humid, active soil fauna, Corg).
  
  Citation: https://doi.org/10.5194/soil-2021-67-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (18 Feb 2022) by Peter Fiener

AR by Frederick Büks on behalf of the Authors (22 Feb 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (04 Mar 2022) by Peter Fiener

RR by Anonymous Referee #1 (18 Mar 2022)

Suggestions for revision or reasons for rejection

Review of manuscript: What comes after the sun? – The integration of soil biogeochemical
weathering into microplastic experiments

General comments
This is a revised version of a manuscript already commented on by the reviewer. The manuscript has been shortened, as recommended. Yet, some of the original concerns remain: the statement that plastics age and therefore that fate and process studies in soils should ideally be done with aged plastics rather than juvenile (pristine/non-aged) plastics, seemingly the main point the paper argues for, seems obvious, raising the question if this point needs such a paper contribution. Furthermore, another shortcoming remains: while the manuscript addresses some of the possible weathering processes (in a rather rudimentary manner), statements or hypotheses for which processes in soils such aging may become relevant are not made. This is a shortcoming since the readers are left to decide for themselves if aging may be relevant to the processes they are interested in studying. To give but one example: while aging / changes in surface properties may affect transport of very small (nm-sized) plastic particles, transport of larger particles (> 10 um) unlikely is affected by surface aging (because particle size and dimensions dominate and will lead to these particles being transported as colloids). There are also some specific comments below that may warrant to be addressed. It would be nice to have more papers in the field of microplastics (in soils) that actually study processes and effects – and fewer papers that are reviews, perspectives, viewpoints, etc on the topic. It seems that the latter have grown out of proportion with respect to the former.

Specific comments:
Line 18: Any plastic particle entering soils will undergo additional changes on its surface. It seems that this is not a mere possibility but, in fact, a given!
Line 19: Decay with enzymes seems limited to specific plastics whereas conventional plastics are resistant to enzymatic attack. This needs to be expressed more clearly considering recent studies claiming that conventional plastics are enzymatically degradable. This is simply not the case.
Line 19: Biotic and abiotic acids? What is meant here? Grammatically incorrect it seems.
Line 20: it clearly is desirable to work with plastic objects that mimic those in nature. But for which types of experiments is this relevant? For instance, persistence of the particles unlikely is changed by these modifications. Transport characteristics also not for larger particles (but maybe for nano-sized particles). Is it possible that all plastic “looks alike” in soils if chemically different materials obtain the same “coating” (ecocorona)? The reviewer would have appreciated a bit more guidance as to which processes are affected rather than a relatively obvious statement that pristine plastic does not equal aged / weathered plastics.
Line 24: What is meant with “young”? Can plastic be young? The authors mean pristine? Non-aged?
Line 41: Sure. But these are studies that used plastics in the mass% range? How realistic is this? And is this not a rather trivial finding that a soil with, lets say 10 mass % of fine ground plastic, is no longer behaving like a soil without 10% of plastic? A soil with 10% more sand or clay (or anything) will change its properties as well.
Line 51: it is true that plastics exposed to the elements undergoes weathering. However, many of the plastics contain additives to prevent chemical transformation. UV stabilizers, pigments, antioxidants … . Is it possible that for many plastics photochemical weathering is small because of these additives? Also, given that plastics differ not only in polymeric composition but also types and concentrations of protective additives, what do the authors suggest? Case-specific weathering of commercial items? Clearly, weathering PE from vendors like Sigma Aldrich or Fisher Scientific, etc would not help if these are, for instance, not photostabilized.
Line 54: what is a microscopic perspective?
Line 55: “Uniformly structured” seems a simplification: for instance, semicrystalline polymers contain amorphous regions and crystalline lamella which behave very differently
Line 58: “photooxidation and indirect photolysis”. This is incorrect. Photooxidation is a chemical reaction that can result from both direct and indirect photolysis.
Line 59: this is a rather crude description of photochemistry. It’s not that photons “hit weak bonds” thereby breaking them. In direct photolysis, photons are absorbed, an excited state is formed which can result in bond cleavage. In indirect photolysis, photochemically produced reactive intermediates form that can attack chemical bonds.
Line 79: recommend instead of advice
Line 83: is gamma irradiation a relevant aging process in the environment?
Line 85: well, any oxidative process would, no? there are many ways of introducing surface oxygen functionality into polymers
Line 93: why “dimmed”. Not dark?
Line 100: Why is soil fauna a “bioreactor”? Sure, they are “alive” and hence “bioreactors”. But most plastics will not be affected chemically when passing through these organisms. If humans swallow stones, would it be correct to say they are “bioreactors”? The reviewer would argue that the stone comes out as it went in (except for some coating). Bioreactors seems to invoke the false impression that the plastic is significantly processed.
Line 105-106: Does this make PS and PE non-persistent? By no means.
Line 106: First, there is relative humidity: a chemical reaction involving water does not need “pure liquid”. Second, which reaction can water perform on conventional plastics? Hydrolysis reactions? No.
Line 114-115: Any surface in a soil will lead to enrichment of specific microorganisms. This per se is not a surprising finding. A leaf added to soil will have a surface microbial community that differs from that in the soil.
Line 134-137: But these polymers remain recalcitrant even if some enzymes (in lab incubations?) oxidize some parts of these polymers
Line 148: soil pH affects the surfaces? How so? Do they contain pH-sensitive acid/base groups?

