Long-term additions of ammonium nitrate to montane forest ecosystems may cause limited soil acidification, even in the presence of soil carbonate

Baer, Thomas; Furrer, Gerhard; Zimmermann, Stephan; Schleppi, Patrick

doi:https://doi.org/10.5194/bg-20-4577-2023

Articles | Volume 20, issue 22

https://doi.org/10.5194/bg-20-4577-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Ecosystem experiments as a window to future carbon, water,...

https://doi.org/10.5194/bg-20-4577-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 22

Research article

|

23 Nov 2023

Research article |

| 23 Nov 2023

Long-term additions of ammonium nitrate to montane forest ecosystems may cause limited soil acidification, even in the presence of soil carbonate

Thomas Baer, Gerhard Furrer, Stephan Zimmermann, and Patrick Schleppi

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Final revised paper (published on 23 Nov 2023)
Supplement to the final revised paper
Preprint (discussion started on 07 Mar 2023)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on bg-2023-38', Anonymous Referee #1, 25 Apr 2023
The current manuscript addresses relevant scientific questions within the scope of the journal: testing the effects of increased N deposition on acidification processes in a well-buffered ecosystem. This is a concept not enough explore so far, which gives high scientific relevance to the paper. The hypothesis are clear, well described, and fully addressed through methods and assumptions, that are clearly explained.
Does the paper address relevant scientific questions within the scope of BG? YES

Does the paper present novel concepts, ideas, tools, or data? YES

Are substantial conclusions reached? YES

Are the scientific methods and assumptions valid and clearly outlined? YES, although a little clarification about the treatments is suggested.

Are the results sufficient to support the interpretations and conclusions? YES

Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? YES

Do the authors give proper credit to related work and clearly indicate their own new/original contribution? YES

Does the title clearly reflect the contents of the paper? YES

Does the abstract provide a concise and complete summary? YES

Is the overall presentation well structured and clear? YES

Is the language fluent and precise? YES

Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? YES

Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? YES, but some suggestions are made to improve or clarify the text and figures.

Are the number and quality of references appropriate? YES

Is the amount and quality of supplementary material appropriate? YES

Minor revisions:
Treatments
line 14-15, Although it is known from previous studies, the text here it is not clear enough about the N addition. Please clarify if N treatment is 22 kgN greater than the ambient N deposition (C treatment), that was 12 kgN at the begiing of the study, or if it represented 22 kgN compared to 12 kgN of ambient deposition.

line 139-145: It could be also clarified a little.

Deposition
Although N deposition and cycling is not key for the study, some further data a discussion on this issue could improve the understanding or, at least, allow further interpretation of the results. Particularly, the nitrate concentration response in runoff could be related to N cycling dynamics that can be connected to saturation processes. Knowing that N leaching is addressed in previous studies, not much information would needed here, but some of it would be appreciated. Particularly, a historic evolution of N deposition comparing both treatments would be appreciated. Moreover, in this study, throughfall deposition (if available) would be of greater relevance than bulk deposition since it represents a direct input into the soil.
line 15, line 92, line 130: the deposition value referred here is from the beginning of the study

line 130: total deposition and N retention in the catchments are also available from previous studies. The authors might consider if these data could be relevant here.

lines 273-280: deposition data from other studies are referred here. Since it is stated that precipitation has not significantly changed in the valley, the changes in N concentration in rain are correctly assumed to be reflected in deposition values. However, some data on measured deposition trends would be appreciate here.

Nitrate leaching
lines 297-299: This is just a question. It is stated that “NO3- can have an acidifying effect if leached together with base cations, leading to their replacement by H+ or Al3+ on the exchange complexes”. This effect is observed in N treatment in the increase of NO3 leaching, that is related to the strong increase in soil acidity on the mounds. Should it be also related to an increased leaching of base cations that matches the NO3 leaching but that cannot be observed in the current graphics?
Figure 1
I understand that trenches in catchment 1 are present in the same way as in catchment 2 (around the entire perimeter) but it is not drawn in the picture.
Please consider the following suggestions
line 43: “taken up” instead of “used”

line 44-45: Something like “together with the increased deposition of H+ caused by NOx” instead of “together with the inflow of H+ from the atmospheric precipitation”

line 47: “Losses of N from NO3- to atmospheric N2”; “denitrification” could be added here.

line 49: “Decomposition of plant residues also leads to a consumption of H+.”

line 77-78: “Thanks to stringent emission regulations in Switzerland and in Europe in general, SO42- concentrations in the precipitation have declined and the pH of precipitation has increased over the last 30 years”; the international Protocols from the LRTAP Convention to reduce sulphur dioxide emissions (or the Convention itself) could be referred here.

line 171: “Samples were taken as 3 cores per plot (5 cm diameter) at irregular intervals of several years.”; sampling years could be added here in brackets.
Citation: https://doi.org/10.5194/bg-2023-38-RC1
- AC1: 'Reply on RC1', Patrick Schleppi, 26 May 2023
  
  We thank Reviewer 1 for carefully reading and commenting our manuscript. We answer his comments in the attached document.
  
