the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Apr 2026
Measurement report: Soil reactive nitrogen gas emissions from the Tibetan Plateau
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 NOx 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 NOx 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 (NOx-independent) daytime atmospheric HONO sources and modulate the regional atmospheric chemical balance by elevating the HONO/NOx 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1913', Anonymous Referee #2, 18 May 2026
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RC2: 'Comment on egusphere-2026-1913', Anonymous Referee #3, 19 May 2026
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?
Citation: https://doi.org/10.5194/egusphere-2026-1913-RC2 -
RC3: 'Comment on egusphere-2026-1913', Anonymous Referee #1, 19 May 2026
Summary. The manuscript by Deng et al. reports on fluxes of reactive nitrogen (Nr = NO, NO2, and HONO) from soil samples collected from sites in the Tibetan Plateau. Fluxes were measured from sieved soil during microcosm experiments. In addition, the flux measurements are accompanied by measurements of soil bio- chemical-physical parameters in an effort to identify drivers of soil emissions. Lastly, the authors carried out experiments to study the impact of air pollution components on soil emissions by ‘fumigating’ soil samples with a mixture of NOx and ozone prior to measuring soil Nr fluxes. The manuscript concludes by using a model to understand the impact of Nr emissions on air quality in the Tibetan Plateau region. Overall, the study provides useful observations of Nr fluxes and characterizes the microbial community in the various soils samples. For this reason, it would be useful to publish these observations and measurements. However, I have some serious concerns about the experimental approach, its environmental relevance, and the rationale behind some of the experiments. Thus, I recommend the authors address the following concerns prior to acceptance.
Major concerns. The most serious concern I have has to do with how the samples were handled prior to measuring Nr fluxes. Following sampling, the authors report storing the soil at -20 degrees C and then thawed within a matter of hours/days prior to sieving and flux measurement. How can we be sure that this this treatment provides a representative sample. Presumably, the freezing can lyse cells and release Nr into the soil matrix. While it is conceivable that the soil may have been exposed to such temperatures naturally during wintertime, in the real environment, soil has months of time to thaw and for microbial communities to change. Have the authors conducted samples on freshly collected soil that has not undergone this treatment to verify that their approach is valid? Without demonstrating this, we cannot trust that these measured Nr and mineralization and nitrification rates are representative of what is happening in nature. For that reason, it should be very clear that these Nr fluxes are observed, and the appropriate disclaimers should be made.
The other concern I have is regarding the rationale behind the “fumigation” studies. In them, the authors exposed soil samples to NOx and Ozone to see what kind of effect these gases have on Nr emissions. A convincing rationale for this experiment is not presented. Also, I don’t see how this experiemnt will yield any useful knowledge as the experiment seems to be flawed. For example, ozone will react extremely rapidly with soil organic matter and effectively be removed as soon as it is introduced into the chamber. If NO doesn’t react with ozone to form NO2 in the chamber, then it may artificially affect the net direction of flux based on the soil compensation point. So, I do not see how results of the fumigation studies could be used to draw any meaningful conclusions.
Specific comments.
Line 86: I am not sure what “sampling via cold chain” is. Please clarify.
Line 121: In addition to instrument limits of detection based on concentration, can the authors please provide limits of detection for fluxes. What is the minimum flux detectable for each gas?
Line 300: replace “detected to emit of” with “found to emit”
Line 348-9: The authors write, “This demonstrates that high NO emissions from cropland soils probably coincide tightly with Actinobacteriota abundances. Are the authors suggesting that the NO emissions are coming from the Actinobacteria. Please explain if so.
Line 373-4: It was found that soil pH tended to increase after fumigation, expecially in forest soils, with an average increase of ~0.53. However, the authors do not explain what the increase in pH is due to. Has this ever been documented in the literature? How sure are the authors that the pH change is not due to some underlying process occurring over time that is independent of the fumigation procedure? The microbe communities will be in constant flux once removed from the environment and subjected to all the lab treatments, so it may not be surprising to see such changes. Do the authors have controls with which to compare soil changes (in pH but also for nitrification and mineralization rates)? If so, those should be included and discussed.
Citation: https://doi.org/10.5194/egusphere-2026-1913-RC3
Data sets
Soil Reactive Nitrogen Gas Emissions from the Tibetan Plateau L. Deng et al. https://doi.org/10.5281/zenodo.19345058
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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
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.
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.
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:
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.