Nitrite consumption and associated isotope changes during a river flood event

Jacob, Juliane; Sanders, Tina; Dähnke, Kirstin

doi:https://doi.org/10.5194/bg-13-5649-2016

Articles | Volume 13, issue 19

https://doi.org/10.5194/bg-13-5649-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-13-5649-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 19

Research article

|

11 Oct 2016

Research article |

| 11 Oct 2016

Nitrite consumption and associated isotope changes during a river flood event

Juliane Jacob, Tina Sanders, and Kirstin Dähnke

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Final revised paper (published on 11 Oct 2016)
Preprint (discussion started on 03 Mar 2016)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Referee comment', Anonymous Referee #1, 23 Mar 2016
- AC1: 'Response Letter bg-2016-75', Juliane Jacob, 27 May 2016
RC2: 'Review of "Nitrification and nitrite isotope fractionation as a case study in a major European River"', Anonymous Referee #2, 28 Mar 2016
- AC2: 'Response Letter bg-2016-75', Juliane Jacob, 27 May 2016
RC3: 'Review of Jacob et a.', Anonymous Referee #3, 06 Apr 2016
- AC3: 'Response Letter bg-2016-75', Juliane Jacob, 27 May 2016

Peer-review completion

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

ED: Reconsider after major revisions (07 Jun 2016) by Helge Niemann

AR by Juliane Jacob on behalf of the Authors (13 Jul 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Jul 2016) by Helge Niemann

RR by Anonymous Referee #1 (04 Aug 2016)

RR by Caitlin Frame (08 Aug 2016)

Suggestions for revision or reasons for rejection

Thanks for this paper, it is a nice data set describing the N dynamics during a river flooding event. I think it should be published, but you need to make additional considerations and analyses before this happens.

Assimilatory reduction of NO3- to NO2- by phytoplankton under light-limited conditions is another possible source of NO2- in addition to ammonia oxidation. Lomas and Lipschultz (2006) review this possibility and Santoro et al (2013, Biogeosciences) include some discussion of this as well. Based on the description of turbidity during the Elba flooding event, it sounds like phytoplankton could indeed be light-limited, perhaps not photosynthetically active, but possibly still able to participate in N cycling. In your paper, you should go through the exercise of considering this possibility, even if you are able to rule it out in the end. I am not certain whether isotope effects for assimilatory NO3- reduction have been published, but they have been reported for dissimilatory NO3- reduction, and so you might, if necessary, use these as proxies for the assimilatory effects. It looks possible to me that towards the end of the event, the δ15N-NO2- is actually tracking the δ15N of the much larger NO3- pool.

If you have any measurements relating to phytoplankton abundance during the flooding event (e.g. chlorophyll concentration), you should include those in Figure 2 as well. If phytoplankton abundances are diluted by the flooding, then it would support your argument that they really are less active.

Page 9, Lines 2-7
I am not sure why you say “ammonium concentrations are below the detection limit at the time of decreasing nitrite concentrations.” Which days are you referring to? It looks to me like ammonium is above your detection limit for all but your last day of measurements in Figure 2. If ammonium is quantitatively converted to NO2- as soon as it is produced, then the isotope effect for ammonia oxidation should not be expressed. However, it does look like there is accumulated NH4+, and so expression of the ammonia oxidation isotope effect should produce nitrite that is depleted in 15N relative to the ammonia. The logic does not really follow in this paragraph. It needs to be rewritten, paying attention to the fact that when you say, for example, that nitrite is isotopically enriched, you also need to mention what that is relative to (i.e. is it enriched relative to previously formed NO2-? The substrate ammonium?).

Page 9, Lines 21-27
You need to provide more details of the analysis that went into the argument in this paragraph. Are you assuming a steady-state NH4+ concentration/isotope composition or is this a closed system with respect to NH4+? It sounds like in your model, NO2- is being simultaneously produced and consumed, so I don’t think that “standard Rayleigh fractionation” is going to describe the system. Can you please draw a diagram showing the N fluxes and isotopic mass balance for the different DIN pools? Are you trying to match your NO2- and NH4+ concentration and isotope data to what you report in Figure 2? I think you need to plot your modeled [NH4+], [NO2-], δ15N-NH4+, and δ15N-NO2- and then compare this to your data. Ideally, show what happens to your model results as you vary the isotope effects of ammonia oxidation and nitrite oxidation.

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ED: Reconsider after major revisions (17 Aug 2016) by Helge Niemann

AR by Juliane Jacob on behalf of the Authors (05 Sep 2016) Author's response Manuscript

ED: Publish subject to minor revisions (Editor review) (19 Sep 2016) by Helge Niemann

AR by Juliane Jacob on behalf of the Authors (20 Sep 2016) Author's response Manuscript

ED: Publish as is (26 Sep 2016) by Helge Niemann

Short summary

During a flood in the Elbe in June 2013, a unique co-occurrence of ammonium, nitrite and nitrate in the water column was found. SPM and nutrient concentrations as well as isotopes were analysed. We calculated an isotope effect ¹⁵ε of −10.0 ± 0.1 ‰ during net nitrite and ¹⁵ε of −4.0 ± 0.1 ‰ and ¹⁸ε of −5.3 ± 0.1 ‰ during net nitrate consumption. A simple box-model calculation results in combined riparian denitrification and 22 to 36 % nitrification.