Export flux of unprocessed atmospheric nitrate from temperate forested catchments: a possible new index for nitrogen saturation

Nakagawa, Fumiko; Tsunogai, Urumu; Obata, Yusuke; Ando, Kenta; Yamashita, Naoyuki; Saito, Tatsuyoshi; Uchiyama, Shigeki; Morohashi, Masayuki; Sase, Hiroyuki

doi:https://doi.org/10.5194/bg-15-7025-2018

Articles | Volume 15, issue 22

https://doi.org/10.5194/bg-15-7025-2018

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

https://doi.org/10.5194/bg-15-7025-2018

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

Articles | Volume 15, issue 22

Research article

|

22 Nov 2018

Research article |

| 22 Nov 2018

Export flux of unprocessed atmospheric nitrate from temperate forested catchments: a possible new index for nitrogen saturation

Fumiko Nakagawa, Urumu Tsunogai, Yusuke Obata, Kenta Ando, Naoyuki Yamashita, Tatsuyoshi Saito, Shigeki Uchiyama, Masayuki Morohashi, and Hiroyuki Sase

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Final revised paper (published on 22 Nov 2018)
Supplement to the final revised paper
Preprint (discussion started on 12 Jul 2018)

Interactive discussion

Status: closed

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

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RC1: 'Review 1', Anonymous Referee #1, 02 Aug 2018
- AC1: 'Reply to your comment', Urumu Tsunogai, 16 Sep 2018
RC2: 'g-2018-258 Submitted on 31 May 2018', Anonymous Referee #2, 04 Aug 2018
- AC2: 'Reply to your comment', Urumu Tsunogai, 16 Sep 2018

Peer-review completion

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

ED: Reconsider after major revisions (17 Sep 2018) by Sébastien Fontaine

AR by Urumu Tsunogai on behalf of the Authors (11 Oct 2018) Manuscript

ED: Referee Nomination & Report Request started (16 Oct 2018) by Sébastien Fontaine

RR by Anonymous Referee #1 (23 Oct 2018)

Suggestions for revision or reasons for rejection

Overall comment:

This is the revised version of the manuscript by Nakagawa et al. entitled “Export flux of unprocessed atmospheric nitrate from temperate forested catchments: a possible new index for nitrogen saturation”. I would like to acknowledge the considerable effort undertaken by the authors to address the various issues I had raised in my previous evaluation. I feel that the revised manuscript is much stronger, more well written, and above all clearer than the first version. The authors did respond to my questions in a way that satisfied me, and I now understand what they were trying to do in this work. Here, they use the Matm/Datm ratio as an independent way to assess the nitrogen saturation status of forested catchments in Japan. They applied this method on three catchments that have been heavily monitored in the past, and for which they had already a fair knowledge of the nitrogen saturation status. They find that the use of the Matm/Datm ratio provides a good indication of the relative saturation level between forested catchments. While I still have a few comments, I don’t see why this manuscript shouldn’t be published in Biogeosciences.

Main comment:

My main interrogation now is regarding your explanation of the difference in the Matm/Datm ratios between the studied catchments: you say that the difference is due to different biological assimilation rates, caused by different nitrogen saturation status. But couldn’t the different biological assimilation rate be caused only by the difference in the vegetation in the forest for each catchment? The tree species abundances are different in KJ, IJ1 and IJ2 forests: could that cause the difference in the Matm/Datm ratios because the trees would have different metabolic rates? And in that case, the Matm/Datm ratio would not be an indicator of N saturation, but just of the retention capacity of a catchment? I think this needs to be at least discussed in your last section.

Specific comments

Section 2.3: Please expand a bit on your methods here. Explain for instance the conditioning of the samples (fridge, freezer, filtration) and if you use acid solution to stop any biological activity, etc.

P.7 line 10 and P.8 line 11: Please refer to Aber et al. (1989) when mentioning N saturation.

Aber, J. D., Nadelhoffer, K. J., Steudler, P. and Melillo, J. M.: Nitrogen saturation in northern forest ecosystems, BioScience, 39(6), 378–386, doi:10.2307/1311067, 1989.

P.15 l.3: Please refer to Bourgeois et al. (2018) when mentioning NO3-atm export from the snowpack to the river.

Bourgeois, I., Savarino, J., Caillon, N., Angot, H., Barbero, A., Delbart, F., Voisin, D. and Clément, J.-C.: Tracing the fate of atmospheric nitrate in a subalpine watershed using Δ17O, Environ. Sci. Technol., 52(10), 5561–5570, doi:10.1021/acs.est.7b02395, 2018.

P.19 l.11: Please remove “huge”, or give an idea of the reservoir of nitrate it represents if you know it. No need to emphasize on the size, we understand the idea of dilution here.

P.21 l.4: Please replace “fractionation” with “enrichment”

Table 1: Maybe add a row with Matm/Datm ratio for each catchment

Referee Report: PDF

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RR by Anonymous Referee #2 (31 Oct 2018)

ED: Publish subject to technical corrections (31 Oct 2018) by Sébastien Fontaine

AR by Urumu Tsunogai on behalf of the Authors (01 Nov 2018) Author's response Manuscript

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Short summary

To clarify the biological processing of nitrate within temperate forested catchments using unprocessed atmospheric nitrate exported from each catchment as a tracer, we continuously monitored stream nitrate concentrations and stable isotopic compositions in three forested catchments for more than 2 years. We concluded that the export flux of unprocessed atmospheric nitrate relative to the deposition flux in each forest ecosystem is applicable as an index for nitrogen saturation.