  Citation: https://doi.org/10.5194/bg-2023-38-AC1
RC2:
'Comment on bg-2023-38', Anonymous Referee #2, 27 Apr 2023

The paper by Baer et al. monitored changes in rainfall, soil, and runoff chemistry (pH, acidity and ions) in the preAlps during more than 20 years during which actions were taken to abate atmospheric pollution in Europe. They also added N to plots to simulate an increase in N deposition. This is an important contribution to understanding the long-term effects of increased N deposition in forest catchments, as well as of the importance of abatements measurements. As such, the current study is a highly valuable contribution to understanding patterns of N deposition and its consequences for the biogeochemistry of European forests.
On the negative aspect, more care should have been taken to avoid incomplete sentences, colloquial expressions, and grammatical errors and typos. Thus, the paper should be carefully revised in this aspect.
Moreover, some questions remain untested. For example, one of the most intriguing results is the increase in the pH of rainfall across time, indicative of reduced acid rain due to political action, but the continuous decrease of soil pH and pH in runoff water across the same time period. I wonder whether there is any temporal correlation among these variables. Or in other words, are shifting pH values across time in rainfall, soil and runoff coupled or decoupled? Are these couplings/decouplings maintained across soil horizons and experimental treatments? The same can be asked for other variables like total acidity, etc. Analyzing these types of questions would, in my opinion, increase the novelty of the paper beyond what could be considered as a very informative and highly valuable report by providing more novel insights.
Other than that, I consider this paper as a very valuable contribution to the field.
L12. Safe for who or what? Please, clarify.
L14. "Increased" instead of "higher".
L16. "At risk of" instead of "endangered"
L20. pH of rainfall, right?
L43, 48, and 55. Not common to show chemical reactions in the introduction.
L94-105. I would suggest rewriting this whole paragraph to improve the flow.
L99-100. The structure of this sentence is a bit awkward; can you rephrase i?
L168-171. This sentence seems out of place here.
L254. Marginally significant what? This part of the sentence is unfinished.
L324. Or no longer have.
L334. We found a leaching peak.
L357. Cutting instead of curring.
L338. Would exhibit changes.

Citation: https://doi.org/10.5194/bg-2023-38-RC2
- AC2: 'Reply on RC2', Patrick Schleppi, 26 May 2023
  
  We thank Reviewer 2 for carefully reading and commenting our manuscript. We answer his comments in the attached document.
  
  Citation: https://doi.org/10.5194/bg-2023-38-AC2

Peer review completion

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

ED: Reconsider after major revisions (06 Jun 2023) by Teresa Gimeno

ED: Reconsider after major revisions (07 Jun 2023) by Anja Rammig (Co-editor-in-chief)

AR by Patrick Schleppi on behalf of the Authors (04 Jul 2023) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (18 Jul 2023) by Teresa Gimeno

Dear Drs. Bär and Schleppi,

Thank you very much for submitting a revised version of your manuscript addressing the comments by the two external reviewers. I believe that the manuscript has improved with respect to the original submission, although, in my opinion, there is still room for improvement. I do not deem it necessary to send this manuscript to be evaluated by the same or additional external, reviewers, but I provide some comments that I hope will be useful to improve the clarity of this manuscript.

As I said before and in line with the comments by the external reviewers, I believe that this manuscript presents the results of a very valuable experiment assessing the impacts of increased N deposition over a long-time scale (> 2 decades!) and at a large spatial scale (catchment). These types of studies are not common and are incredibly valuable to assess how our terrestrial and water ecosystems will evolve under future global change scenarios.

After reading and evaluating the manuscript for the second time, there are two things that are not clear to me yet. First, I am not sure whether the griding and felling treatment was an explicit part of the experimental design. It appears that this was an intentional treatment that was replicated in the control and in the experimental plots. Also, it seems like the results of this treatment were presented in detail in another study, but here it appears that this experimental treatment still had a very significant effect on the acidity of not only soil, but also of runoff water. However, this does not become clear until the reader finishes the discussion. Therefore, it might be necessary to add a section in the introduction explaining how reducing basal area would have a feedback mechanism on soil and runoff properties derived from decreased N demand from plant uptake and decreased litter input. These mechanisms are invoked in the discussion without having been introduced properly. In addition, the authors would need to consider incorporating this treatment explicitly in their experimental design and statistical analyses.

Second, the experimental design and measurement methodologies are not explained clearly. It is not clear why while there were two catchments, the experimental plots where rainwater (with or without extra N) were outside these catchments. The spatial details and chronology of the study are not explained clearly. Furthermore, there seem to be some inconsistencies in how water and soil samples were measured. The information regarding the analytical methodologies is hidden in different sections of the methods, but I think that in this study this is critical information and deserves a sub-section of its own. Also, in this sub-section, the authors should clarify potential changes in instruments or methodologies that were (understandably) implemented over the course of such a long-term experiment. Also, why are there experimental pairs of plots outside the monitored catchment? And why are there no experimental pairs of plots inside catchment two?

