General Comments:

This paper advanced knowledge regarding mobilized POC biodegradability as a result of Arctic thaw slumps.  The identification of the rate of biodegradability of slump-mobilized POC answers an important piece of the lateral carbon flux puzzle in this region, making this paper very worthy of publication.  Ultimately, this paper also answers the separate question by proxy, that there is a trend for slump-mobilized DOC to decrease during incubation despite the low biodegradability of POC and TOC.  The knowledge gap is clearly stated, and the introduction does a comprehensive job of outlining the main question.  The paper also detailed comprehensive experimentation over the course of many years in order to answer a series of related, nested questions.  There are a few modifications and clarifications that I have outlined below that I believe would help to heighten the considerable impact of this paper’s findings. I have broken up my main points into four bullets below.  Line edits and more detailed questions follow in the Specific Comments and Technical Comments sections.

• Relative to what the degradation of organic carbon would have been had it remained frozen in the Arctic tundra, oxidative loss of 4% POC per month could be significant. At this rate, this accounts for a potential loss of 16% over the course of a 4-month growing season.  When scaled up across the Arctic or scaled up over many years, this represents a considerable C degradation pathway.  I believe it is important for the author to contrast this 4% loss with the alternative, had POC not been mobilized by thaw.  For instance, in the absence of thaw slumps mobilizing this POC, can we assume negligible loss of permafrost organic carbon over the same timescale?  The paper’s tone does not do this impactful result justice.  I would re-casting the significance for C cycling in contrast to the degradation rate in the absence of slump-induced C mobilization.  Though the biodegradability may be low, it is still quite important at the rate of 4% per month. 
• This paper includes many great experiments; however, the conclusion is a little truncated. Echoing point 1, the conclusion is a good place to reiterate the significance of the main finding, of 4% POC loss per month.  Further, I would suggest taking a stronger stand on each of your experiments and the text of the discussion, where you parse out the relative importance of each of the potential POC sequestration pathways (abiotic and biotic).  The conclusion would be improved if it included more regarding the vulnerability of this mobilized and subsequently sequestered C.  Will the sequestered C be vulnerable to a faster rate of decomposition? What is the most important sequestration mechanism in the second to last sentence (L308)?
• The supplemental flow charts (S5-S7) are incredibly useful in aiding the reader’s understanding of the experimental design. Within the text itself, the location/code of individual slumps and the treatment codes distract the reader from the main findings beyond Figure 1, which orients the reader to each slump location and code.  In general, I am curious why the authors did not combine the slumps that were similar in their analyses, treating them as replicates of one another (with the exception of the slump with some encroachment reported).  As a general suggestion overall, if it is possible to remove all reference to specific site acronyms (rather, refer to each site as site 1, 2, 3, etc.) and to refer to the treatments with complete description, e.g., “unfiltered upstream” rather than by acronym “UU”, I believe the text clarity and readability would be greatly improved.  This is a minor update that I believe would have a major impact. 
• <1 week incubation times seem very short. For soil incubations, this short time would be considered a disturbance measurement since there are artifacts from handling and setting up an experiment in conditions away from the field. Furthermore, POC from older carbon permafrost soil (as evidenced by radiocarbon age) would likely have a slow turnover time, which by nature takes longer to measure rates. Please add some visible text to the discussion and conclusion that the short term experiments might be limited in both detecting the actual rate of change (Type II statistical error), and the role that the novel lab conditions may play during this time period.

Specific Comments:

• This paper is primarily focused on POC not POC and DOC, however, the ultimate findings suggest that POC fractions studied have lower biodegradability than DOC and I believe that contrasting the two broader size classes of stream OC and how they may interact could be of use given the ultimate findings (e.g., increased POC mineral input into streams has the potential to increase DOC sorption). I also believe that POC and DOC transport is an important aspect of lateral carbon fluxes worthy of mentioning early on in the abstract, albeit briefly.  Transport of carbon is the initial mechanism that allows for mineralization into CO₂ and re-sequestration into sediments. 

L11: Mineralization as CO₂ and sedimentation are two POC fates, but this sentence does not address re-sequestration of stream C by aquatic plants or transportation downstream (though transportation is not an ultimate, chemical fate).  I believe 1) it would be useful if the abstract jumped right into POC as this is the primary focus of the paper’s research OR 2) for the abstract to include mention of transportation as the mechanism allowing for soil organic carbon to become transported POC, mineralized CO₂, or re-sequestered sediment within stream systems.

