High nitrate variability on an Alaskan permafrost hillslope dominated by alder shrubs

McCaully, Rachael E.; Arendt, Carli A.; Newman, Brent D.; Salmon, Verity G.; Heikoop, Jeffrey M.; Wilson, Cathy J.; Sevanto, Sanna; Wales, Nathan A.; Perkins, George B.; Marina, Oana C.; Wullschleger, Stan D.

doi:https://doi.org/10.5194/tc-16-1889-2022

Articles | Volume 16, issue 5

https://doi.org/10.5194/tc-16-1889-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-16-1889-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 5

Research article

|

19 May 2022

Research article |

| 19 May 2022

High nitrate variability on an Alaskan permafrost hillslope dominated by alder shrubs

Rachael E. McCaully, Carli A. Arendt, Brent D. Newman, Verity G. Salmon, Jeffrey M. Heikoop, Cathy J. Wilson, Sanna Sevanto, Nathan A. Wales, George B. Perkins, Oana C. Marina, and Stan D. Wullschleger

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Final revised paper (published on 19 May 2022)
Supplement to the final revised paper
Preprint (discussion started on 11 Jun 2021)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on tc-2021-166', Anonymous Referee #1, 17 Jul 2021

Please find my comments in the attached pdf. With kind regards

Citation: https://doi.org/10.5194/tc-2021-166-RC1
- AC1: 'Reply on RC1', Carli Arendt, 24 Sep 2021
  
  The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2021-166/tc-2021-166-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/tc-2021-166-AC1
RC2:
'Comment on tc-2021-166', Anonymous Referee #2, 15 Aug 2021

Please see the enclosed pdf with comments to the authors

Citation: https://doi.org/10.5194/tc-2021-166-RC2
- AC2: 'Reply on RC2', Carli Arendt, 24 Sep 2021
  
  The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2021-166/tc-2021-166-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/tc-2021-166-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (29 Sep 2021) by Ylva Sjöberg

AR by Carli Arendt on behalf of the Authors (08 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (22 Nov 2021) by Ylva Sjöberg

RR by Anonymous Referee #1 (11 Dec 2021)

Suggestions for revision or reasons for rejection

I see that the authors tried to restructure the text, but the MS in the presented state cannot be published, sorry.
The structure is still not satisfying: several repetitions (on flushing by rainfall), part of the discussion is presented like result section, etc.
Because the whole MS needs to be restructured again, I will not give here a specific line- per-line comment. And please (a detail), do not reply to reviewers by using the same sentence/expressions all the time, this is more than boring.

Here, my main reason why the MS cannot be accepted for publication:
1) There is a general and deep misunderstanding, an article needs to be written and condensed to a unit, important pieces of information have to be incorporated in the article and not presented in the Supplementary Material or in a co-located study or somewhere (by the way, this was stated already in the first review). You cannot formulate statements in the abstract and the data are fully absent in the article. The reader of your article MUST find your “main story” in the MS, not in the Supplementary Material.

2) The data presented as main result in the abstract, are simply not new:
“Soil pore water collected within alder shrubland had an average NO3--N (nitrogen from nitrate) concentration of 4.27 ± 8.02 mg L-1 and differed significantly from locations outside alder shrubland (0.23 ± 0.83 mg L-1; p < 0.05).”
I suggested that you express the nitrate as Nitrate-N = NO3-N (without a minus sign) to enhance the comparability to other N concentrations (e.g., NH4-N), but then you have to recalculate it:
For example: 4 mg L-1 NO3- = 0.90 mg L-1 NO3-N!
[The concentrations in the text and in the figures are NOW all wrong]
Even correctly calculated, this difference can be expected as alder are N2-fixing plants (write N2-fixing not N-fixing).
3) The design remains weak, when precipitation events seem to be the main driver for flushing events, but the precipitation has not been measured on site, there is no valid proof.

4) Several references are still not cited correctly (Alnus viridis encroachment in the Alps occurs in the montane vegetation belt, not in the alpine, see Bühlmann et al. 2014, line 54) and more important: please make sure that you are not employing any plagiarism when using whole and/or "very similar" sentences from other articles (e.g., Salmon et al. 2019).

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RR by Anonymous Referee #2 (11 Dec 2021)

RR by Anonymous Referee #3 (18 Jan 2022)

Suggestions for revision or reasons for rejection

The aim of this study is to the N cycle and soil NO3- concentrations along a topographic gradient in a permafrost area with a vegetation cover of nitrogen fixing Alnus viridis spp fruticose. The main conclusion from the work emphasize the temporal variation in soil NO3- within and downslope from the alder shrub-land, where soil NO3- is supposedly being flushed downslope during precipitation events. This is an interesting and relevant topic of research to investigate N-cycle and transport-processes at landscape scales in permafrost regions, and the manuscript holds important data that I believe should be made available to the science community.
This manuscript has previously been revised in response to two extensive peer-reviews and I find that the authors have responded comprehensive and careful to the criticism raised. Meanwhile, I also find that the manuscript in it’s current form can be further improved and streamlined to clarify and emphasize even stronger the outcome of this study. My main concern is specifically the speculation about precipitation driven downslope nitrate transport, combined with the isotopic observations that I recommend to moderate and soften. See comments below.

