The authors present methane flux measurements from a bison herd in North America measured with the eddy covariance technique. While several studies in the past – none however for wild ruminants – have used this technique, these have also highlighted potential caveats when using the technique at hand (presence and frequency of animals within the footprint, ability to detect points sources of GHG emissions, disentangling the soil from the animal signal etc.). At the same time the technique may be “easier and less invasive” than complex tracer techniques (SF6) or animal respiration chambers. The latter are still seen as the gold standard to estimate methane emissions from ruminants and yet these are less likely to be applicable to rangelands as well as to wild animals.

Both reviewers think the manuscript could be a valuable contribution to Biogeosciences. At the same time, and particularly reviewer 1 remains critical for several reasons: 1) lack of providing sufficient metadata ie feeds provided to bison, bison status (weight, age etc), 2) the choice of the footprint model and 3) data cleaning procedures during pre-processing. As many reviewers do, reviewer 1 also requests - if possible - to add more data as the current observation window is relatively small and covers a winter period only. Reviewer 2, on the other hand is less critical while also requesting for more information.

The authors respond in detail on how to address the majority of the issues raised and thus I want to encourage them to submit a revised version of the manuscript which will then be reassessed by the reviewer before a final decision can be made.

I am looking forward to receiving your contribution. With kind regards and stay healthy

Lutz Merbold

Dear Paul Stoy et al.

your revised manuscript "Methane efflux from an American bison herd" has been seen by two independent reviewers. Both reviewers think that the study is innovative, interesting and valuable despite the relatively short time period of observations.
Yet, both reviewers are also critical, with reviewer 1 questioning the approach taken to assess spatial uncertainty and reviewer 2 being particularly critical about the use of the two footprint models and the assumption on where methane is emitted from ie difference in height between snow surface vs bison head resulting in a major revisions suggestion. Given the additional comments made, the paper is not yet ready for publication.
While this is the second round of reviews, both reviewers as well as myself share the opinion of this being a valuable study, Thus, I ask you to address the issues raised by clarifying and possible simplifying the manuscript as well as by removing the speculative bits.

with kind regards

Lutz Merbold

Dear Paul Stoy et al.,

I have now received the third review for your revised manuscript on methane emissions from bison. While the reviewer is not completely convinced by the responses you have given in your previous response I am confident the currently remaining issues can be clarified, particularly in terms of explaining which errors you are showing based on how many data points and how these were derived. Similarly, I suggest to address the importance of specific biological parameters on CH4 emissions from ruminants (ie. diet quality not just quantity). Of course, there aren't many studies on bison available and this seems to be a scoping study to extend future measurements, I would like to point towards comparing your results to either "similar" species such as buffalo or cattle that are grazed even during colder periods in the year. You may further think about stressing more strongly that your system is a mix between a grazed and and a feedlot systems, given the supplementation with hay.
With this, I am looking forward to receiving your revised manuscript.

with kind regards

Lutz Merbold

Dear Paul Stoy and co-authors,

thank you for providing the detailed responses to the remaining concerns raised by the reviewers during the last round of revisions. Particularly the rewritten section on uncertainty analysis as well as the section where you put the results from bison into context is most beneficial for the future readers of this manuscript. With this, I think you have covered all issues raised and addressed these thoroughly and sufficiently. Over the three iterations, the positive manuscript evaluation provided by the reviewers further underlines the manuscript improvement.
Thus, I am glad to inform you about the acceptance of your manuscript and wish you all a relaxed and most importantly healthy festive season.

with kind regards
Lutz Merbold