Suggestion 1: “Upon thaw, permafrost particulate organic carbon (POC) may be mineralized into CO₂…”

Suggestion 2: “Upon thaw, permafrost carbon entering and transported within streams may be…”

L30-35: Transport is covered in this section, I believe it should be mentioned in the abstract, briefly as is presented in Specific Comment #1 above.  The dichotomy of fates as it relates to the transport trajectory (transport vs deposition according to size and density fractions) is ultimately relevant to the study findings.

L32: It is probably worth mentioning that anoxia reduces overall mineralization rates but also shifts carbon loss towards methane (Schaedel et al. 2017 Nature Climate Change)

L40: Might be helpful to discuss different sources of POC in slump affected- and non-affected streams so reader can understand why lability might go up/down.

L127: circumneutral-pH, in my experience, pH of many Arctic water tracts is closer to pH5 than pH7. 

L190: for clarity, identifying SE particles as slump SE would be useful and parallel HA slump particles later in the sentence.  However, see point #3 in the general comments.

L184: most organic matter is partially oxidized because it has oxygen molecules. For example, glucose has a lot of oxygen molecules.  Would this line be expected to be a 1:2 line instead of a 1:1 line?  Most organic matter has oxygen as a part of it, does this change the heterotopic respiration line of 1:1?

L250-255: Some DOC may be decreasing as it is converted to CO₂ alongside consumption of O₂ as shown in Figure 3F and mentioned in L215.  I would propose DOC declines as a possible reason for O₂ consumption mentioned in L250-255. 

Figure 4: Please note, MQ water has been found to carry a baseline amount of DOC, typically below the standard detection limits of a TICTOC but enough to impact radiocarbon measurements (0.5 ppm) if MQ water is used to generated standards.  

Supplemental Information:

Page 3: Do you suspect that the varying incubation timing (7, 11, 8, and 27 days) has any impact on the resulting POC degradation?

Unresolved general question: How did you store your samples prior to analysis? How many days were they stored once collected, were they refrigerated, frozen, or acidified? Were they stored in the dark? These questions impact the ultimate degradation of the C within the samples.

Technical Corrections:

Table 1: In my copy of the manuscript, Table 1 text is too large for the cells, with overhanging letters in the first four columns.

L56: 1) removing the slump site identifiers entirely from the text regarding the 2016 and 2019 experiments or 2) Identifying which three slump sites were used (HA, HB, HD) in 2015 would be useful for the reader and would mirror the identification of slumps SE and FM3 in the 2016 and 2019 experiments in line 61 and 62, respectively (see comment on L59, below). However, see point #3 in the general comments.

L59: Site HD-UP is introduced in the text before the reader is oriented to what site “HD” represents; supplemental Figure S5 does not portray HD-UP, I believe this should be updated to Figure S4 and HD could be introduced in Line 56 as mentioned above. However, see point #3 in the general comments.

Figure 1: It would be beneficial to the reader to identify slump SE on the larger map as well as in the map inset (slumps HB, HA, FM3, and HD are all identified on the larger map, but SE is missing).  Indicating that SE, HB, HA, FM3, and HD are slumps on the map key would be useful. Within the inset, SE-IN is identified. Should UP, DN-1, and DN-2 also be described with the SE- prefix in the inset? I would suggest labeling the entire inset as the slump SE transect and omitting the label SE- from the IN location. However, see point #3 in the general comments.

L81-83: The settling component of the 2015 experiment is distinct from the 2015 incubation experiment.  I believe this would be best organized in a subsection, rather than grouping the incubation and settling together by year in one paragraph, as variation over year is not a factor of interest in the overall paper.

L85: Slump SE is referred to in this section however the 2015 sites were not mentioned by name in the previous section (2.2.1).  I’d recommend consistency between the sections.  However, see point #3 in the general comments.

General Comments

The study utilizes experimental aerobic incubations of sediments taken from slump affected streams in the Peel Plateau to investigate if the potential of mineralization of slump derived POC varies from that or POC in non-impacted streams, and to quantify the biodegradability of slump POC fractions relative to their transport potential.

