The review and syntheses article titled "VOC emissions from soil cover in boreal and temperate natural ecosystems of the Northern Hemisphere" by Isidorov and Zaitsev is a comprehensive review covering several decades of resarch on this topic. It is extremely important work, which highlights the knowledge gaps on which research should focus and also will encourage model developers to consider processes that have been overlooked for too long in their calculations.

While the review is well written and very interesting to read, I have been wondering about its structure and the flow of information. I understand that it is no small task to organise the very large amount of information included in this review that is often all interconnected. Nevertheless, in its current form, it seems that readers will have to go back and forth unnecessarily between chapters. For this reason, even though I absolutely do recommend Biogeosciences to publish the review, I would like to see a minor revision of its structure. 

In my view, the chapters presented are not sufficiently clearly defined and overlap heavily. While it is a very interesting and comprehensive review to read, it was not clear to me what was the logic in the way the information is presented. More specifically, this is what I have been thinking:

- Do litter-decomposing microbes (chapter IV) consitute a biotic control of leaf litter decomposition? If so, why is it dicussed separately from chapter II?

- Every chapter seems to provide information about the qualitative composition of emissions, not only chapter III.

- Laboratory and field measurements (chapter V) seem to inform conclusions on the other topics as well.

Here are some suggestions on how the review could be presented with slightly different (sub)sections:

1) Introduction: It currently makes an excellent job to present the topic and its relevance. It also estbalish important definitions for the comprehension of the literature presented. Not much need for changes here.

2) Start the review with living soil cover (LSC), lines 370-398, which is similar to the very common emission studies from trees. Include here a comparison of the LSC's and trees' biomasses and their respective potential contributions to VOC emissions (both qualitative and quantitative information). Field and laboratory measurements on LSC should be mentioned here.

3) From there, move on to leaf litter decomposition, first describing abiotic processing, such as the very important work about the influence of UV radiation on the emissions and other environmental factors (e.g. temperature, soil humidity). Once more, qualitative and quantitative information from both field work and laboratory experiments supporting it can be included here.

4) Continue with the even more complex biotic processes from larger ones (e.g. herbivory) to smaller ones (e.g. fungi, microbes) with the same ideas as for the previous chapter: include qualitative and quantitative results from relevant field work and laboratory experiments.

5) Conclusions: Just like the introduction, the conclusions are in their current form well written and highlight the knowledge gaps that should be focused on (e.g. processes, ecosystems) and the fact that VOC consumption by the same LSC and litter has not been investigated sufficiently.

In a similar fashion, the current supplementary is simply one section covering "Litter and LSC biomass distribution". Why not make two distinct sections (for isntance LSC first, then litter production) and link them the the suggested new (sub)sections, highlighting that, in combination with information from the main text, this is a good starting point for the geophysical modelling community.

Minor technical comment:

- While the Introduction chapter is numbered with the arabic number 1, other chapters use roman numbers. This should be fixed for consistency.

Isidorov and Zaitsev review VOC emissions from leaf litter. The topic is timely and the review is comprehensive. Perhaps too comprehensive; the manuscript would benefit from more efficient text in places and removing unnecessary sections, but I admit that I like the more conversational tone. I recommend it be accepted with minor revisions noted below.

96: is this annual production? 

115: can it be said with certainty what is and is not common in this case?

121 and elsewhere: the dot to signify multiplication is not needed in my opinion.

159: tropical forests should be mentioned as well, if not only for completeness e.g. https://www.nature.com/articles/s41467-018-04658-y

167: this paragraph is unnecessary. There's enough justification of litter mass and perhaps these points can be integrated elsewhere.

205: is Zimmer et al. the relevant reference here?

211: Trowbridge et al. covered this topic for the case of soil fungi: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JG005479

239: does photodegredation result in VOC flux from litter? The photodegredation wection was a bit long and speculative and distracted from the main theme. It would be better shortened.

for section II, subsections IIa and IIb for abiotic then biotic controls could help the reader navigate all of this material.

445 and a number of paragraphs afterward focus mostly on decomposition over time, which is interesting of course but it is unclear how this entire section contributes to a review of VOC emissions which remain largely uninvestigated as noted on line 485. Shortening this section to focus briefly on microbial changes during the decomposition process would help focus on the topic of the review.

Section V is great and makes key points about global representativeness.

