Measurement report: Soil reactive nitrogen gas emissions from the Tibetan Plateau

Deng, Lingling; Chen, Yuhan; Ye, Chunxiang; Wang, Rui; Wang, Ruhai; Fu, Zehua; Zhao, Hongfang; Wu, Dianming

doi:10.5194/egusphere-2026-1913

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https://doi.org/10.5194/egusphere-2026-1913

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27 Apr 2026

| 27 Apr 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Measurement report: Soil reactive nitrogen gas emissions from the Tibetan Plateau

Lingling Deng, Yuhan Chen, Chunxiang Ye, Rui Wang, Ruhai Wang, Zehua Fu, Hongfang Zhao, and Dianming Wu

Abstract. The Tibetan Plateau, highly sensitive to climate change, exerts strong atmospheric oxidation capacity partly through rapid cycling of atmospheric reactive nitrogen (Nr). Soil Nr emissions play a crucial role in atmospheric nitrogen cycling and oxidation capacity, yet their emissions on the Tibetan Plateau remain poorly quantified. Combining dynamic chamber measurements, laboratory analysis, and a parameterized model, we assessed the characteristics, driving factors, spatial distribution and annual emissions of soil Nr in the Tibetan Plateau. We found that the optimum soil fluxes of nitrous acid (HONO), nitric oxide (NO), nitrogen dioxide (NO₂), and ammonia (NH₃) were 21.6 ± 8.4, 43.7 ± 14.7, 15.8 ± 1.3, and 190.0 ± 116.3 ng N m⁻² s⁻¹, respectively. These emissions were mainly influenced by soil pH, nutrient content, and microbial community composition. After exposure to atmospheric NO_x and ozone (O₃), Nr emissions from forest soils were enhanced but those from croplands and grasslands were suppressed. The estimated annual emissions of soil HONO, NO, and NO_x from Tibetan Plateau to be 7.0 ± 3.4 Gg N yr⁻¹, 11.6 ± 7.8 Gg N yr⁻¹, and 20.3 ± 7.0 Gg N yr⁻¹, respectively. Soil HONO emissions contribute approximately 10.5 % of the external (NO_x-independent) daytime atmospheric HONO sources and modulate the regional atmospheric chemical balance by elevating the HONO/NO_x ratio. Our results provide the first integrated quantification of soil Nr emissions on the Tibetan Plateau and emphasize their importance for regional nitrogen cycling and atmospheric oxidation capacity.

Received: 03 Apr 2026 – Discussion started: 27 Apr 2026

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Lingling Deng, Yuhan Chen, Chunxiang Ye, Rui Wang, Ruhai Wang, Zehua Fu, Hongfang Zhao, and Dianming Wu

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RC1:
'Comment on egusphere-2026-1913', Anonymous Referee #2, 18 May 2026 reply
The manuscript entitled “Measurement report: Soil reactive nitrogen gas emissions from the Tibetan Plateau” by Deng et al. presents a comprehensive investigation of soil reactive nitrogen (Nr) gas emissions from Tibetan Plateau soils. The study combines soil microbial functional gene data, soil physicochemical properties, and Nr gas emissions measured during wetting–drying cycles in laboratory dynamic chamber experiments. These datasets are then integrated into a parametric model to (i) characterise differences in Nr emission patterns across land‐use types and (ii) estimate the contribution of Tibetan Plateau soils to the missing daytime HONO source via a simple upscaling approach.
The manuscript is concise, well-structured, and generally clearly written. The authors manage to harmonise a complex set of data—ranging from microbial functional genes to dynamic chamber flux measurements—into a coherent picture of soil Nr emissions and their underlying mechanisms. Their conclusions align well with the last decade of soil HONO research and provide (albeit statistical but) mechanistic insight into the contribution of microbial processes and chemical transformations of soil N to reactive N gas emissions.
The upscaling approach is necessarily simplified and comes with limitations, but the systematic consideration of multiple land‐use types is a clear strength of this work and significantly enhances its value. While I think the fumigation experiment could be more prominently showcased in the main text (see Comment 2 below), I also understand the authors’ concern that this could dilute the main narrative.
Overall, I find the study robust and well-presented, thus I recommend publication after minor revision.
Below, I list a few major and several minor comments that I hope will help to further strengthen the manuscript.
Major comments
Optimum flux vs. integrated flux patterns (Fig. 3)

The comparison between optimum flux and integrated flux across land use types is particularly interesting. In several cases, these two metrics appear to show similar patterns across land uses, while the behavior of HONO and NO₂ differs more markedly.
I encourage the authors to explicitly discuss these similarities and differences in the Results and/or Discussion sections, in connection with Fig. 3.

For example, under which land use types do optimum and integrated fluxes track each other closely? Where do they diverge? How do these patterns differ? and what might this imply about the temporal dynamics of production and consumption processes?

Fumigation experiment in the main text

The fumigation experiment provides important insight into microbial N balance and potential changes in atmospheric N input that altered microbial activity across different land uses.
At the moment, this appears mainly in the Supplementary Information and is only briefly referenced.

I recommend including at least one key figure or summary panel from the fumigation experiment in the main text (e.g., a figure showing how fumigation affects HONO/NO emissions or relevant N processes across land-use types), even if this information is not directly used in the upscaling.

A concise description of the main fumigation results would help readers better appreciate the mechanistic link between microbial dynamics and Nr emissions and would make this valuable part of the work more visible.