Introduction
Line 31: A recent paper studies lateral N-transport in a permafrost landscape and demonstrates the function of lateral N-transport for plant uptake and growth. Rasmussen, L. H., et al. (2022). "Nitrogen transport in a tundra landscape: the effects of early and late growing season lateral N inputs on arctic soil and plant N pools and N2O fluxes." Biogeochemistry 157(1): 69-84.

2.6 Statistical analyses
Line 194: The method for determining normal distribution needs to be mentioned.
Since data were all normally distributed, why was non-parametric test applied rather than more powerful parametric statistical tests?
Line 205: Calcium, Sodium and Chloride statistical analysis is explained. Confusing as these ions have not been described in previous section on chemical analysis of soil water (only that cations and anions in general were analyzed)?

Results
3.1 Soil depth and moisture, line 216: the equation percent dry/wet weight is not gravimetric soil moisture content but rather dry matter content. Please, specify or give correct equation.
3.2 Phase 1:… line 228: It’s rather unfortunate that the local rain gauge malfunctioned at the time of the ongoing field work. Perhaps it could be relevant though to somehow provide indications for the amount of precipitation deposited during this event? The term …brief precipitation… is rather vague. Was it a short, intense rain event, or more kind of extended drizzle?
3.3 Phase 2:… line 245: Data for A4 shrubland NO3- during 2017 are not in Fig 3?
Line 254: ..up to 20-m downslope of the shrubland… with reference to Fig. 3c. The distance downslope in meters is in fact not possible to see from Fig 3c, which pictures within, downslope and seep. The numbers for ‘downslope’ according to the Figure are actually quite low, and not elevated as the text suggests (line 253).
Line 262: You refer to Fig. 4b, but sine the Fe changed along both transects I assume it should be both Fig. 4a and 4b?
Generally, Figure 4 is difficult to read. The log-scale makes the column to appear quite similar in height and difficult to reveal patterns. Also, a scale along the X-axis would be useful.

Discussion
4.2 Effects of precipitation…: Generally, I think this section is too speculative as data on precipitation is missing. How much rain did actually fell in these periods, and would that enable a downward transport of water (and solutes) given the actual conditions of soil wetness prior to rain and soil infiltration rates etc. I really suggest these speculations being removed from the manuscript. Along this line, I also find the data on isotopic observations somewhat over-interpreted; e.g. no “real-time” data on nitrate in precipitation or isotopic values of nitrate and water in precipitation is presented to support the statement about precipitation isotopic imprints on soil water NO3. Also, with reference to Fig S2, is the predicted denitrification driven shift in isotopic values significant as this is based on very few data points? If not, I suggest to remove this line. (later in line 331 it’s referred to as a trend in data).
4.3. Effects of redox…: Line 328: It’s not clear to me how the lack of mobility of NO3 beyond the first 20-30 m downslope can be seen from Fig. 4 as this shows no downslope-scale (see comment above).
4.4 Spatial and temporal… Line 344: As for my previous comments, I find the interpretations on soil NO3 in relation to precipitation basically unsupported as no precipitation data are presented. Moreover, in Fig. S1 it is shown that soil NO3 and soil moisture was not correlated, which somehow is conflicting the statement that “…the notable day-to-day changes in soil NO3…was driven primarily by the presence of rainfall.”
4.5 Future research. Line 357, see aforementioned paper by Rasmussen et al., 2022.

Supplement
Isotopic insights: The inclusion of isotopic water and nitrate data is acknowledged as these can be important indicators for dominant processes in the present site. However, I do on the other hand also recommend data being interpreted and extrapolated with due attention to the fact that inputs and characteristics of e.g. NO3 in precipitation is not established in this study – see comment above.
Please, explain how 15N in TDN was measured.

Coefficient of Variability: Table S6 (first line in section) should refer to Table S3.

Table S2. It’s briefly mentioned (section 2.5) that DO, pH and conductivity didn’t correlate with NO3-, but did the authors assess if there’s any particular spatial or temporal pattern in variation of these parameters? Maybe worth to mention.

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ED: Publish subject to revisions (further review by editor and referees) (01 Mar 2022) by Ylva Sjöberg

AR by Carli Arendt on behalf of the Authors (19 Apr 2022) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (02 May 2022) by Ylva Sjöberg

AR by Carli Arendt on behalf of the Authors (03 May 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (03 May 2022) by Ylva Sjöberg

AR by Carli Arendt on behalf of the Authors (03 May 2022) Author's response Manuscript

Short summary

Degrading permafrost and shrub expansion are critically important to tundra biogeochemistry. We observed significant variability in soil pore water NO₃-N in an alder-dominated permafrost hillslope in Alaska. Proximity to alder shrubs and the presence or absence of topographic gradients and precipitation events strongly influence NO₃-N availability and mobility. The highly dynamic nature of labile N on small spatiotemporal scales has implications for nutrient responses to a warming Arctic.