Several experiments involving samples collected over different sites and seasons. In 2015 samples were collected from sediments in streams near and within different slump sites to test if slumps affected the biodegradability of POC. Water samples were also collected above, within and downstream of slumps. Water samples UU unfiltered upstream (in situ POC) relative to filtered upstream water to which slump POC was added (SU). In 2016 samples of sediment were collected near the SE slump, upstream, in the slump, and downstream of the slump – to test variability in biodegradability with transport.  In 2019 sediments were collected within and downstream of slump FM3 for follow-up experiments.

The authors conclude that there is minimal (4%) mineralization (oxidation) or POC over 1 month incubations. The authors propose that these low rates may be due in part to protection by adsorption to mineral particles. Additionally, the authors propose that the surrounding mineral rich tills promote inorganic C sequestration via chemolithoautotrophic processes.

This study involves the application of a carefully executed field sampling design, combined with carefully designed laboratory experiments and sophisticated analytical tools to address a very important knowledge gap in our understanding of the biogeochemical controls on the fate of particulate carbon released from permafrost thaw and disturbances. This is a study very worthy of publication. The methods are well detailed and documented, and overall the results are very well presented, although I have some concerns and suggestions about results. This is a very data/results rich paper.

My only substantive concern is the brevity of the conclusions. I feel the authors have missed the opportunity to really put these findings in to context. The authors show that these systems release and move a lot of carbon, but that the GHG emission potential is minimal – this is very significant, and should be discussed in the context of other work that suggests these abrupt thaw events might account for a large part of emissions from thawing permafrost - For example, the authors should discuss the meaning of their results in the context of the findings of Turetsky et al.’s Nature Geoscience, 2020 article (https://doi.org/10.1038/s41561-019-0526-0).

Specific Comments

This was a remarkably comprehensive and carefully designed and executed series of experiments.  Although, I really appreciate the very concise explanations of the experiments in section 2.2, the subsequent results (sample acronyms and experiments) were hard to keep straight, until I read through the supplemental and saw Figures S5, S6, S7.  I strongly recommend including Figures S5, S6, and S7 (or maybe some reduced form of one or two these) in the methods section of the main paper. These are great illustrations. I realize that space constraints might make this an editorial decision, however I found these figures were critical to clearly communicating the methods and experimental design. These will also help the reader keep the acronyms for the samples straight.

Table 1 is a really great help for summarizing the findings, and the limitations. However, it is a bit difficult to follow in places, I suggest presenting this table in landscape format, so that it is not as crowded. Again I realize this may be an editorial decision, however some rearrangement or reformatting is required to really maximize the readability of this important table.

 Results.

In section 3.1 the authors report that the %change in POC is lower where the slump particles were added, and that this is likely due to the fact that the particle concentrations were so high in those samples that the % change is small. The % changes is potentially masking the importance of the magnitude the change in total mass of POC. It would be useful (more useful) to provide tables (or figures), in not in the main paper, then at least as a supplement that illustrates the changes in DOC, POC and TOC in terms of total g of C. Perhaps the point can be made at least in part by referring to the data as shown in Figure 3a and/or 3d for the DTOC.

Similarly, for the fractionated vs. unfractionated experiment. It would be helpful to show somewhere (e.g. Table B2) how the mass of C is distributed across the size fractions, to know where the greatest total C losses and gains are occurring, and thus to better interpret the % changes in terms of effect of size fractions on mass of C lost/gained. Perhaps this can be done in part by citing Figure 3b – which shows that the greatest change in C is due to the <20um fraction?

The references to Appendices seemed odd to me. I am not familiar with Journals that support appendices, so I didn’t even know where to look for them at first – I was happy to see they were at the end of the main document.. I think they definitely ought to appear in the main paper somehow – rather than in a supplemental -given that these data are very important in terms of the support they lend to the findings. The only exeption might be the material in Appendix C, which could go in the supplemental if necessary.

Technical Corrections

Line 50: I suggest including some years to provide reader with more confined age of the Pleistocene age tills in this area, if known.

Line 52: Insert “the” after comma following “Thus, the relative..”

Line 53:  Delete “Variations in”, start sentence with “Source composition can also vary…”

Line 56 : Since there are 4 sites on the map in figure 1, I suggest inserting the site names of the 3 sites in brackets in this sentence to clarify the sites sampled for this experiment “In 2015, …within three slump sites (HA, HB, HD)

Figure 1: label all panels in the figure. E.g. the map should be labelled as panel a) then the headwall units panel b); and the sampling site locations panel c).

