the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Aug 2025
Environmental conditions rather than nitrogen availability limit nitrous oxide (N2O) fluxes from a temperate birch forest
Abstract. Forest ecosystems play an important role in the terrestrial nitrogen (N) cycle, accounting for over a quarter of the land area of the Earth. However, our understanding of nitrogen dynamics in forest systems is limited. The consequences of N deposition to forest ecosystems are often overlooked. In this study, dry deposition of NH3 was replicated over a two-year period in a temperate semi-natural birch forest via a unique custom-built automated NH3 release system to investigate the impact on emissions of the greenhouse gas nitrous oxide (N2O). This study provides evidence that in both natural forest soils (in-situ) and soils under controlled laboratory conditions (ex-situ), the substantial addition of reduced N compounds (NH3/NH4+) had no direct impact on N2O emissions. Emissions of N2O from these soils were dependant on the meeting of several additional thresholds, below which N2O producing activity was constrained. When environmental conditions in-situ were considered warm and wet (soil temperature >12 °C and volumetric water content >20 %), emissions of N2O were an order of magnitude higher than when either of these thresholds was not met, regardless of exposure to NH3 deposition. Ex-situ experiments indicated that microbial activity in the soils was highly constrained by the availability of labile carbon. The addition of glucose to these soils resulted in a considerable increase in N2O emissions after N application. While cumulative NH3 deposition to the in-situ soils was relatively large over the measurement period, there was no accumulation of mineral N observed in the soil, suggesting plant-uptake of N was able to mitigate N loading. The implication of these results is that forest ecosystems may be able to mitigate localised NH3 pollution plumes, in the short-term at least, without incurring an N2O penalty. However, the long-term impacts of N enhancement remain unclear and further long-term field experiments are required to examine the impact of prolonged exposure to high quantities of N deposition to forest soils.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-3233', Anonymous Referee #1, 02 Sep 2025
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AC1: 'Reply on RC1', Galina Toteva, 19 Sep 2025
We are grateful to Reviewer 1 for their constructive comments. Please find below our responses.
- L167, please correct information around analytical uncertainties-one number for N2O and one for CH4
Thank you for highlighting a potential misinterpretation around the analytical uncertainty of N2O and CH4. I have clarified this in text. Please see below.
“The analytical uncertainty in flux methodology was calculated to be ±0.05 nmol m−2 s−1 for N2O fluxes and ±0.58 nmol m−2 s−1 for CH4 fluxes (Cowan et al., 2025).”
- Some of the highest observed N2O fluxes occurred in July and August 2021, prior to the start of NH3 I cannot find any narrative or explanation for this in the discussion section. Do you have any idea why this happened? What were the environmental conditions on the experimental site during this time?
Yes indeed, the period of the highest observed N2O fluxes coincided with a period of warm and wet conditions, which supports the dual temperature – moisture threshold suggested by this study. I have added this in lines 461 to 466.
“This is consistent with the findings of this study which suggest that environmental factors had a more pronounced effect on N2O fluxes relative to the experimental treatment (gaseous NH3 addition). For instance, the highest in-situ N2O fluxes were observed between June and August 2021 (Fig. 2) which corresponded to a relatively warm (soil temperature > 12 °C) and wet period (VWC > 20 %) (Fig. S4). In contrast, N2O fluxes during June to August 2022 were relatively low (median flux < 0.05 nmol N2O m−2 s−1), which could potentially be explained by a period of drought that summer (VWC < 15%). These findings suggest a dual temperature-moisture threshold which could be controlling soil N2O fluxes.”
- In L507-508 you say that observations in this study are consistent with the IPCC EF for non-agricultural soils. However, there is no mention anywhere in text what an indicative EF from the current experiment could be. Could you stipulate based on data from the control and impact chambers?
N2O fluxes have been calculated as a proportion of NH3 dry deposition, which corresponded to 0.38% from control areas and 0.05% from impact areas (please see Table 1 below). According to the IPCC methodology fluxes from control areas are subtracted from fluxes from impact areas. In this case, this would result in a negative value. Even though fluxes of N2O were low throughout the study period (both in-situ and ex-situ), there was no evidence of uptake of N2O by the soil and previous work by Cowan et al.(2014) have suggested that negative N2O fluxes are often a methodological artefact. Presenting a negative emission factor could be misleading to the reader, which is why we have been reluctant to include these calculations in the manuscript.
Table 1 Fluxes of N2O as a proportion of NH3 dry deposition.
