In this paper, the authors present results from a unique study comparing soil carbon stock and 14C values to soil respiration fluxes and 14C values during soil incubation of soils from 3 sites with distinct climate to up to 6 meters depth. What is truly unique about this study is the depth to which the authors sampled and incubated soils. I also find the depth increments to be impresively fine, adding to the value of these data (especially for modeling). The authors found that even at several m depth, microbial respiration during incubations is fueled by relatively younger C. While this finding is not unique, the comparison of sites across a climate gradient and the depth to which this study sampled in the soil profile is quite unique.

The paper is reasonably well written, but could be improved with some editing and revisions to the figures. These are relatively minor issues. More concerning, however, is the lack of statistical analyses or descriptions to support the authors' interpretations and conclusions. This maniscript needs some description of the statistical analyses used to provide the reported errors (are they standard deviation or standard error? something else?) and at minimum some simple statistical tests to look at differences between the sites and the 14C of TOC vs respired CO2-C. This could be simple: A t-test to test the statistical significance of the TOC vs CO2 could be done easily by calculated the difference and testing if it is different from 0 - this could be a paired test so the same site/depth are compared to one another.

More detailed comments are below:

The title is great - I love that the main finding is right there!

L 19 "higher than of the soil" is missing a "that" or something similar

L 23 "strong microbial decomposition" - what is meant by "strong"? Perhaps a different word is better.

L 24 "which is likely due to stabilization" - I do not follow this logic. Do we know that decomposition stops at depth because of stabilization? I do not know that that we know that for sure. Perhaps "posssibly" instead of "likely" or "partly" or be more specific about what you mean exactly.

L 30 "topsoils" are not generally thought of as the top meter (often this is the surface or A horizon, maybe the top 10 cm or 20 cm or even 30 cm, but certainly not the top meter!) Consider just saying in the top meter here and elsewhere, "top meter" is not much longer than "topsoil".

L 35 the following paragraph is confusing to me - do you mean in the field? Incubations? Can you be more specific?

L 43 you need references to back up the statement that the total CO2 is largely composed of CO2 respired by roots. And what do you mean by largely? Half? Can you be more specific? What did these studies find?

L 44 This paragraph would be a good place to clarify that you did laboratory incubations. It is not clear that you used incubations until the methods on L 92.

L 137 This looks like a total DNA extraction with no clean up to remove plant or animal DNA - is this true? If so you should not call it microbial DNA. If you did remove plant and animal DNA please explain this more clearly. I think you can still use total DNA as a proxy for microbial DNA and biomass but you should be clear that this is what you have.

L 149 use "flushed" or "scrubbed" instead of "rinsed"

L 152-153 I don't follow - samples were collected at multiple time points over 11 or 8 months and checked for CO2 concentration? It sounded like it was collected for 14C as it was written but the results seem to have only one time point for each site. I don't think you need to provide so much detail here - you can just say CO2 concentrations were monitored and you collected the samples for 14C when there was enough or when you cutoff the incubation (at 11 months?) Please clarify. There is also detailed description of the respiration rate sampling earlier, but those data are not presented - why is that?

L 167 what is the reported analytical uncertainty? were soils pretreated for carbonates? were there carbonates (especially at the arid site?)

L 172 I know what you mean, but the phrasing is awkward. You could say "...model to the data, but there was not enough data to constrain such a model" that said you could make assumptions or do a 2 compartment model with the respiration data, if you wanted to. (I don't think you do though!)

L175 - there is no description of statistics for the results! How were trends and differences assessed? Are these differences statistically significant? What are the reported uncertainties? Standard deviation or error or something else? Where is the respiration rate data?

L 204 Why do you say that most C respired by microorganisms is likely directly derived by roots? Can you be more specific? You might consider refering at some point to Phillips et al 2013: https://bg.copernicus.org/articles/10/7999/2013/

L 223 add citations (you could use Phillips et al 2013 but there are likely others)

L 227 What "interaction" do you refer to? Can you be more specific? This sentence is very awkwardly worded, please revise.

L 233 This paragraph is difficult to follow - is it about decomposition or about 13C? It seems to go back and forth but not clearly explain how the two are connected.

L 237 What do you mean by "this"? Stabilization would not cause enrichment in 13C.

L 240 I do not follow how this is connected to 13C or the rest of the paragraph, it feels disconnected from the rest.

L 244-6 it would only be similar if C was cycling the same way, it seems like C cycles more slowly at the arid site and you would expect to see a smaller Suess effect there, which you do see. I would just cut this last sentence and reword the first part of the previous sentence to something like, “it is possible this is partly a result of the dilution of atmospheric 13C…..”

L 253 Earlier in the paper suggests that 13C of roots were used for this interpretation too, but it looks like no? Are those data helpful here? Were there carbonates anywhere (it looks like maybe not, but it would be helpful if you could clarify this).

L 250 There is no discussion about the differences in vegetation causing some of the differences between sites. You should at least consider these differences since there is definitely a difference in vegetation cover across sites.

L 254 can you add some more citations? Maybe https://doi.org/10.1007/s10533-020-00725-z

L 260 by "a different carbon pool" do you mean "a portion of the TOC pool"?

L 263 don't you think this is mostly because of water? these sites are so dry!

