This is a nice paper about microbial C, N and P cycling in the litter layer of forest and tundra ecosystems. I have two suggestions and some rather minor comments.

Concerning Fig. 8, I suggest adding a second panel that shows the percentage of respired 13C (derived from glucose-6-phosphate). Based on this figure, the authors could discuss the loss of C in comparison to the loss of P. 

It seems that the authors conclude that the main driver of the differences in microbial element cycling between forest and tundra is the difference in litter stoichiometry between these two ecosystems. This raises further questions. For example, about how the plants are able to acquire higher nutrient contents in the forest than in the tundra, and specifically how the forests are able to build up a larger N stock than the tundra ecosystems. I think that the (formation of the) larger N stock in the forest ecosystems is very relevant for the discussion of microbial N cycling in these ecosystems. I would like to read some lines of text about this, including a short outlook about how this might change in response to global warming and what might happen with these element ratios when the treelines shift. 

Further comments

L.50-60 the transport of nutrients (nitrogen and phosphorus) into decomposing litter by fungi is also be very important during the initial decomposition stages. 

L. 59 Whether “overflow respiration“ really occurs in soil microorganisms is debatable. It might be a process that can only be observed under extreme conditions in the lab. 

L. 71 and 81 It is not clear what exactly “litter quality” is. This term requires an explanation.

L. 95 In hypothesis 1, the term “litter quality” requires an explanation.

L. 98 What is “microbial functioning”? This sounds a little awkward. I guess the authors mean microbial ecophysiology or something related 

L. 105 see above

Section 2.1 It would be helpful to see a map and a few photos of the sites. 

L. 218/219 Was the glucose-6-phosphate uniformly labelled or was it only labelled in one C position?

L. 125 How was this done randomly. Please explain. 

Fig. 1 The label on the y-axis is not clear. This should be improved and further explained in the caption.

Fig. 3 and corresponding text. Please indicate whether these ratios are based on mass or number of moles. It is confusing that in some parts of the text, it is indicated that these are molar ratios while in other parts the authors simply refer to ratios. 

Fig. 8 I suggest to add a second panel to the figure that shows the percentage of respired 13C (derived from glucose-6-phosphate). This would allow the authors to discuss loss of C in comparison to loss of P. 

Fig. 450/451 This sentence is not entirely clear. Specifically it is not clear what exactly “decreasing C:N:P ratios” refers to and what “effects” refers to. 

L. 455 replace “mineral” by “inorganic”

L. 470 Microbial mineralization of what? I guess glucose-6-phosphate but it might be good to clarify this. 

L. 474 It seems that rather the lab studies are biased (or artificial) because they exclude nutrient import and export. 

L. 484 is mineralized (not becomes mineralized)

L. 502 Please replace “very likely” by “which might have been” (since this is rather speculative).

L. 505 see above. What is “microbial functioning”?

L. 552 remove “in the tundra”.

l. 556 This is not correct. While the litter has a lower nutrient content in the tundra than in the forest, 100% of the nutrients are released in both ecosystems after several decades

L. 572 remove “worldwide”