Minor comments
Line 52. Please clarify the phrase “nitrate leaching” as:

“nitrate leaching to groundwater/aquifers”
Lines 82–85. The description of the sampling is currently somewhat ambiguous. It appears that: larger composite samples were taken for the dynamic chamber experiments (three subsamples in 10m x 10m grid, sampled diagonally and homogenised? If so, how many kilograms approximately?), and smaller subsamples (e.g. 1–2 g) were used for microbial analyses.
Please clarify:
Exactly how the soil samples were collected (e.g., “three subsamples per 10m x 10m plot, taken diagonally across the plot, from 0–5 cm depth, then homogenised to form one composite sample per plot”), and why the number of samples are different across land use types

At what stage homogenisation occurred, and

Whether the microbial subsamples (1–2 g) were taken from the same homogenised 0–5 cm composite used for the chamber experiments.

Line 94 – The phrase “measured by a continuous flow analyser” is too vague, since the analyser is an instrument, not a method. Please specify the analytical method used (e.g., colorimetric method for nitrate and ammonium, specific reagents/reactions, detection wavelengths), or at least provide a reference to a standard method.
Line 130 – Since a three‐valve rotary system is employed to alternate between chambers, the current flux calculation equation may not fully account for the time interval between chamber switching. Please clarify whether any interpolation was used to estimate fluxes between switching events.
If fluxes were assumed constant over certain intervals, this assumption should be stated. Furthermore, the flux equation should be reformulated to correctly reflect the time-averaging or interpolation procedure inherent to the switching system.
Line 148 – The choice of a 5-day fumigation period should be briefly justified:
Was this based on previous studies, preliminary tests, or methodological constraints? How sensitive do the authors expect their results to be to the duration of fumigation (e.g,. would shorter or longer fumigation times yield qualitatively similar effects on the microbial community and Nr emissions)?

A short rationale here would help readers interpret the fumigation results and understand the scope of inference.
Data availability – Zenodo repository

In the Zenodo repository, it would be helpful if file names were provided in English.

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Citation: https://doi.org/10.5194/egusphere-2026-1913-RC1
RC2:
'Comment on egusphere-2026-1913', Anonymous Referee #3, 19 May 2026 reply
General Comment
Reactive nitrogen species play important roles in atmospheric chemistry; however, soil emissions of Nr remain poorly constrained. In this study, Deng et al. collected soil samples from different landscapes across the Tibetan Plateau. Emissions of HONO, NOx, and NH3 from these soils were measured, and their relationships with soil properties and microbial community distributions were investigated. In addition, the impacts of NOx and O3 fumigation were examined. Finally, the laboratory results were combined with a parameterized model to estimate annual emissions of these Nr species. Considering the importance of Nr sources for atmospheric chemistry over the Tibetan Plateau, this study provides valuable insights into the Nr budget on the Tibetan Plateau and its potential atmospheric impacts.

Major Comments
Soil–atmosphere exchange of reactive nitrogen is typically bidirectional (Bao et al., 2022). However, by using dry Nr-free air as the carrier gas, only net emissions can be observed. In addition, surface water exchange strongly affects Nr emissions (Xue et al., 2024). Under dry-air conditions, surface water evaporation is likely maximized, which may enhance emissions compared with natural conditions. Related discussion is needed.

In the field, wet–dry cycles are regulated by water exchange at the soil–air interface and are influenced by environmental factors such as temperature and relative humidity. How does the current study account for these processes when extrapolating laboratory results to field conditions?

Considerable NO2 emissions were observed. However, neither nitrification nor denitrification mechanisms fully explain direct NO2 emissions. This should be further discussed.

The effects of NO/NOx/O3 exposure on Nr emissions were investigated, and impacts were observed. However, were these effects incorporated into the estimation of annual emissions?

The derived Nr emission fluxes are based solely on laboratory experiments. These estimates should be compared with published field measurements and/or top-down estimates to evaluate their representativeness and reliability.

Minor Comments
L62–64: This conclusion remains controversial. Please also consider the discussions and comments published after the paper appeared.

L77: It would be helpful to include a map showing all sampling sites.

L137: Why do the first two treatments focus on NO, whereas the other two focus on NOx + O3? Since NOx reacts rapidly with O3, the actual concentrations inside the chamber may differ substantially from the initial values.

L217: BLH usually exhibits strong diurnal variation rather than remaining constant.

L363–364: Nr emissions were generally observed from soil based on results of this study. Why did cropland and grassland soils act as sinks for HONO and NO?

Reply
Citation: https://doi.org/10.5194/egusphere-2026-1913-RC2

Lingling Deng, Yuhan Chen, Chunxiang Ye, Rui Wang, Ruhai Wang, Zehua Fu, Hongfang Zhao, and Dianming Wu

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https://doi.org/10.5194/egusphere-2026-1913-supplement

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Soil Reactive Nitrogen Gas Emissions from the Tibetan Plateau L. Deng et al. https://doi.org/10.5281/zenodo.19345058

Lingling Deng, Yuhan Chen, Chunxiang Ye, Rui Wang, Ruhai Wang, Zehua Fu, Hongfang Zhao, and Dianming Wu

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Short summary

Soil reactive nitrogen (Nr) gas emissions from the Tibetan Plateau remain poorly quantified, especially nitrous acid (HONO) and nitrogen oxides (NO_x). We find soil HONO and NO_x emissions are substantial across different land use types, with annual emissions of 7.0 ± 3.4 and 20.3 ± 7.0 Gg N yr⁻¹, respectively. Soil HONO emissions contribute approximately 10.5 % of the external daytime atmospheric HONO sources and modulate the regional atmospheric chemical balance by elevating the HONO/NO_x ratio.


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