Line 78: it is unclear how much water was used, the serum bottles were 120ml, but does this mean you used 120ml of water + 2 ml of slump runoff? Insert sample volume to be clear how much headspace was left in the bottles. e.g. “we incubated <xx ml> unfiltered upstream ….”

Lines 85-87: The sieving process could use some additional explanation. It is not at all clear how such a small sample volume (0.5ml) could be sieved. Also were the fractions weighed? How did you know the mass of each fraction added (or the concentrations) of the final 60ml solution?

Line 106: Delete “First,” and start this sentence with “To assess…”

Line108: Insert the volume of sample used, so that water vs. headspace volume in the 60ml bottles is clear. “We incubated “XX mL” of sample in 60mL glass BOD bottles

Line 113: The bracket should include reference to equations 1a, 1b. “…(eqns. 1-4)…” x

Line 114: Indicate the methods used to quantify N species and sulfate, and/or refer to citation or supplemental where this is explained at end of sentence.

Line 113: Since your goal as stated at the start of section 2.2.4 is to asses O2 losses and OC gains, and since not all the equations (1-4) contribute to generating OC. You should insert “could consume O₂” in this sentence.  E.g. “…could consume O₂ and/or generate OC, …”.

Results:

Line 141. I am not familiar with having appendices in journal articles. I suggest adding material from Appendix A to the main paper or the supplemental.

Line 145: It would be helpful to show the DSUVA254, in the paper or in the supplemental.

line 160: Figure 2 caption,  you say “measured (point) and modelled (line) O2” but there are no points visible in panels a-c.

Lines 185-191: Minor point, but you use lower case letters to identify the panels in Figure 3, yet in the text you cite Figure 3A, 3C etc. using upper case. I suggest that you should use lower case letters in the in text citations (Figure 3a, 3c…) to be consistent with the figures.

Line 202: Figure 3 caption, note the 1:1 line in panel (f) is solid vs. other panels where it is dashed. Is there a reason why this one is different? If so explain this in the caption, if not edit so that it is dashed as in the other panels.

Line 215 and 218 – I suggest replacing “balancing to” with “resulting in”Line 218: Insert “in sterilized bottles” after TIC.

Line 223 and 227 – use lower case letters in reference to figure panels, so that these are consistent with how they appear in the figure.

Line 228-230: I think this sentence requires rewording to clarify the message the authors are making. The authors suggest that the increase in simple compounds in sterilized samples “cautions against assuming the sterilized treatment is a true abiotic control of organic matter changes”.  This seems to suggest that you are calling into question the fact that your sterile samples were truly sterile, which I don’t think is the intent. I think you mean to indicate that the changes in DOM could be entirely due to the sterilization process itself, hence the change in DOM composition of the sterile samples cannot be considered a “control” or “baseline” of the of DOM if there had been no biological activity in the samples. I suggest maybe a simple correction, delete

“PC1 separated DOM…proportion of simple compounds,” Given that the sterilization process itself could increase the proportion of simple compounds, the results caution against …. “.

Discussion:

Line 261-262: … indicate that CO₂ production ceased by the end of …” this sentence requires a citation to support this statement.

Line 282:  you suggest that chemolithoautotrophy as a possible mechanism for counterbalancing OC mineralization.  Can you discuss or provide evidence to support that these reactions are likely/possible in these environments - i.e. are the thermodynamics/redox conditions consistent with environments where these chemolithoautotrophic processes (organisms) are known to occur?

Conclusions

These findings of the low biolability of the permafrost POC are so important, yet the significance is not raised at all in the conclusions. These findings need to be put into context in the conclusions to better highlight their significance – as stated above especially with respect to Turetsky et al 2020.  It seems to be broadly accepted that these large abrupt permafrost thaw events are likely to have strong positive feedbacks on atmospheric C and climate. Your study calls this into question – I think you need to highlight this. 

Supplemental Information

Page 3, Para 2: line 4:  “Material the passes through “  replace “the” with “that”

Page 3, Para 2: line 6:  “…. Material passed through the filter discard and…”should be “discarded”

Page 3, Para 2: line 9:  INSERT “from the 0.5 mL sample” between “particles” and “through the …”

Section 3.3 paragraph 1 line 2: a 0.7 micron filter is not standard for these analyses. Can you comment on why you used this pore size, and also what if any effect the larger pore size might have relative to standard measures?

Figure S5 – since you didn’t use both time points, I suggest removing the one you didn’t use. Also indicate the time of the timepoint (30 days?) on this and other figures or in the captions.