L 270 this comparison to permafrost seems very out of place here. You could develop this comparison by adding more context to the Intro and to the Discussion (with citations). If you think this important it could make the paper more interesting.

L 273 what do you mean by "tightly connected"? Can you be more specific? It seems like the deeper soil is connected to the shallow soil, but are the shallow soils really connected to the deeper soils?

L 277 where will the data be? the paper has been accepted as a discussion paper, where can the data be found?

Figure 2 for figure c, you could add a 1:1 line to show where the points would fall if respired C = TOC (since the axis are not the same)

Figure 3 It doesn't seem like the axis need to go to -16 delta 13C. Can you plot these across a smaller range to make it easier to see the differences? Maybe -22 to -30 permil?

Figure 4 It is difficult to compare the panels arranged this way - maybe you could plott all 3 sites together with an inset or break in the axis to help show the differences? Something more like figure 5?

In this preprint the authors use carbon (C) isotopes ( ¹⁴C and ¹³C) to determine the source and age of respired soil carbon in three sites along a climatic gradient in the Coastal Cordillera of Chile. They find that respired CO2-C was of more recent origin than soil organic C and that C in deeper soils was older than surface C. They then use these results along with total DNA extracted from the soils to conclude that microbial decomposition is primarily of new carbon, rather than old in these soils. I found this manuscript to be an important contribution to our understanding of the origin and age of microbially-processed soil carbon and generally an interesting well-written piece. My chief concerns are to do with the authors’ use of total DNA as equivalent to microbial DNA, and therefor a proxy for microbial activity (Line 137), in some of the authors statistical methods relating respired total ¹⁴C to respired ¹⁴C (Figure 2c), and the interpretations regarding the influence of primary productivity (Line 228, conclusions, and final line in abstract), which I have detailed below.

Specific comments below:

Line 49-50: It would be nice to include an explanation for why the arid site was dug less deeply than the other two, perhaps in the methods section 2.2.

Line 73: Do the authors have any estimation of the differences in isotopes that might be due to the differences in sampling year and how that compares to the ranges seen in their results? This is particularly important for the ¹³C-isotope results. I could see those as being particularly sensitive to differences in temperature and moisture conditions between the two sampling years.

Line 114: For methodological transparency, the authors should include a brief explanation about how they determined the pre-incubation period for each environment.

Line 137: Rather than referring to this measurement as microbial DNA, the authors should refer to it as total DNA. It likely not only includes DNA from microbial sources (e.g. microbial eukaryotes, fungi, and prokaryotes), but also DNA from non-microbial sources such as plant roots or soil arthropods. Although proportionally the non-microbial DNA is likely to be low in comparison, it is likely to vary among soil types (based on the amount of vegetation and moisture of the soil) and depth. Therefore it likely that the soils not only contain to contain different proportions of microbial:non-microbial DNA in different soils but also at different depths. The authors therefore should be cautious in their interpretation of total DNA as a proxy for microbial activity and should adjust their discussion accordingly. One relatively easy experimental way around this, would be to use qPCR to quantify the abundance of 16S and ITS gene copies in each soil sample's DNA. These numbers would be a more accurate quantification of microbial abundance at least, even though not all organisms possessing those genes are likely to have been active during incubation.

Section 2.7 – not having much disciplinary expertise in this area, I defer to others who do in evaluation of the methods. However, I appreciate the explanation in the last paragraph about accurate estimation of ages, which is helpful for non-experts in ¹⁴C dating such as myself. It is also helpful that the authors imply that the carbon they observe likely is derived from the last 1000 years, and would be further helpful for their non-expert audience to know where the estimate of 1000 years comes from. Furthermore in their discussion, the authors describe a similar study in permafrost that was able to estimate ages using ¹⁴C, if there is any way to get at least a range of ages from this data to compare to that study, I would find it very useful in interpreting the results.

Figure 2c – The R² value of the arid linear model is quite low compared to the mediterranean and humid sites. I wonder if a simple linear model is even appropriate for this relationship as it seems that depth, along with other co-correlates are at play. In fact, for the humid site as well, the mean and variance appear to be related with higher mean values having less variance at more shallow sites which violates the assumption of equal variance of residuals which means that their estimated slopes may be incorrect. The authors should address this low fit in the text and potentially may find some helpful solutions in this guide: https://academic.macewan.ca/burok/Stat378/notes/remedies.pdf

Line 228, conclusions, and final line in abstract: I’m not sure the authors have made a very strong argument that processes of the soil are highly dependent on primary productivity aboveground. Although the results do indicate that newer carbon is being respired, an alternative explanation could be that the newer carbon is simply being re-cycled among the microbial community as the community turns over. The incubations are not directly measuring the influence of primary productivity since they are plant-free incubations and there aren’t measurements for primary productivity at each site. I recommend a more conservative interpretation.

Section 4.2: The authors may also want to consider the process of priming in their interpretation of this observation (see for example Bernard et al. 2022 for a nice review on the subject - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2435.14038).

Technical corrections:

Line 98 – Typo, I believe “weight” should be “weighed”

Line 118 – typo in the specification of volume of gas: there are extra spaces between the number and the units. Additionally the units of liters should be abbreviated with a capital “L” rather than lowercase “l”.