Stable isotopes of nitrate reveal different nitrogen processing mechanisms in streams across a land use gradient during wet and dry periods

Wong, Wei Wen; Pottage, Jesse; Warry, Fiona Y.; Reich, Paul; Roberts, Keryn L.; Grace, Michael R.; Cook, Perran L. M.

doi:https://doi.org/10.5194/bg-15-3953-2018

Articles | Volume 15, issue 13

https://doi.org/10.5194/bg-15-3953-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-15-3953-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 13

Research article

|

02 Jul 2018

Research article |

| 02 Jul 2018

Stable isotopes of nitrate reveal different nitrogen processing mechanisms in streams across a land use gradient during wet and dry periods

Wei Wen Wong, Jesse Pottage, Fiona Y. Warry, Paul Reich, Keryn L. Roberts, Michael R. Grace, and Perran L. M. Cook

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Final revised paper (published on 02 Jul 2018)
Supplement to the final revised paper
Preprint (discussion started on 04 Oct 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of bg-2017-240', Anonymous Referee #2, 02 Nov 2017
- AC1: 'Response to reviewer 2', Wei Wen Wong, 13 Nov 2017
RC2: 'Reviewer Comment: Wong et al', Anonymous Referee #3, 15 Feb 2018
- AC2: 'Response to reviewer 3', Wei Wen Wong, 16 Mar 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (20 Mar 2018) by Manmohan Sarin

AR by Wei Wen Wong on behalf of the Authors (23 Mar 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Mar 2018) by Manmohan Sarin

RR by Anonymous Referee #2 (16 Apr 2018)

RR by Anonymous Referee #3 (18 Apr 2018)

Suggestions for revision or reasons for rejection

The revised manuscript by Wong and colleagues is much improved; laying out their arguments more clearly for the none expert reader and I thank the authors for taking the time to address both mine and the other reviewers concerns. I have a few relatively minor suggestions prior to publication, mainly asking for a number of newly added sentences to be clarified.

Scetion 2.1 Study area: please refer to Figure S2 throughout this section, to support your description.

Page 5, Line 22: Please provide the precision / reproducibility of the elemental analyzer measurements.

Page 7, Lines 3 to 6: ‘minimal isotopic fractionation of both processes’ is this really the case for nitrification. Based on the literature ammonia oxidation / nitrification can be associated with a large isotope effect (e.g. Mariotti et al, 1981; Casciotti et al, 2003 and Santoro and Casciotti, 2011). Surely it is the tight coupling of processes that is resulting in no isotope effect being expressed and hence the 15N of organic matter / ammonium and nitrate being similar.

Page 7, Lines 12 to 15: Here the authors are discussing how nitrate produced from nitrification can be fractionated by subsequent processes such as denitrification and this is of course correct. However, I am confused how this fits into this section about calculating the 18O of newly produced nitrate from nitrification. Put your use of equation one in context, tight coupling of the steps of nitrification, little exchange or nitrite accumulation and why not to use the equation presented by Buchwald and Casciotti, 2010. Most of these points the authors already highlight, but the text needs to reformulated to make this clearer, and I would suggest to remove or clarify the discussion of denitrification / a subsequent fractionation process.

Page 10, Lines 5 to 16
- Until now in the manuscript you have discussed that denitrification results in a 2:1 pattern. Now you have switched to phrases such as ‘trajectory of 1’, put this in context for the reader, 2:1 versus 1:1 and how Granger and Wankel, 2016 are trying to reconcile this.
- ‘anammox is still disputable’ what are the authors referring to here, that anammox has not been observed in your system?

Page 11, Lines 5 to 7 and Figure 9: Here you are also using a 1:1 line and this needs to be explained for the reader and also highlighted in the caption of Figure 9 (the whole caption of Figure needs to be looked at, as currently information is missing, it is correct in the reviewer response). Surely you can also exclude denitrification here, due to the high levels of DO?

Page 11, Lines 22 to 24: Should this be Figure 9? Otherwise I do not understand this sentence as currently written.

Figure 5: Reference is missing from caption, but is present in the reviewer response, please make sure it is in the final version. I missed this in my last review, but I can see that in this figure you are showing negative isotope effects, whereas in the text they are positive (e.g. volatilization, Page 7, Line 23). I know the use of positive versus negative varies in the literature, but please be consistent with your use throughout the manuscript.

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ED: Publish subject to minor revisions (review by editor) (01 May 2018) by Manmohan Sarin

AR by Wei Wen Wong on behalf of the Authors (22 May 2018) Author's response Manuscript

ED: Publish subject to technical corrections (01 Jun 2018) by Manmohan Sarin

AR by Wei Wen Wong on behalf of the Authors (06 Jun 2018) Author's response Manuscript

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Short summary

Over-enrichment of nitrate can pose substantial risk to the quality of freshwater ecosystems. Hence, understanding the dynamics of nitrate is the key to better management of waterways. This study evaluates the relationship between the effects of land use and rainfall on the major sources and processing of nitrate within and between five streams in five catchments spanning an agricultural land use gradient. We found that rainfall exerted significant control over the fate of nitrate.