Global soil nitrous oxide emissions in a dynamic carbon-nitrogen model

Huang, Y.; Gerber, S.

doi:https://doi.org/10.5194/bg-12-6405-2015

Articles | Volume 12, issue 21

https://doi.org/10.5194/bg-12-6405-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-12-6405-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 21

Research article

|

10 Nov 2015

Research article |

| 10 Nov 2015

Global soil nitrous oxide emissions in a dynamic carbon-nitrogen model

Y. Huang and S. Gerber

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Final revised paper (published on 10 Nov 2015)
Preprint (discussion started on 11 Feb 2015)

Interactive discussion

Status: closed

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

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- Supplement

RC C258: 'Review of Huang & Gerber, 2015, BGD', Benjamin Stocker, 19 Feb 2015
- AC C3279: 'Response to Beni Stocker's review', Stefan Gerber, 03 Jul 2015
SC C370: 'Too little, too late: Interactive comment on Huang & Gerber's N2O MS', Iain Colin Prentice, 28 Feb 2015
- AC C3300: 'Response to Colin Prentice's comments', Stefan Gerber, 03 Jul 2015
RC C493: 'Interactive comment on "Global soil nitrous oxide emissions in a dynamic carbon-nitrogen model" by Y.Y. Huang and S. Gerber', Lex Bouwman, 04 Mar 2015
- AC C3303: 'Response to A.F. Bouwman's Review', Stefan Gerber, 03 Jul 2015
- AC C3314: '2nd (and improved) response to Lex Bouwman's review', Stefan Gerber, 03 Jul 2015
RC C648: 'Review of Huang and Gerber, 2015', Anonymous Referee #3, 17 Mar 2015
- AC C3307: 'Response to Anonymous Referee', Stefan Gerber, 03 Jul 2015

Peer-review completion

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

ED: Reconsider after major revisions (16 Jul 2015) by Sönke Zaehle

AR by Stefan Gerber on behalf of the Authors (21 Aug 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Aug 2015) by Sönke Zaehle

RR by Anonymous Referee #1 (31 Aug 2015)

Suggestions for revision or reasons for rejection

Review of revised manuscript Huang & Gerber, 2015, BG

I thank the authors for their constructive responses and the efforts put into revising their original manuscript. Several major concerns were raised by the reviewers (see below), most of them by more than one reviewer, which underlines their significance. The authors took on the task of addressing these issues, but
didn't "re-invent the study". As a general impression, the limited modifications made in the revised manuscript contrast with the authors' acknowledgment of the weaknesses of their initial submission. However, I also acknowledge that model evaluation w.r.t. N2O emissions remains a particulary challenging task, which shouldn't prevent incremental progress (by studies like this one) before "all problems are solved".

In the following, I'm trying to list what were in my view some key points of criticism raised by the different reviewers, and how I judge the authors' respective responses.

*Novelty and scope*
Regarding this aspect, the authors write "Since we build largely on existing parameterization of nitrification-denitrification processes, our focus relies on the evaluation of these processes if tranferred to a different model." (p.34,l.14/15). The aspect of how a similar implementation of one module (inorganic N dynamics following a DNDC-type model) yields different results when integrated within different coupled C-N cycling models (comparison to DyN-LPJ and O-CN), could be strengthened. This leaves model evaluation as the most important aspect of this paper.

I like the track of evaluating sensitivity to WFPS, also considering the interesting model parametrisations of the N2O yield factor as a function of WFPS (which is an innovation, considering that other models use a constant factor). I find this aspect of the paper particularly interesting as the authors use empirical parametrisations to calculate the N2O yield factor from nitrification and denitrification (section 2.1.2.2) and provide insights of N2O sensitivity to soil moisture. However, WFPS itself is not benchmarked and observed soil moisture at the measurement sites (I assume/hope this is measured along with N2O) was not used in the model for the scale evaluation across the season (Fig. 5). Therefore, we still don't know if the relationship of N2O to WFPS is correct. I consider the seasonal analysis provided by Fig. 5 as a valuable addition. It may even provide more insights if the analysis of N2O and WFPS is coupled.

*Suitability for BG (vs. GMD)*
The authors' responses to this point do not convince me, but it is not my task to make a final judgment. The authors agree that the main scope of this paper is model evaluation (see above "novely and scope"). I think the revisions even reinfoce this aspect (model description moved from appendix into main text).

*Subject to model performance of entire C-N cycling and sensitivity analysis*
I appreciated the added descrition of general characteristics and key processes in LM3V-N. This section is now rather technical with a few equations provided. This could be improved by providing a more general description of the model characteristics. For example, in the responses the authors note that LM3V-N simulates very tight N cycling as opposed to other models, and correspondingly small BNF ("empirical" BNF rate of 108 TgN/yr: what reference?). Chosen yield factors of N2O from nitrification/denitrification are rather high, probably necessary due to the tight N cycling.

The sensitivity analysis to other model parts (BNF, fire, DON, and gaseous losses, N uptake rate, and N2O yield factors) underlines what may be expected anyway. The resulting changes in N2O emissions from varying parameters by factor 10 are on the same order of magnitude as the absolute value using "standard" values. The analysis is not used in the light of observational constraints and therefore provide limited new insights. I acknowledge that this point is a challenge for all C-N models attempting to simulate N2O emissions. However, this manuscript doesn't offer a clear way forward, leaving this as a weakness of the paper.

*Poorly documented equations and parameter values and placement of model description in Appendix*
This has been improved and the equations are moved into the main text. However, in some instances, it was still not clear where exactly the equations and parameter values were drawn from (e.g., Section 2.1.2.2: Gaseous partitions from nitrification-denitrification).
Another aspect that confused me: How do these empirical functions fit in with the statement that single constant factor of 0.4% is for the N2O yield from nitrification? This number doesn't appear in Eq. 14, 15, 16 - I must have misunderstood something...

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RR by Anonymous Referee #3 (07 Sep 2015)

ED: Publish subject to minor revisions (Editor review) (25 Sep 2015) by Sönke Zaehle

AR by Stefan Gerber on behalf of the Authors (09 Oct 2015) Author's response Manuscript

ED: Publish subject to technical corrections (14 Oct 2015) by Sönke Zaehle

AR by Stefan Gerber on behalf of the Authors (21 Oct 2015) Author's response Manuscript

Short summary

The prediction of the greenhouse gas N2O from natural soils globally is sensitive to the representation of soil water. Factors that regulate nitrogen retention and nitrogen limitation, including fire and biological nitrogen fixation are further influencing the N2O gas production. Responses to warming and CO2 increase are strongly controlled by tropical soils. Therefore extrapolation of mostly extra-tropical field studies the globe warrants caution.