In line with these comments, one wonders whether the observed decrease in pH of runoff water in control plots would be simply due to the addition of extra rainwater, which already has a baseline concentration of N forms. How do these trends compare to similar measurements of pH in runoff water from similar catchments?

In addition, I provide some more comments that I hope they will improve the clarity of the language, which can still be polished. Comments that I consider are more relevant are separated by an extra line space. Line numbers (L) refer to the version with track changes.

L11 N deposition is not exclusive to forests. Also “forest ecosystems” can be shortened to “forests”.
L12-13 what do you mean by “sensitive receptors of the environment”.
L20 name these “soil acidification processes”.
L20-21 I do not understand the addition of the clause “while previous reports…” I think you could remove this from the abstract.
L23 the sentence “In spite of…” is repetitive, this same idea is given in the previous sentence.
L23 Remove “First”.
L25 what does “they” refer to here?
L31 Close your abstract with a brief conclusion or future perspective derived from your study.
L33 remove “more than”.
L36 more concise language: remove “are a” and change “of” by “from”
L38 more concise language: “and deposited in dry or wet form”.
L40 New paragraph starting with “As large regions…”
L54 provide a citation for this.
L62-64 I am not sure this is very relevant for this study and you could consider removing it.
L116 “at a site”
L125 “a deeper water table” instead of a “lower laying water table” (unless you meant a “more shallow water table”, which is not entirely clear here).
L130 specify the net rate of which process.
L136 remove “their”.
L146 & 179 provide manufacturer details.

L141 Here is where you should present the experimental design in detail and how the treatments were applied (currently L180-191)

L140 I assume these values of N deposition refer to the levels at the beginning of the experimental treatment, please clarify this.

L142 According to the responses to the reviewers’ comments, one of the catchments was not delimited by a trench, could you clarify this?

L166-167 Can you provide the maker, model and manufacturer details of the rain gauges. Also, how did these gauges collected and measured precipitation? Were the two rain gauges located in the same open area? Where was this area with respect to the experimental and control plot?

L174-177 How consistent were measurements taken more than two decades apart? How can you relate potential drifts with changes in measurement instruments, technology and their precision?

L190 what does “it” refer to here?
L192 what do you mean here by “in general”?
L214 provide the corresponding package citations for this function.
L215-216 This belongs to the results section, also, what are “periods” here?
L232 I do not understand this, I thought the variable “treatment” was included as a fixed factor on the models testing for the trends of soil acidity.
L245 remove “highly” and “atmospheric” (precipitation is by definition atmospheric).
L246 what do you mean here by “at the beginning of the project”? Maybe use instead “at the start of the experimental treatment” or “pretreatment measurements”?
L247 remove “almost”.
L247 & 250 These rates of change should be accompanied by an error estimate.
L248 what do you mean here by “weather events”?
L251 where do you show this result? Please refer to the corresponding table or figure.
L263-269 replicates instead of replications. Also, consider writing your results in past tense.
L269 “were the most acidic” instead of “have the highest acidity”.
L307 add “of precipitation water” after NO3-.
L319 pH instead of “it”.
L325 remove “likely”
L333-334 I am not sure I understand this, soil acidification has to be intrinsically linked to the concentration of H+.
L339 remove “in greater detail”.
348-349 I am not sure I follow how decreased N demand by the trees has a feedback effect on the concentration of Ca+, could you explain this?
L349 But I thought the felling resulted in a decrease in N demand, this is contradictory.
L350-351 Please provide a citation to support this claim of low nitrification rates.
L364 what soil property is buffered?
L371 provide citations and examples. Also consider “lack” instead of “don’t contain”.
L376 how would deeper rooting help buffer acidification due to increased N deposition?
L376-379 This line of argumentation linking plant litter, denitrification and cation transport between moulds and depressions is not clear.
L383 “peaked”
L416 “emissions of N and S oxides”

Figure 1. Add a legend indicated what are the blue and red circles.

Figure 2-6. Please, increase font size of all labels and axes.

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ED: Reconsider after major revisions (27 Jul 2023) by Anja Rammig (Co-editor-in-chief)

AR by Patrick Schleppi on behalf of the Authors (31 Aug 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (13 Sep 2023) by Teresa Gimeno

ED: Publish subject to technical corrections (21 Sep 2023) by Anja Rammig (Co-editor-in-chief)

AR by Patrick Schleppi on behalf of the Authors (29 Sep 2023) Author's response Manuscript

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Short summary

Nitrogen (N) deposition to forest ecosystems is a matter of concern because it affects their nutrient status and makes their soil acidic. We observed an ongoing acidification in a montane forest in central Switzerland even if the subsoil of this site contains carbonates and is thus well buffered. We experimentally added N to simulate a higher pollution, and this increased the acidification. After 25 years of study, however, we can see the first signs of recovery, also under higher N deposition.