Abiotic versus biotic controls on soil nitrogen cycling in drylands along a 3200 km transect

Liu, Dongwei; Zhu, Weixing; Wang, Xiaobo; Pan, Yuepeng; Wang, Chao; Xi, Dan; Bai, Edith; Wang, Yuesi; Han, Xingguo; Fang, Yunting

doi:https://doi.org/10.5194/bg-14-989-2017

Articles | Volume 14, issue 4

https://doi.org/10.5194/bg-14-989-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-14-989-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 4

Research article

|

03 Mar 2017

Research article |

| 03 Mar 2017

Abiotic versus biotic controls on soil nitrogen cycling in drylands along a 3200 km transect

Dongwei Liu, Weixing Zhu, Xiaobo Wang, Yuepeng Pan, Chao Wang, Dan Xi, Edith Bai, Yuesi Wang, Xingguo Han, and Yunting Fang

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Final revised paper (published on 03 Mar 2017)
Supplement to the final revised paper
Preprint (discussion started on 10 Jun 2016)
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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', Anonymous Referee #1, 15 Jun 2016
- AC1: 'Responses to Referee #1', Dongwei Liu, 01 Sep 2016
RC2: 'Review BG-2016-226', Anonymous Referee #2, 08 Jul 2016
- AC2: 'Responses to Referee #2', Dongwei Liu, 01 Sep 2016

Peer-review completion

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

ED: Reconsider after major revisions (20 Sep 2016) by Michael Weintraub

AR by Dongwei Liu on behalf of the Authors (24 Oct 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (02 Nov 2016) by Michael Weintraub

RR by Anonymous Referee #1 (04 Nov 2016)

RR by Anonymous Referee #3 (08 Nov 2016)

Suggestions for revision or reasons for rejection

There's not a lot to say here. It's an excellent study that seems to have benefited from excellent reviews and editorial management. I mainly point out typos below with a few other minor comments. Also I am often critical of figures but these ones are beautifully done. Overall A+ for all involved in the process, especially study authors.

L 74. typo: controlling = control
L70 microbially produced
L95 Antarctica
L 112: 3200 km
L 176. recommend change "pointed out" to "suggest" or "indicate." If using suggest, "could be" can be removed.
L 177. Rewriting suggestion: Thereafter, we refer to the areas with MAP from 36 mm to 102 mm (15 sites) and from 142 mm to 436 mm (21 sites) as the arid zone and the semiarid zone, respectively.
L 182. missing a space after NH4+
L 285. cool finding. I might mention this in the abstract. I think part of this is that anammox is so energetically inefficient it will occur really slowly and so might not have a huge impact on fluxes.
L 295. last sentence here doesn't add to paper. make more specific about what should be done or delete.
L 312. yes very good analysis, very interesting. I've seen in mountain systems with rich soils that have lots of organic matter the opposite--almost all NO3 of biological origin except for a big flush of atmospheric N from snowmelt. This contrast in a dryland makes a lot of sense.
L 319. Another point I might make here is that lack of water means that the highly mobile NO3- ions don't leach into streams and groundwater as much as in more mesic areas.
L 342. Sentence not grammatical. If I understand the point of the sentence, I might rewrite the second phrase to say something like "..., the expected d15N value for NH4+ derived from fixation."
L 344. change "research" to "study"
L 367. effects
L 369. compared
L 373. pools
L. 373. "Ammonification (N mineralization) BOTH supplies NH4 + for plant uptake and favourS soil nitrification"
L 374. enrich THE remaining
L 377. I am in favor of deleting the final throwaway line about effects of climate change. A stronger concluding sentence would summarize the key findings of the study, which show that readily interpretable broad-scale patterns of N cycling can be seen across a wide gradient of arid systems.

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RR by Anonymous Referee #4 (14 Nov 2016)

Suggestions for revision or reasons for rejection

Dear Mike,

I have reviewed the manuscript titled: “Abiotic versus biotic controls on soil nitrogen cycling in drylands along a 3200 km transect.” Overall, I think the manuscript provides useful data and information about processes controlling N cycling—it advances understanding of N cycling. I am confident it will be of use to the scientific community exploring how aridity impacts ecosystem N dynamics.

I think it would be useful for a native English speaker to do one more round of editing. I think the data and scope of the paper are great and this begs for the additional editing work to increase overall clarity.

I have a few concerns regarding interpretation of results and highlight below issues that I think require attention before publishing.

L62. I suggest reminding the audience what the authors mean by “open” N cycling.

L63-65. Good example of a sentence that can use editing

L68. I suggest clarifying this sentence as there are drylands in which hydrological N losses are larger than gaseous losses (at least on an annual basis and especially during a wet year).

L74. “That” can be deleted

L130. Perhaps clarify these were frozen. Refrigerator, at least to me, means +4 C.

L167. Please specify that SPSS is software.

L175. Figure 4 is out of order because figure 3 has not been mentioned yet.

L179. This sentence repeats the same information in L173

L200. It would be useful to edit the sentence so that “positive” is not perhaps interpreted as slope.

L208. But the 15N-NO3- could have also been enriched through other processes not involving NO3- losses. For example, during the transformation of NH4+ to NO3-, NO2- could have produced NO, enriching the leftover N oxidized to NO3-.

L224. But this could have also been produced by NO loss enriching NO2- during nitrification. Can the authors separate between these two processes and be sure of denitrification alone?

L232. This is a rough transition because the authors had been discussing denitrification. Nitrification appears suddenly and abruptly.

