Decadal fates and impacts of nitrogen additions on temperate forest carbon storage: a data–model comparison

Cheng, Susan J.; Hess, Peter G.; Wieder, William R.; Thomas, R. Quinn; Nadelhoffer, Knute J.; Vira, Julius; Lombardozzi, Danica L.; Gundersen, Per; Fernandez, Ivan J.; Schleppi, Patrick; Gruselle, Marie-Cécile; Moldan, Filip; Goodale, Christine L.

doi:https://doi.org/10.5194/bg-16-2771-2019

Articles | Volume 16, issue 13

https://doi.org/10.5194/bg-16-2771-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-16-2771-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 13

Research article

|

16 Jul 2019

Research article |

| 16 Jul 2019

Decadal fates and impacts of nitrogen additions on temperate forest carbon storage: a data–model comparison

Susan J. Cheng, Peter G. Hess, William R. Wieder, R. Quinn Thomas, Knute J. Nadelhoffer, Julius Vira, Danica L. Lombardozzi, Per Gundersen, Ivan J. Fernandez, Patrick Schleppi, Marie-Cécile Gruselle, Filip Moldan, and Christine L. Goodale

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Final revised paper (published on 16 Jul 2019)
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Preprint (discussion started on 19 Dec 2018)
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Interactive discussion

Status: closed

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

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RC1: 'Review of Cheng et al. 2018', Anonymous Referee #1, 03 Jan 2019
RC2: 'Review for Chent et al., 2018 (BG) "Decadal impacts of nitrogen additions on temperate forest carbon sinks: A data-model comparison"', Benjamin Stocker, 24 Jan 2019
RC3: 'Interactive comment on Cheng et al.', Anonymous Referee #3, 30 Jan 2019
AC1: 'Response to all reviewers', Susan Cheng, 27 Feb 2019

Peer-review completion

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

ED: Reconsider after major revisions (06 Mar 2019) by Sönke Zaehle

AR by Susan Cheng on behalf of the Authors (07 Apr 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (15 Apr 2019) by Sönke Zaehle

RR by Benjamin Stocker (26 Apr 2019)

Suggestions for revision or reasons for rejection

Most points that I raised regarding the initial submission of this manuscript have been addressed satisfactorily. However, I recommend addressing the following remaining points (referring to numbers in the author's response document):

12. The point I was trying to make was that the 15N added is well mixed in the model in mineral N pools (irrespectively of its depth), whereas in the field, it may first be immobilised in the litter layer. I was trying to argue that in the model, natural 15N has the same probability as experimentally added 15N for being taken up by the plant (well-mixed mineral N pool), whereas in the field, it may not. The author’s response and added text does not fully address this point.

13. Added text addresses my point well. I didn’t understand, however, whether the entries ‘0’ in Table 1 (Amount of N fertilizer, g N m-2 yr-1) were wrong in the first submission and have been changed now to 0.5.

14. Greatly appreciated. No units are given in Table S1. Data that is of interest for use in other research should be made available following the FAIR principles (see https://libereurope.eu/wp-content/uploads/2017/12/LIBER-FAIR-Data.pdf). Here, it may be made available as a CSV file in a “tidy” format, i.e. one variable per column, one entry per row (no merged cells), and deposited on a public repository (e.g., Zenodo) with a DOI.

15. Ok.

16. Added text under 4.1 (author response to comment 40) is helpful as a statement regarding how to improve the representation of N cycling in CLM.

17. Ok.

18. I don’t agree with the author’s argument. Following the definition I proposed, then, if for two systems, Nin is the same (hence Nout is the same in steady-state) and NPP is the same, then their “openness” is the same, too (unless the C:N ratio of new production and/or the resorption efficiency). I am not sure if I understand correctly how the index was calculated here to estimate how often N cycles through plants before being lost fro the system. I assume it’s the residence time of N in at the ecosystem level (Nplant+Nsoil+Nmineral / Nin), divided by the residence time of N in the plant (Nplant / (Nuptake+Nfix) = Nplant / Nlitterfall). Is this the case? Please specify this more precisely in the manuscript.

19. Ok.

20. This is a point that I have been thinking about and I may not understand what’s the best answer. The author’s response doesn’t solve my puzzle either. When doing an ANOVA, you consider the uncertainty of two samples. The uncertainty from the observations is both the result of measurement error and variability in the system. In the model, it’s only variability in the system, but has no error (it’s a deterministic model, not stochastic). Is it permissible to treat variability in the model as an uncertainty and do an ANOVA. I’m not an expert on this, but have doubts.

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ED: Publish subject to minor revisions (review by editor) (07 May 2019) by Sönke Zaehle

Dear authors,

many thanks for your revised manuscript. This version has now been seen by one of the original reviewers and myself, and we agree that the manuscript has improved in response to the comments. The reviewer raises a couple of points that you will need to addressed before this manuscript can be accepted for publication. In particular, the reviewer raises the important point of making the data set that you have compiled accessible to the community according to the FAIR principle, which applies to both table S1 and S3, which I ask you to follow.

In addition, I would recommend to reconsider the presentation of the two CLM5 versions. From your responses I take that the main purpose is to show that there are important weaknesses of CLM5 irrespective of the openness of the N cycle, however, this entire issue does not become very clear in the abstract nor in the introduction (Why are two different versions of CLM5 used? Why is this important?). To make the manuscript more appealing to non-CLM5 users, I think you should introduce the reasoning for analysing the two versions in the introduction, to lay the ground for your recommendations in the conclusion section.

In terms of terminology, I'm slightly concerned about the loose use of N inputs lumping together N fixation and N deposition (and then making inferences about CLM5 needing improved representation of N inputs, when N deposition is not modelled by CLM5, so its improvement is merely one of choosing better input data, not improving the model), and denitrification as the only source of N gas losses (what about volatilisation and nitrification?). Please carefully revise the wording of the manuscript.

As a minor point (since it very likely does not matter for the main results), it is unclear to me why N deposition changes between the different model variants

Best wishes,
Sönke Zaehle

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AR by Susan Cheng on behalf of the Authors (04 Jun 2019) Author's response Manuscript

ED: Publish as is (09 Jun 2019) by Sönke Zaehle

AR by Susan Cheng on behalf of the Authors (18 Jun 2019) Author's response Manuscript

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Short summary

Nitrogen deposition and fertilizer can change how much carbon is stored in plants and soils. Understanding how much added nitrogen is recovered in plants or soils is critical to estimating the size of the future land carbon sink. We compared how nitrogen additions are recovered in modeled soil and plant stocks against data from long-term nitrogen addition experiments. We found that the model simulates recovery of added nitrogen into soils through a different process than found in the field.