General comments

The paper presents the results of experiments to study the possible effect of microplastic fibres on soil properties and soil erosion. Even if there already some studies focused on changes in soil properties, it is surly one of the first studies to tackle this subject, at least with respect to water soil erosion. Overall, I find this an informative study on an important subject: How does MP effect soil and potential soil erosion? In my view the experimental design is sound, laid out well, and the results are overall well presented and be summarized. I would like to thank the authors for this interesting study and the results. When discussing the results, I miss the clarification that this study is about laboratory experiments that may not always reflect reality. We are still in the knowledge level of estimating.

Specific comments

Nevertheless, I have four fundamental critics to the paper which should be clearified:

• This study deals with microplastic fibers – as written in the title. This is not always clear stated in the paper. Especially the introduction is very generally about MP and not focused on fibers. Since the present study deals purely with fibers, I would also go into more detail on fibers in the introduction. Also in the methods and results/discussion it is often not clear you talk about microplastic at all or microplastic fibers in detail. It always has to be clear stated.
• The preparation of the fibers (size/shape) used in the experiment are described in the methods. After this description the fibers are mixed with soil via a blender. This makes the size/shape of the used fibers (still fibers at all?) unclear again and that is dangerous for the entire interpretation of the results. I expect a statement in the methods that justifies the use of a blender and how you deal with the lost information about size and shape of the fibers.
• Can the method described in this study for selecting the water-stable aggregates be cited? And with it results are compared? To my knowledge, in soil research, soil aggregates <53 µm are referred to as water-stable. In this study it is aggregates up to > 2 mm. I don't think that all water stable aggregates identified by different methods and defined with different sizes can be compared 1x1 in the discussion. On the one hand, I expect an explanatory statement on the aggregate selecting method used in the method section. On the other hand, I expect a realistic classification of the results (water-stable) in previous literature on soil erosion and soil aggregates in the discussion.
• I would be more open with the final conclusions of this study. In the discussion and conclusion, everything needs to be considered in context that the results are based on a microscale laboratory experiment and may not present natural conditions. I generally lack the courage to "don't know". It should be dealt with more consciously that the results provide estimates and do not yet close any knowledge gaps. Even with the results of this study, we still know too little about the effect of micoplastics on the soil ecosystem.

Here are some more suggestions I have for the authors:

Material and Methods:

• The description of the differences in the soils could be more precise. In the description clarify with comments like "this is the soil with most soil carbon" or with indication by value. Or shorten the text and refer to the table for comparisons.
• The information that the recorded procedure could not be continued because of the pandemic does weak your method description. I would leave that out and accordingly only say that the samples were incubated for 6 months. If you are sure that your samples were okay, I see no reason to be suspicious of this statement.
• The soil samples are dried to determine the BD. I assume at a temperature of 105 ° C. Can it be confirmed that this temperature has no effect on the MP-soil interaction? Even if polyester has a melting temperature of 150 ° C, e.g. 100% polyester fabric must not be washed above 40 ° C, as the fibers can then shrink or melt. Melting would, however, affect the analyzed parameters especially soil aggregation. I would add a sentence to this context to the methods.
• Fibers with a diameter of >2 mm were prepared. Are you sure they can move down to a sieve to 0,106 mm? Is it not obvious you will "analyse" a rise in water stable soil aggregation and by that overestimate this effect? You have an idea about the movement behavior of the fibers in the sieves?

Discussion

• Broaden your discussion due to the soil erosion. At present the discussion focuses on the differences between soils. While that is perfectly fine (but always be focused on fibers) it would be good to place the topic of the rain simulations also in a somewhat broader framework. The size of the plots you studied implies that you mainly measured interrill erosion. This also implies that your results may would change depending on scale. You should discuss this. What are the implications at a real landscape scale?

Technical corrections

There are more small comments and some textual suggestions in the annotated pdf attached.

In their present work, the authors analyse the alteration of soil structural, soil hydrological and erosion parameters after addition of plastic microfibers. In the first of two experiments, disaggregated soil samples from three locations with different texture (Lt3, Ls2 and Slu, following KA5) were incubated for 6 month to compare the re-formation of total and water-stable aggregates, macroporosity/air capacity and plant available water capacity between treatments with and without microplastic (MP). In a second experiment with similar preparations, the erodibility of soil samples was tested in runoff containers by measuring surface water runoff, drainage and sediment loss. These parameters are known to be influenced by a loss of soil structure and water holding capacity. The combination of both experiments is therfore a good approach to analyse cross-scale effects of MP contamination in soils. Furthermore, MP fibers are a major part of plastic brought into agricultural fields by the application of sewage sludge and have been shown to be adverse to soil structure. The applied concentration of 0.5 % w/w only apperas in highly contaminated areas, but is reasonable. On the other hand, the authors used juvenile MP, which has surface characteristics very different from weathered MP found in the environment. This work can make a significant contribution to our knowledge of the effects of MP on soil structure and landscape erosion. However, from my point of view some improvement is needed. In the following I would like to list a few points which I hope will be helpful for you.

Abstract

Line 20: Please describe in more detail.

Introduction

Line 28: „Improper disposal“ sounds a little bit like littering, but MP in remote areas is more from textile fibers or intended application followed by dispersal.

