This paper by Grant et al presents a comprehensive study where they measured the 14C contents of many soil organic matter fractions with depth, from the specific grassland soil. These are very useful data to gain more insight in organic matter cycling in soils. A great amount of work lies behind it, for which I compliment the authors.   Although the data are good, I think the paper would improve by a somewhat better presentation, and some more in depth discussions that can be structured better. I have given comments on the attached pdf, but some main and additional points here:

The influence of the bomb spike on 14C contents (especially important for the the upper layer) is missing in the discussion.

The authors discuss the relative differences between the various fractions (size, density, compound class), but not not discuss the depth aspect much, nor do they even try to quantify turnover times, which is one unique aspect that these 14C data allow. This is a missed chance.

A discussion of the source of the organic matter: besides leaf litter from above, which is then worked downwards by bioturbation and percolation, there should also be some root litter. Where does this go?

The structure of the discussion. This goes a little criss-cross through the data, size and compound fractions and I had to re-orient myself each time what figure and data to look at. One approach could be to try to answer certain questions/hypotheses using the data, and place this in perspective to data and insights from the literature. What is the most exciting news learned from this study?

In this manuscript, the authors describe radiocarbon data collected from a wide variety of soil carbon fractions following various definitional methods. This was done in an attempt to identify faster and slower cycling carbon pools following our understanding of bioavailability and persistence of soil carbon. The data are compelling and clearly presented, and the interpretation is well-founded. Thank you for the opportunity to review this research. This is a very strong manuscript that should be published following minor corrections.

Throughout the manuscript, there is some lack of narrative and propulsive sentence structure. The data here are novel and very important to the current understanding of soil carbon cycling, especially in the “mineral-associated/heavy” fraction and the diverse cycling rates therein. If these data can be presented in a stronger, more assertive fashion, these data could support a strong narrative. I may sound like a broken record on this, but it is the primary thing that could be improved on in an otherwise very strong manuscript. If the writing is tightened up and a bit more forward-leaning, this could be a high-impact paper.

Something that could possibly strengthen the discussion throughout is how “bulk soil” is referred to. Since we’re discussing radiocarbon, the “bulk soil” 14C is the mean of all C atoms in the soil. If an extracted C fraction is enriched, than it is cycling faster than the average C atom, or the average of the bulk soil. Of course, this is not a normal distribution. But if an extracted C pool has a higher radiocarbon content, that pool cycles more quickly than the soil C on average. Perhaps I haven’t articulated it well here either, but the framing of these C pools and being faster or slower than the mean i.e. bulk soil could support the narrative.

Below are line-by-line comments:

L43-50: I think this paragraph could better introduce why you want to specifically refer to compounds. I think a couple more references here and more active justification would serve this paragraph well.

L58-60: Switching this sentence to more active voice would clarify it and more strongly identify the persistent research question.

L70: The sentence that starts with “Each of these…” could better introduce these compounds as biologically valuable C pools, both of which we might assume to be quite fast-cycling. I think this sentence and the one on line 73 could add to the story of C decomposition in soil; a competition between microbial decomposition and stabilization, and the processes and soil characteristics that may determine those outcomes. I know this paragraph ends with a sentence along these lines, so maybe this could be reshuffled to create more narrative.

L73: I think there is something missing from this sentence; that, or the “not only” can be removed.

L81: “can be” is repeated.

L81: While “wholistic” is correct, “holistic” is more commonly used. Your choice.

L83: “of” is missing between “understanding” and “carbon”

L91: This probably was supposed to be “coastal”.

L246, 252: Add a + before positive radiocarbon values or remove them elsewhere, in the interest of consistency.

L272: “each of which” may flow better than just “which” on its own

L279-281: This sentence is doesn’t pack the punch that the last sentence of this first discussion paragraph could have to propel the discussion. Tie it in with the data directly, either why you see this, or why you don’t. This information is available from the figures, but it should be clear whether the data is behaving as you’d expect or otherwise.

L283: Maybe personal preference, but prepositions at the end of sentences should be avoided, “from which they were extracted”.

L284-285: This sentence could be tidied up to be easier to read and more propulsive.

L288: Maybe add “preferentially” here, to highlight that they are microbially active because of their N content making them more appealing

292-307: This is well reasoned, well done.

319: This is an great description of what WEOC is

L323: Maybe add “mobile” or something similar to this list

L324: Could this also be that the FLF fraction is the remainder of more “recalcitrant” C that persists after the more bioavailable C is decomposed (and possibly shows up in WEOC and POC)?

L326-328: These first two sentences could be streamlined for readability.

330-331: Why could this be the case? Due to closer contact with microbes and dissolved C under lower water conditions? Of course, if water content decreases enough then this could de facto preserve C as it cannot be accessed by microbes. Does this also imply abiotic oxidation? This sentence could be strengthened with one or two references and more context for why this observation would be expected.

332-335: Great!

350-351: This is true, but slightly redundant. What may be more interesting is that the sand size fraction is older (in order to produce a mean value in the bulk soil, if bulk – silt – clay = sand). We could assume the silt+clay to be more microbially active due to higher C content etc, and thus have enriched 14C, but the sand story is also interesting.

354-360: This is the first time AI is discussed, which is not inherently a problem but it is slightly jarring since the interpretation of AI data hasn’t been established yet.

363-366: What do you think is the source of the older TLE that is implied by the more depleted 14C in bulk soil TLE compared with DF TLE?

Upon looking at the figure, this may have been written incorrectly. It appears that DF TLE is always more depleted than bulk soil TLE except at the surface. This makes much more sense. One would not expect a source of older TLE in the FLF of OLF fractions, e.g. Be sure to correct this in the text.

I think this is a crucial paragraph, and is important in pushing our understanding of diverse, multi-level pools and highlights the benefits of compound-specific 14C analysis. Make sure it comes across with a strong statement.

379: The comma after “these” can be removed.

387: Indicative of aromatics

393-394: Interesting that the petrogenic rocks are still relatively modern. Do you think this a mixture of rock and more modern sources? Or rock-derived C being incorporated the biologic C cycle on the rock surface? There is some evidence of this occurring.

412-414: Good explanation.

415-425: Discussion of methodology limitations is often lacking, and this serves as a nice comparison of methodologies.

Section 4.5: If there is a recommendation to be made about size vs density separation, I think this is a good place to put it. Obviously there are pros and cons to each, but don’t shy away from suggesting one is better for research question X vs Y.

436: Abbreviate to SOC.