I wish to thank the authors for their replies and making an updated manuscript. I have taken some time to go over the discussions and revisions and find my self torn on the suitability of this paper. I welcome the general focus and analysis in the context of GHG emissions in particular, and the sections on retention time and surface-groundwater connectivity are relevant and topical. Therefore, I think the topic and content could ultimately be suitable for publication in HESS, however currently the writing lacks focus, is repetitive, and in general the paper currently lacks in-depth critical review and synthesis of the published literature in this area – the latter is an essential element for a HESS review article. For these reasons, the paper is unsuitable in its present form and needs a major overhaul. Below are some high level points that need to be addressed:

• Lack of focus – Both the abstract and introduction (and elsewhere) contain significant “text book” information without a clear narrative or focus. Some points to consider:
o The abstract doesn't state the aim of the paper until 2/3 of the way down (the first 2/3 is all background info), and the aim is to provide a “review and discussion of C and N dynamics”. No clear findings are presented except that at the end of the abstract it is stated that further examination of N transformations are necessary. This needs to be sharpened.
o The introduction highlights the motivation (that CW performance is unclear and the emission of GHG is unclear) which is good, but no clear aim is listed and no comment about the rationale for the approach adopted during the review is made and it is left to the reader to guess what is the focus is. An aim (or multiple aims) is still essential for a critical review, eg. to understand the range of variability in X, or the dominant processes controlling Y, or to identify knowledge gaps.
o Following from this, it is not clear if the authors are focusing entirely on wastewater wetlands or other types (eg. stormwater treatment systems?). Currently the abstract hints the paper is considering wastewater systems, but the introduction is more ambiguous mentioning wastewater and stormwater systems and others (page 3 - line 77) with no further clarification. A comment in the replies seems to indicate the focus is broadly covering all wetlands. If so, do CW systems perform equally well at removing N under these different contexts? Are the some generalizable findings about how different types work? Or if the review is focusing on one type then the title should reflect that.
o How does this review build on or differ from other review works on CW’s that have been made?
o There is a partial treatment of C in the paper, relevant to GHG emissions. In response to a reviewer comment, C was removed from the title, but I am not sure the solution of removing C from the title addresses the underlying problem – the paper superficially treats C cycling. If GHG’s are a core focus of the paper, then C needs to be treated equally with N.
o A symptom of the above issues is seen in the repetition of ideas throughout the paper. This is as bad as including two exactly identical statements in one paragraph (mid way through page 3), but also more generally concepts of isotopes and pollutant swapping are repeated several times in different sections. The section in the introduction from lines 104-150 is essentially going into the detail that is later reviewed in the dedicated sections.

• Need for more depth and detail – There are quite a few superficial statements that could benefit from being more precise or more detailed, and also several areas come to mind where I was looking for the review to go into … some examples:
o “the efficiency of OM removal is often satisfactory, but N removal is generally limited“ – OM is also consisting of N as DON/PON so this sort of statement is ambiguous. Presumably you are referring the fact that TN goes down but DIN does not. If so, then try to be specific and say that the wetlands are removing PON and not removing NO3/NH4, and provide an explanation or interpretation, eg sedimentation of particles is occurring but unsuitable conditions and/or lack of retention time for denitrification to occur etc (see comment on synthesis below)
o Nr is introduced but not explained or defined – is it DIN? DON is reactive too.
o Little treatment is given with respect to plant types;
o Limited discussion on measuring surface-atmosphere fluxes of GHG directly (eg. via eddy correlation or chamber work) despite GHG being a focus;
o Significance of different types of soils in the soil section;
o Description of modelling needs to undertake nutrient budgets;
o Significance of management actions on GHG;
o C:N ratios and organic matter quality.
o How do we go about scaling GHG emissions up to regional / global scales
o Role of temperature
o etc

• Insufficient synthesis – As mentioned by reviewer 2, the paper needs to be more than “he found this, she found that…” (ie ‘PhD introduction’) style of review. In a response to reviewer 2 some changes have been made but this is insufficient in my view - there should be some clear “take home messages” emerging from the different sections of the analysis that readers can easily walk away with. The authors could improve the paper in this regard by critically synthesizing the results from the literature in the form of tables and graphs and then undertaking analysis of this information that has been collated to compute means and standard deviations (e.g., some suggestions: mean TN and Nr reduction efficiency categorised for different wetland types/sizes, range of N2O efflux rates reported, denitrification rates as a function of retention time etc) of what has been reported in different contexts (surface/subsurface; wastewater/stormwater). Ideally the product of the analysis should be a refined conceptual model, with clear statements of specific experimental work (or modelling work) that needs to be undertaken (beyond simple statements like “further studies are required on XXXX general area”). Finally, the discussion should talk about how we could engineer the systems to improve efficiency, based on the learnings from the present analysis.