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RR by Anonymous Referee #2 (18 Mar 2022)

Suggestions for revision or reasons for rejection

The authors achieved major improvements in the current manuscript, and I am still convinced their recommendation to include “biogeochemical” weathering as a factor in microplastic stud-ies in soils is a valid point. However, since it is now supposed to be a perspective article, it needs more focus on the message the authors want to bring about. It still has lengthy paragraphs of review character that should be shortened to the information relevant to substantiate the au-thors’ view point. In addition, the main title and the section titles do not reflect the article’s view point character and the authors’ intent to call for a new methodologic approach in the microplas-tics research community. In the following, I will specify points I would recommend to change be-fore acceptance.

- Title should reflect the intention of the paper: an appeal to where research should be di-rected
- Consider to use the more common term “pristine” instead of “juvenile”
- Some expressions/wordings are a little clunky in parts and quite a few grammar errors and typos occur throughout. I’d recommend a revision by a native speaker to ensure the language is easy to read and inambiguous, and also to shorten the sentences.
- Line 13: replace “MP fraction” with “MP size fraction”
- Line 24: why this title? Rather, a point should be made that studies so far have widely ne-glected the dimension of time of plastic exposure to environmental factors
- Line 53 ff: instead of the following two sections divided into photooxidative and “biogeo-chemical” weathering, I would recommend to craft this into one section. In such, the au-thors can get to the point, using less words, which types of weathering relevant for soil environments have been considered in past studies and which have not. That said, me-chanical effects that have been assigned solely to the underground part (that is how I in-terpret “dimmed world of soil fauna…”) might happen as well at the surface, e.g. during abrasion with soil particles caused by wind/water erosion.
- Line 72: Please provide a link to this initiative and translate for the international reader-ship
- Line 105: I am not sure I understand the meaning of “to an eminent degree beyond the proportion of additives”
- Lines 137 ff: I am missing the argument that plastic is unlikely to serve as a major sub-strate for microbes due to their large molecular size, high chemical stability, and low bio-availability (Oberbeckmann and Labrenz 2020)
- Table 1: Why put (?) if there are references?
- Line 156: replace “mechanical treatment through biota” with “biotic effects”
- Line 163 ff: This title is not very meaningful, and also this section has too much “review-character”. For a view point paper, the authors should focus more on what they think is needed for future studies, but not putting too much detail into the techniques of weather-ing simulation that have been applied in the past.

Finally, I would like to share some thoughts I had during this second review that the authors might like to reflect upon and include in the manuscript at their discretion: In the end it is stated that weathered or aged microplastics should be produced large-scale for effect experiments, if future research will show that “biogeochemical” factors like acids, mechanical stress, and en-zymes in the soil change the particles’ surface and behavior to a significant degree.
1. If this was the case, would it not have tremendous implications on the quantification methods currently used? The stresses are presumably much stronger in the extraction set-ting compared to a soil in situ. In fact, can some of the separation methods be considered accelerated weathering (e.g. breaking down soil aggregates in a mortar, using acidic rea-gents, enzymatic digestion)? Or can we assume that these effects are negligible in situ, if microplastic particles resist the harsher conditions in the lab?
2. However, in the case that microplastics are altered to a significant degree once they en-ter below the soil surface, why should weathering simulation methods be established be-fore we even know if these processes alter microplastics behavior and impacts, e.g. on organisms or soil structure? Should the community not prioritize on effect studies that compare pristine with weathered particles regarding their effects on soil biota, accumula-tion, transport, etc.? What about an alternative approach of studying the effects in soils that have been contaminated under real conditions in field studies of soils with high “leg-acy” microplastic contamination and controls with low levels (e.g., in long term sewage sludge or compost trials)?

References:
Oberbeckmann, Sonja and Matthias Labrenz
2020 Marine Microbial Assemblages on Microplastics: Diversity, Adaptation, and Role in Degradation. Annual review of marine science, 12:209–232.

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ED: Revision (02 Apr 2022) by Peter Fiener

AR by Frederick Büks on behalf of the Authors (08 Apr 2022) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (05 May 2022) by Peter Fiener

ED: Publish subject to technical corrections (05 May 2022) by John Quinton(Executive Editor)

AR by Frederick Büks on behalf of the Authors (10 May 2022) Author's response Manuscript

Short summary

The adverse effect of microplastic (MP) on soil biota and soil structure depends on MP particle size and surface characteristics. Since weathering plays a major role in the genesis of these, it must be considered in both the analysis of environmental MP and the production of artificial MP for laboratory experiments. This work integrates recent findings on adverse effects and the genesis of its surface characteristics and discusses how to reproduce them to obtain closer-to-nature designer MP.