Dear Dr Stefansdottir, dear Bjarni,

Thanks you for uploading a revised manuscript and brief response to the referees' comments. I could not see a response to the comments I had made in my e-mail to you on the 24th July (for reference again below). Please add a response to these points and make appropriate changes in the manuscript.

With kind regards,
Jens-Arne Subke



From: Jens-Arne Subke
Sent: 24 July 2014 15:07
To: 'bjarni@lbhi.is'; 'bekind@simnet.is'
Subject: Peer review completion bg-2014-127

Dear Dr Stefansdottir, dear Bjarni,

You will have received an e-mail from the editorial system to inform you of the end of peer review following the reviews received. Both reports are very short, and broadly supportive. Please address the few points made, e.g. by discussing the likely role of arbuscular mycorrhizas in your system.

In addition to the two reports, I have further points and one clarification in particular that I would like you to address, as it could impact on the interpretation of your results.

My main question concerns the calculation of N on the basis of density and dune area, as the main difference in dunes of different age is the size, so an increase in DON and DOC per area would result simply from an increase in sediment depth. You state that the tephra sands have a DON content of less than 0.01%. Looking at you results in table 1, it would follow that given a volume of 176 m3, and a bulk density of 1.16 (using dune #7 as an example), we would expect a dune mass of about 200 t, and hence a DON amount of around 20.4 kg. So for a dune area of 154 m2, a background DON concentration of around 132 g DON m-2 could be expected. I realise that the estimate of 0.01% is a coarse upper value, but my point is that the accumulation of N would result simply from substrate accretion and increased dune height, when expressing DOC and DON densities on an area basis, so the estimate of DOC and DON accumulation due to plant growth should take this into account. Can you propose how your analysis can adjust for this effect, and indicate whether you think your conclusions regarding N transfer from outside areas remains valid?

Some detailed comments:
Abstract: Rather than stating first that Surtsey is a good place to work, you should focus on the scientific background of your study. There is no indication of why it is worth knowing the spatial and temporal variation in organic matter, N accumulation etc. in dune systems. I appreciate that it is a good place to do science and address ecosystem development issues, but your motivation for the paper should be scientific, and this should be stated first. Please re-work the opening of the abstract to reflect this.
6596, l. 1: Delete “then”.
6596, l. 27: You say you express results on an area basis, but give per-volume units; next line states per-volume, but has units or per-mass…
6597, l. 22: add “an” before “idea”
6601, l.18/19: Better: “These roots are most likely to originate from…”
Fig. 3: Please indicate whether soil depth in control areas outside dunes was also 75 cm.

Dear Dr Stefnsdottir, dear Bjarni,

Many thanks for addressing my additional points in the latest revision. Regarding the ectomycorrhizal comment by one referee, I had hoped that you might want to explore some previous work on the role of ECM in dune C accumulation, but that is not central to your paper, so ok. I am happy for this manuscript to now be published in Biogeosciences, subject to very minor edits:

Page 9, lines 24/25: Please delete the rhetorical question. The argument follows well also without it, no need to "guide" the reader in this way. I find it can sound patronising to use questions like this in papers.

In Figure 3, please put "Leymus" in the x-axis title in italics. I was not sure why there was a break in the y-axis in the top panel, and scaling was to 800. Can this be removed?

Many thanks, and best regards,
Jens.