General comment

N2O is one of the most important greenhouse gases, and the ocean plays an important role it the global budget. The production of this gas in the water column will be boosted at low oxygen concentration but consumed when the oxygen concentration drop to certain criteria. Therefore the water with low oxygen concentration is generally important source for N2O. Base on the information provided by the authors. The Baltic Sea is good spot for N2O biogeochemical cycle study in the low oxygen concentration, and the authors use state-of-the-art technique such as isotopemer analysis to reveal the possible N2O production, which will of course promote our understanding of the N2O dynamics in this study region. Unfortunately, I think there are some major issues in this manuscript, and I think the author should make more effort to revise it before it is acceptable for the Journal. One of my biggest concern is that the authors divide the water column in two types of water, oxic and suboxic, and the suboxic water also divided in two groups bases on whether or not there is sulfide detectable. However, I can’t not tell is this a reasonable way since the there are very limit information for the dissolve oxygen distribution patterns in the study section, only section with small figure in figure 2a, and a station profile figure 2b. However, from Figure 2 and Figure 3, I suppose that there may be annoxic or oxygen-deficient layer presented in the study region. So, the concentration of oxygen should be clearly displayed, and better categorized, this will provide some very important information for the readers. Secondly, I think the manuscript lacks figure of hydrographic parameters such as temperature and salinity, since the authors mentioned the hydrographic process such as MBI, and it may provide some important information such as the the authors mentioned in the manuscript, like, how the oxygen “intrude” the region. Finally, as a whole, I don’t think that the authors give the readers a very clear and solid conclusion, there may be many reasons lead to this situation, including poor preparing of the figure, which in turn lead to incomplete description of phenomenons, and skill of writing, which stop the authors from well describe the phenomenon, and so on. Hence, I think that the authors should carefully reanalysis the dataset, redraw the figures and reorganized the logic and language of this manuscript. A major revision is needed before the paper is suitable for publication by the journal.

Specific comment

Line 55, signature of the nitrate or nitrite... I think reference is needed here.

Line 69 5-15nM..., if this is general range, I suggest a wilder range since the polar ocean may have higher N2O concentration

Line 75-81 for the first question, I feel there is inconsistent with last paragraph. In the last paragraph, it seems there are colleagues concluded that nitrification is the predominated N2O source, whereas the authors want to answer the dominant pathways, any new insight we can obtain in this study?

Line 82 this Manuscript is not a methodology paper, so I think this question is put forward inappropriate here, if the authors is not confident to use the method here, they should carry out a study to estimate of effectiveness of this method in advance.

Line 130 Is this means that the sample is bubble free before capping, generally, when butyl stoppers and aluminium crimps was capped, it will easily introduce bubble, so i guess bubble free should be make sure after capping?

Figure 2.  as mentioned before, hydrographic figures should be provided here, and the oxygen profile of each stations should be added to figure 3

Line 174 O2 between 9-2umol? Typo?

Figure 3, the scale of x y axis of each figure should agree with each other, so it is easier for the reader to compare them.

Line 229 “Significant” is a word used only after statistic data analysis performed, it generally should followed with a criteria such as p value

Line 264 I think the first sentence should be rewritten

Line 396 the conclusion should be rewritten, there is no solid conclusion in current format, for example, the sentence  “our results demonstrated the spatial variability...” what kind of variability? Moreover, half of the paragraph is about future work, generally, it will only be one or two sentence for future work

This manuscript by Bardhan et al. reports N2O concentration and multi-isotope abundance from the central Baltic Sea. They found that, in oxic waters, N2O accumulates with isotopic signatures indicating ammonia oxidation-derived N2O source; in suboxic or anoxic waters, they found elevated isotopes signatures and attributed the N2O processes (small concentration) to consumption by denitrification or even chemo-denitrification. Overall, this publication is easy to follow. However, as its current form, this study does not deliver a strong enough conclusion.

• With samples during one cruise, the study heavily relies on the fragmented isotope results (some are not available due to low concentration) for discussion of N2O processes. Little information from other parameters are implemented for supporting such explanations, including salinity, temperature and even DOC/DOM contents. In addition, more side-by-side comparison with other N2O isotope studies and summary in figures/tables may be necessary.
• The discussion was formulated in a simple and thus uncertain way. I am not totally sure hydrogen sulfide detection is enough for identifying specific regions of biogeochemical cycling in the ocean water. Further, the interpretation of the isotope signatures need to be revised: instead of calculating kinetic isotope effects, it is highly important to consider both N2O production and consumption at every station. Even for large production of N2O (net concentration), consumption is always happening.

Line 12: “δ values” should not be written as “d”.

Line 60-64: Strong reduction of N₂O will also result in enrichment of SP.

Line 228-230: Keeling plot approach is based on the assumption of simple mixing between source and background, which is clearly not the case here. The production and consumption processes may be quite different from station to station and long the vertical profiles.

Line 379-395: Regarding NosZ I and II genes and the likely regulatory mechanism on N2O is beyond the scope of the study and the collect observation evidence. I suggest to leave them out.