I do think this is possible but I do acknowledge it will require some more substantial effort. Given the significance of these recommended changes, I would expect a major revision of the paper would be required that would then be subject to a further round of review.

Dear Authors,

First my apologies for the delay in getting the reviewers comments in. I now have received two new sets of comments, which are copied below. In summary Rev 1 rated the revised manuscript as poor in terms of significance and quality, and outlined several reasons for this. Rev 2 (who had reviewed the earlier version) is more positive and saw the improvements, but also still raised concerns about the quality of the presentation and readability.

For the paper to be published it is important that the concerns raised in the Rev1 comments below are adequately addressed. If you feel this is too onerous then we can cancel the review process so you can resubmit elsewhere (if the review process is cancelled in this way we can arrange credit of your fee for your next submission). However, I would hope that you are willing to undertake these revisions and further develop the paper to ensure the review fully covers the state of the art literature.

REVIEWER 1:
Overall, I think the review has been much improved after the previous reviewer comments. However, I think there and two key problems with this review, particularly in relation to the nitrogen cycle.
First, it does not really synthesise our understanding much beyond the classic texts of Kadlec and Wallace (Treatment Wetlands) and Reddy and DeLaune (Biogeochemistry of Wetlands), which are not even cited! I think the article should be more upfront about the existence of these works and specifically address how this review synthesizes the state of play beyond these texts. For example the discussion of HLR and HRT on nitrogen removal from lines 200 – 207 is well covered in Kadlec and Wallace.
Second, the article seems to have missed many recent relevant articles in the field, and many statements are vague, incorrect and not referenced. A few examples of this are given below.
A key open question is the relative amount of nitrogen denitrified compared to that assimilated by plants. A recent study has shown that the amount of nitrogen that is lost through denitrification relative to assimilation depends strongly on nitrogen loading
Payne, E. G. I. and others 2014. Biotic assimilation or denitrification? The division of nitrogen between temporary storage and permanent removal in stormwater biofiltration systems. Plos One 9: e90890.
Payne et al have covered much of the discussion on nitrogen cycling processes in a review in biofilters (effectively ephemeral vertical flow wetlands).
Payne, E. G. I., T. D. Fletcher, P. L. M. Cook, B. E. Hatt, and A. Deletic. 2014. Processes and drivers of nitrogen removal in water biofiltration. Critical Reviews in Environmental Science and Technology 44: 796-846.
In my view this previously published review addresses some of the key questions on nitrogen cycling posed in this manuscript, particularly on the subject of the effect of plant species on nutrient removal which the authors claim is not well covered in the literature (line 170 -).
Some of the synthesis on nitrogen cycling is vague and is not resolved for example the discussion on the effect of C:N ratio on denitrification and DNRA lines 188 - 191
Lines 284. The idea of anoxic microsites is one that is used a lot with little evidence. The work of Minett et al offers some insights based on actual measurements within the rhizosphere.
Minett, D., P. L. M. Cook, A. Kessler, and T. R. Cavagnaro. 2013. Macrophyte roots alter the spatiotemporal dynamics of sediment oxygen consumption. Ecol. Eng. 58: 414-422.
Line 291 An important statement with no reference
Line 301 No reference is provided, I think the general paradigm is that DNRA is favoured at high organic matter loading or highly reducing conditions.

REVIEWER 2:
I have been reading over this review paper. It has undergone a very major improvement since my first read. And I very much like their new figure, which is a great summary of what the different wetland elements do to the different nutrient species. I am also impressed by their tables showing the range of emission rates for the different nutrients. Just the type of tables we frequently need and bemoan the fact no-one has compiled.

Overall I believe it is a very useful paper, and would be good for HESS to publish it.

It still requires a science editor to go through it. Still quite a few awkward sentences and phrases. Much improved from the last read, but still enough in there to be jarring for the reader. Mistakes with grammar, tenses etc.

So I guess I would say "very good job with the improvements", but corrections required, specifically with the English.

Dear Authors

Thank you for the revised version and reply to the previous reviewer comments. I have spent a while reading this revised version, and feel this is inching closer to being suitable for publication but still requires some careful editing. I have not sent this for re-review but rather provided some review myself to help clarify the required changes to the text of the paper. I realise this is frustrating given the effort you have put in to date, however, HESS is a high profile journal in the hydrological sciences and we strive to ensure the editorial standards of all accepted papers are of the highest quality.

I have outlined some specific points below and ask that you please take a careful look in all sections of the paper with these issues in mind.