The manuscript by Isidorov and Zaitsev aims to provide an overview of what is known about the contribution of litter and soil cover to ecosystem bVOC fluxes and emphasize the need to study these sources and their controls so to improve predictive models for atmospheric chemistry and our understanding of species interactions mediated by these emissions. The review focuses on northern, boreal forests and is comprehensive in scope and very-well cited. My main issues have to do with structure, synthesis, and the impact of the tables and figures. First, the way the text is structured into sections/sub-sections and the lack of clear topic sentences made it challenging to follow the logic and key points of the paper. There are a number of areas where the text can be re-worked for brevity and more streamlined by providing only important details or results of studies without so much extraneous details. At times, the text becomes quite “listy” and references are related, but not well-integrated in making an overarching statement or generalizable comment. This is related to my other main concern, which was the lack of substantial synthesis despite the incredible amount of literature cited. What can we say with some certainty that might hold true across biomes in these regions and how much is site, species, and climate specific? An outline of the major knowledge gaps should be explicit at the end, but it might also be helpful to touch on these at the end of each section, presenting what we do know followed immediately with what limitations exist and how they might be addressed. Finally, a number of the tables and figures appear to simply re-package data from a single paper and it’s unclear why those studies were chosen and thus, the impact of these data displays are minimal.

Specific comments:

L18: Repetitive with L11, consider deleting

L40: Delete “to” after the comma

L40-44: Consider making this one topic sentence by combining and streamlining. E.g. Terrestrial living vegetation is the main source of atmospheric VOCs that significantly affect chemical processes in the boundary layer. These emissions have garnered considerable attention due to the majority of them being highly reactive, and thus, impacting atmospheric chemistry…”

L80: It’s unclear why methyl chloride is emphasized here, instead, you could use the Bahlmann et al. citation to support the statement that significant biological sources remain elusive.

L84: What does the acronym TOHRE stand for?

L87: identify and quantitatively characterize is repetitive, could simply say “identify and quantify.” Also, it’s unclear how we can characterize unknown reactive VOCs…this is a point that could be elaborated upon, and other well-known processes and effects (like the production and destruction of ozone, etc.) could be more streamlined and concise.

L108: This sentence is unclear; the concentration of many secondary metabolites is significantly reduced compared to what? Fresh and living leaves?

L134: While interesting, the text in this paragraph can be significantly reduced and streamlined. Also, it is unclear how the natural source of reactive VOCs has remained unaccounted for and unquantified if the Zimmerman et al. (1978) paper reported the magnitude of VOCs from leaf litter. Is it that this has remained relatively unexplored since? Please clarify.

L154: Specify “these important components” as leaf litter and soil cover and consider moving the definitions of these terms (L172-186) up after L154 and end the paragraph with L154-158.

L187-189. This sentence is quite long and the clause “available information regarding the emission rates of separate VOCs” seems out of place and not well integrated. Consider revising for brevity/clarity.

L217: Change “they” to “that”

L233: Please provide a citation for this statement.

L226: The relative proportion or importance. It would make sense that a lot or most of these processes are occurring all the time, but perhaps at different rates and interacting/overlapping in various ways depending on prevailing environmental conditions. This makes it sounds like processes like leaching appear and disappear, where in fact there is always that potential, but it becomes more and less important during certain times of the year and under specific conditions. Also, this topic sentence is inconsistent with the content. The biotic processes are mentioned, but this paragraph only focuses on abiotic factors. L236 is vague and not effective in supporting the rest of the text in this section.

L239: More pronounced distinction of what? Please specify and construct a more robust topic sentence.

L254: What does “with the influence of experimental conditions determined” really mean? Please clarify. Did they test in the absence of O2 and then in humid air? Also, L257-261 seem irrelevant considering the initial speculation was made on very limited data.

L283-286: Please provide a citation for this and it also seems like a bit of a no sequitur between paragraphs.

L331: Please write out LSC in words to remind reader of the acronym.

L408: This entire paragraph is about one study, and instead, could be briefly summarized to follow up on what was stated on L405, emphasizing that VOCs after sufficient decomposition by microbes were likely the products of their metabolism and point out these specific classes et.g., C3-C8 carbonyl compounds, lower alcohols, etc.

L445: the microbial succession section can be simplified and some information from the preceding paragraph incorporated within (instead of it standing alone with lots of extraneous information). Also, L459 is vague and uninformative as no other details are provided. The following sentence is equally confusing in terms of whether these patterns are generalizable or are site/ecotype specific.

L470: I think this sentence is missing something at the end (a verb?).

L630: There are studies on soil cover and below canopy VOCs from deciduous US forests that might be relevant here (See Stoy et al. and Trowbridge et al. 2021).

I’m not quite sure I see the relevance of Table 1.

The inclusion of Tables 7 and 8 are also confusing: Why list all the compound emission presented from one paper? Might this be a copyright issue? The same can be said for Figures 1 and 2 (reproduced data from the author’s 2010 paper). Why are these being specifically highlighted when the review should be synthesizing and compiling data from many sources.