Line 35: Is it „sewage sludge“ (without comma) or do you mean „sewage, sludge“ in the sense of „waste water and sewage sludge“? There are also other entry pathways like coatings of seeds and fertilizers (very important) as well as irrigation from natural water bodies.

Line 38: Slow degradation – please add a reference.

Line 42: … percolation, cryoturbation, peloturbation.

Line 46: They are related, cancel „more or less strictly“.

Line 56: This work is more on changed soil properties than the underlying processes.

Lines 58-59: Is the expected impact assumed to be positive or adverse? Why do you assume that the fragile soils (I think you mean the sandy Alfisol) are suffering the most from MP? I guess, that soils with a high capability of aggregation would be more susceptible. Please deduce your hypothesis.

Material and Methods

In general: The comparison of only the t=6 month data is sufficiant, but t=0 data would be nice.

Line 68: What is „good“?

Line 69: What are „other elements of fertility“?

Line 81: N concentrations change very fastly after sampling due to microbial processes. Is that measurement important for the underlying work?

Table 1: It would be helpful to add a column „texture“ using KA5 (Lt3, Ls2, Slu) or SSM.

Line 96: Are the soil macroaggregates entirely disaggregated by the blender? And do you expect significant comminution of fibers during the blending?

Figure 1, a): Why do the 3 soils have 4 colors?

Line 108-109: I think, a „dry-out event“ applied to all samples is no problem in this case, and I agree with referee #1 to leave that out.

Line 112-113: Why 8 control replicates and only 4 with MP?

Line 114-141: This section is a bit confusing. Why do you measure so many θ values and use van Genuchten, if you only need the saturated value, field capacity and the permanent wilting point? What is „the value determined at h=-1 m after oven-drying“ (after oven-drying, there is pF 6.5)? Why is there an oven-drying in between (which probably disturbs binding factors within the soil aggregates), and can you ensure that the re-packing of samples do not destroy soil structure (and thereby alters the soil hydrological properties)?

Lines 135-136: Macroporosity and air capacity are both defined as the pore space of macropores (>50µm), which are permanently drained due to their low matrix potential. This pore diameter corresponds to pF 1.8. Please explain your choice of pF 1 (0.1 m hydrostatic head) and pF 2 (1 m hydrostatic head).

Line 143: Here, the measurement of soil structure needs a clarification. What is the parameter? (I think, water stable plus water labile aggregates?)

Line 144: Dry-sieving can destroy macroaggregates especially in sandy soils. The subsequent application of wet-sieving to the same samples can lead to artifacts, as the less stable WSA could have been destroyed and the %WSA reduced.

Line 146: If the soil macroaggregates were entirely disaggregated, also the other size classes >250µm give information on newly built macroaggregates.

Line 154: The WSNFA(%) is a tricky parameter. The oberserved increase of WSNFA(%), that comes along with the decrease of NFA(%), can be caused by both, the increase of WSA or a loss of water labile aggregates (WLA) faster than the loss of WSA. The different implications of these two cases should be clearly discussed. Please also reconsider your choice for the WSNFA indicator.

Equations (1), (5) and (7): Why „A“ and „P“ for weight instead of m?

Equation (6): Please explain the additional information given by the S index.

Line 169: Why „Therefore, …“?

Line 200: It might be helpful to add that 33.4.mm h-1 is a heavy rain event.

Results

Additional Table: What do you think about an additional table containing the trends of all measured soil properties in both experiments?

In general: Is the degree of aggregation after six months within the control samples similar to the degree of aggregation observed in the field? If not, there might be further development of soil structure. Please consider that in MP samples, due to the short term approach, a delayed development of soil structure towards uncontaminated samples cannot be excluded by the experiments. This is important for assessing the environmental risk.

Discussion

In general: The relation between soil texture, soil structure and the influence of MP on aggregation and water holding capacity on the one hand and between the results of the first and second experiment on the other hand are only sparsely discussed by the authors. Starting with a clear-cut hypothesis, the observed alterations should be discussed in terms of their consistency. E.g., the loss of total aggregation in the sandy soil comes along with a loss of macroporosity/air capacity and an increase of water holding capacity, which increases the ratio of percolation to surface runoff and, thus, reduces erosion. What about the more loamy soils? What could be the effect on the landscape scale?

Line 289-290 and 292-293: The Entisol (Ls2) shows alterations very similar to the Vertisol (Lt2). Although both soils differ in their clay content, they are loams, that have a tendency to build stronger soil aggregates. In contrast, the Alfisol (Slu) has a low clay content. Sandy soils do not aggregate very well. Perhaps the explanation of the effects rather benefits from this distinction then from putting Entisol and Alfisol into one class.

Line 306-307: Why should MP, that is occluded within microaggregates, reduce their ability to bond with other microaggregates?

Line 343: Is there a possibility that microplastics prevent soil erosion without having a positive effect on the aggregate structure, e.g. by simply lying around in the way of the runoff?

Conclusion

From my point of view, the conclusion should contain considerations about the effects on the landscape scale (because we have contaminated the whole world with microplastics), long term effects and the role of lower MP concentrations (because common concetrations are some orders of magnitude below the applied 0.5 % w/w, https://soil.copernicus.org/articles/6/649/2020/).

Best regards,

Dr. Frederick Büks (Chair of Soil Science, Technische Universität Berlin)

(If you have further questions, don’t hesitate to ask.)