Kind Regards
Matt Hipsey



Specific comments:

The new sentence at the end of the abstract is confusing to me. Pollution swapping is not defined (can this be done after first sentence when you mention transformation to other reactive forms?). Also, the qualifying statement “While nutrient cycles have been …” is better for the intro as opposed to the abstract.
Nonetheless, the sentence I think should be reworded as “… in situ in
transformation and fate of the transformation products …” is not clear.

The changes to paragraph 1 (in response to reviewer 1) I also feel could be reworded. To support the statement that “less is known about constructed wetlands” a general text book on constructed wetlands is cited. Similarly the Kadlec and Wallace book is about treatment wetlands which are generally constructed wetlands and certainly this is a focus of the book. It is now cited when you make the point about general books on wetlands. Therefore, overall I find the assertion that there are no books on constructed wetlands (and that “…It emerges that we may know quite a number of aspects related to wetlands but not so much about constructed wetlands…”) to be highly arguable and this section could be more accurate in setting up the motivation for this study. I also think the wording of the last sentence here is quite a negative assessment saying that studies have not delineated pathways and not adopted appropriate designs – could this be qualified to say “rarely quantified all relevant pathways”, for example? I think the first two sentences of paragraph 2 are good to make clear the current deficiency, therefore requiring this study.

As I interpreted it, the main reason reviewer 1 mentioned the text books was because of “general” content appearing in later sections.
Eg. “Soils with high permeability enhance downward nutrient movement to groundwater. High cation exchange materials in soil enhance NH4+ fixation by the soil matrix.”
“soils of CWs represent organic C and Nr-rich systems, where the products of the continuously occurring biogeochemical processes can be transported to fresh waters and to the atmosphere”.
“In low redox conditions with limited DO, methanogens can consume DOC and thus it is conducive to CH4 production”
There are lots of these well known facts throughout the sections that may be useful for introducing a sub-section and setting context, but currently appear as findings of the review.

In the last example, the 3 sentences relating to methane production could be more concise and avoid the general statement by rewording ...:

“Carbon mineralization in sediments depends on the redox chemistry of soil, availability and quality of C (labile or recalcitrant) and temperature. In CWs, C cycling is very complex due to the changes in redox chemistry, which regulates production and consumption of CO2 and CH4 (Brix et al., 2001). In low redox conditions with limited DO, methanogens can consume DOC and thus it is conducive to CH4 production. “

may be more concise/specific as:

“The rate of carbon mineralization in CW sediments depends on the redox chemistry of soil, the bio-availability of organic C and temperature. In particular, areas of sediment subjected to prolonged low redox conditions (e.g, -XXXXmV) are conducive to methanogens and rates of CH4 exceeding YYYYmg/m3/day (Brix et al., 2001), but this is highly variable depending on C:N ratio of the influent water and wetland seasonality. For example … “

This sort of sentence would allow you to be more concise further down this paragraph, where there are awkward sentences (Line 350-357) that don't follow a clear logic. Currently the sentence “Only a limited number of studies have considered CH4 and CO2 efflux from CWs (e.g. Mander et al., 2008)” is in the middle of a section about temperature and seems out of place.

I have noticed many more examples like these and have indicated this as I believe this is what Reviewer 1 was referring to but has not been addressed in this revision. Most I feel don't change the substance of the paper and can easily be dealt with by careful editing.

The other issue that needs careful attention is the many assertions. For example:

“ A fluctuating water table in CWs has significant impacts on GHG dynamics. Pulsing hydrologic regimes decreases CH4 but increases N2O emissions. In aerobic and anaerobic conditions caused by pulsing hydrology, incomplete nitrification and denitrification increase N2O emissions”

This is interesting but is not supported with a citation (as a side note it is also not clear to me how this is related to aquatic plants, which is the topic of the paragraph).

In the next section:
“Surface emissions of GHG from CWs are well recognised and have been commonly measured by chamber methods. The GHG produced in CWs can also be transported to the groundwater with the percolating water and emitted to the
atmosphere upon discharge to surface waters.”

Similarly requires substantiation by referencing appropriate studies, or changing the wording to indicate it is your suggestion. Also, I notice that in the early section there was only a limited number of studies (Mander et al 2008), but now it is stated that it is well recognized - so the sections seem to contradict each other.

Dear Authors
The revised submission looks much improved and now reads very well and I believe will make a useful contribution to the literature. I just noticed a few places that need a little more proof reading which I encourage you to do during the paper finalisation process. Thank you for your patience wit the editorial process and I look forward to seeing your paper finalised online.
Regards
Matt Hipsey