L247-249. To the best of my knowledge, the abiotic reduction of NO3- to NO2- (i.e., the “ferrous wheel hypothesis”) has not been confirmed within the context of “natural” soils—if it has, please provide references. Therefore, I suggest minimizing speculation and concentrating on NO2- rather than NO3-; NO2- can react abiotically to produce NO (Heil, J., Vereecken, H., Bruggemann, N., 2016. A review of chemical reactions of nitrification intermediates and their role in nitrogen cycling and nitrogen trace gas formation in soil. European Journal of Soil Science 67, 23-39)

Perhaps a mechanism for this NO3- reduction may involve coupled biotic-abiotic processes. I think linking these two papers together might work very well in this manuscript:

Roco, C.A., Bergaust, L.L., Shapleigh, J.P., Yavitt, J.B., 2016. Reduction of nitrate to nitrite by microbes under oxic conditions. Soil Biology & Biochemistry 100, 1-8.

Homyak, P.M., Kamiyama, M., Sickman, J.O., Schimel, J.P., 2016. Acidity and organic matter promote abiotic nitric oxide production in drying soils. Global Change Biology 10.1111/gcb.13507.

By linking these papers it may be possible to explain the increasing δ15N-NO3- in the arid zone as precipitation increases. In extremely arid regions the NO3- signature is dominated by atmospheric deposition but as soils become wetter, the biological signal begins to enrich δ15N-NO3-.

L 267. Please clarify: “while simultaneously NH4+ was also consumed.” Why would a gradual depletion in 15N-NH4+ suggest consumption? This is opposite to our understanding.

L 266-284. I have problems understanding how the decrease in 15N-NH4+ as precipitation increases can be explained by plant consumption. I agree that if N is limiting then isotope discrimination would be low (i.e., both 14 and 15 N would be taken up, say at the same rate). However, to cause a decrease in δ15N-NH4+ as precipitation increases you must consume more 15N than 14N or inject more 14N into the system. That mineralization caused fractionation sounds more plausible to me, but this is not well stated and it is confusing. It is critical to the paper to address this well. The shape of figure 3a is controlled by both 15N-NH4+ and bulk soil 15N. I suggest the authors systematically how 15N-NH4+ changes relative to the bulk soil. Doing so would help clarify against figure 2e.

L300. It would be useful to remind the reader that it is one O from air and two O’s from water. The ratio somewhat implies it but it is not clear.

L315-316. Since figure 5a does not show elevated NO3- concentrations, I think it would be useful to clarify that the concentrations are high because these measurements were made in arid regions. Is this right?

L325. For the definition of heterotrophic nitrification to work better, it is best to modify this sentence so that the definition is located in the middle of the sentence—as a reminder—rather than as a definition. By starting the sentence with the definition it implies it is a new term not well known by readers of the journal.

L340-345. The argument about biological soil crusts (BSC) needs to be better developed. Supposedly BSCs contribute to NH4+ inputs as precipitation decreases. However, figure 4 shows the complete opposite: that BSC gene abundance increases in wetter regions and are less important as precipitation decreases. Please elaborate. And this becomes a problem because the authors use the gene abundance data to suggest that nitrification is not likely to dominate N cycling in arid regions. So for nitrification the data are true but somehow for BSCs we are asked to not believe it.

Also, I am confused as to what is meant by “notice that biological N fixation provided NH4+ with the δ15N value around zero.” Is the reader supposed to be looking at data showing rates of N fixation in relation to δ15N-NH4+? If so, I don’t see those data in the manuscript.

L359. I think the authors have a nice study and great dataset such that they do not need to justify its importance by saying they were first to make this measurement—that is not too useful. What is useful is what we have learned from it.

L366. Perhaps say “15N-enriched” instead.

L600. Figure 1 legend: there is an extra “and”

L609. Figure 3 legend: the first two sentences are identical and can be condensed.

Figure 8. I suggest adding N losses via NO during the transformation of NH4+ to NO3- in both the arid and semiarid zone. The legend of figure 8 requires this information be present. If the authors wish to omit these processes from figure 8, they must rewrite the legend to be specific as to why they are excluded. The authors are showing several processes removing N from the system (NH3 volatilization, annamox, denitrification) and by that logic NO losses should be identified during the transformation of NH4+ to NO3- . Or at least acknowledge that nitrification can produce losses

A suggestion to increase overall clarity: I suggest ending paragraphs with a synthetic “take-home” message summarizing and concluding the paragraph. Ending paragraphs with citations, reinforcing information already known, does not help highlight how this paper is advancing our knowledge.

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ED: Publish subject to minor revisions (Editor review) (09 Dec 2016) by Michael Weintraub

AR by Dongwei Liu on behalf of the Authors (23 Dec 2016) Author's response Manuscript

ED: Publish subject to minor revisions (Editor review) (12 Jan 2017) by Michael Weintraub

AR by Dongwei Liu on behalf of the Authors (24 Jan 2017) Author's response Manuscript

ED: Publish as is (12 Feb 2017) by Michael Weintraub

AR by Dongwei Liu on behalf of the Authors (14 Feb 2017) Manuscript

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Short summary

The use of ¹⁵N natural abundance of soil ammonium and nitrate demonstrates a clear shifting contribution from abiotic to biotic controls on N cycling along a 3200 km dryland transect in northern China, with a threshold at mean annual precipitation of 100 mm. Abiotic factors were the main driver below threshold, shown by the accumulation of atmospheric N and NH₃ losses. In the area above threshold, soil N cycling was controlled mainly by biological factors, e.g., plant uptake and denitrification.