Chamber type
Mean NH3 deposition, kg N ha-1 yr-1
Mean N2O flux, nmol m-2 s-1
N2O flux, kg N ha-1 yr-1
N2O as a % from N deposition
control
2.55
0.048
0.009
0.38
impact
20.55
0.054
0.011
0.05
Citation: https://doi.org/10.5194/egusphere-2025-3233-AC1
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AC1: 'Reply on RC1', Galina Toteva, 19 Sep 2025
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RC2: 'Comment on egusphere-2025-3233', Anonymous Referee #2, 08 Sep 2025
This study assesses the effect of NH3 deposition on N2O emissions in a temperate birch forest. NH3 deposition was simulated in the forest for 2 years throuhg an NH3 release system, and N2O emissions subsequently measured. Additionally, they performed laboratory studies with soils from the same forest, where they simulated N deposition under the different amounts of C. The study shows that NH3 deposition did not lead to an increase in N2O emissions in the forest, which only occurred in the ex-situ/laboratory experiemnts in the presence of labile C and under warm and moist conditions. This shows that forest may be able to mitigate NH3 pollution, at least on the short term, without leading to N2O penalies.
The topic is relevant and very well fits the scope of the journal. The study is well designed, and very well presented. The methodoloy used is appropiate to address the question. The soil incubations performed on top of the field trial are an excellent approach to understand the mechanisms that justify the observed resesults. The figures are clear and the article is very well written. Therefore, I only have very minor suggestions:
- Please clarify in the methods why the addition of N In the lab is done with both NH4+ and NH4NO3, instead of opting to only apply NH4+, which better simulates the addition of NH3 as it was done in the field. This comes clear in the results, in order to check the effects of both reduced and oxidized N forms, but not before.
- In Fig 2 it could be indicated that the NH3 release system was not working in July 22, as the previous 2 months N2O fluxes were high, and higher in the impact areas.
- Is there a reason in the field for the higher fluxes, before the start of the application of the NH3 treatment, in the control areas? Fluxes reached very high levels in June-July 2021, which did not happen again. Can meteorologiocal conditions (temperture or soil moisture) at that time help explaining these extremely large fluxes?
- L384-385 and Table 2: I do not think the abbreviation AN was explained before, please clarify that this stands for NH4NO3.
Citation: https://doi.org/10.5194/egusphere-2025-3233-RC2 -
AC2: 'Reply on RC2', Galina Toteva, 19 Sep 2025
We are grateful to the Reviewer for their constructive comments. Please find our responses below.
- Please clarify in the methods why the addition of N In the lab is done with both NH4+ and NH4NO3, instead of opting to only apply NH4+, which better simulates the addition of NH3 as it was done in the field. This comes clear in the results, in order to check the effects of both reduced and oxidized N forms, but not before.
A clarification has been added in the methods section (line 186) as to why N was added both in the form of NH4+ and NH4NO3.
“N was applied in the form of either NH4+ in aqueous solution (Cowan et al., 2024) or NH4NO3 in order to study the effects of reduced and oxidised N forms.”
- In Fig 2 it could be indicated that the NH3 release system was not working in July 22, as the previous 2 months N2O fluxes were high, and higher in the impact areas.
Fig.2 and its caption were edited by adding an asterisk to indicate the temporary issue with the NH3 release system in July 2022.
Figure 2 Fluxes of N2O over the duration of the experiment. Fluxes measured from control area (blue) and area where the impact of NH3 deposition was expected (orange) are shown. The horizontal dashed lines mark the minimum detectable flux. The vertical line denotes the start of the NH3 release. An asterisk (*) in July 2022 indicates that the NH3 release system was inactive for most of the month due to technical issues.
- Is there a reason in the field for the higher fluxes, before the start of the application of the NH3 treatment, in the control areas? Fluxes reached very high levels in June-July 2021, which did not happen again. Can meteorologiocal conditions (temperture or soil moisture) at that time help explaining these extremely large fluxes?
Yes indeed, the period of the highest observed N2O fluxes coincided with a period of warm and wet conditions, which supports the dual temperature – moisture threshold suggested by this study. I have added this in lines 461 to 466. This was also addressed as part of Reviewer 1, comment #2.
“This is consistent with the findings of this study which suggest that environmental factors had a more pronounced effect on N2O fluxes relative to the experimental treatment (gaseous NH3 addition). For instance, the highest in-situ N2O fluxes were observed between June and August 2021 (Fig. 2) which corresponded to a relatively warm (soil temperature > 12 °C) and wet period (VWC > 20 %) (Fig. S4). In contrast, N2O fluxes during June to August 2022 were relatively low (median flux < 0.05 nmol N2O m−2 s−1), which could potentially be explained by a period of drought that summer (VWC < 15%). These findings suggest a dual temperature-moisture threshold which could be controlling soil N2O fluxes.”
- L384-385 and Table 2: I do not think the abbreviation AN was explained before, please clarify that this stands for NH4NO3.
Table 2 was edited by explaining the meaning of the AN abbreviation. Please see below.
“Nitrogen was applied either in the form of ammonium (NH4+) or ammonium nitrate (AN, NH4NO3).”
Citation: https://doi.org/10.5194/egusphere-2025-3233-AC2
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I have thoroughly enjoyed reviewing this manuscript describing experiments to quantify impact of NH3 deposition on N2O emissions from forest soil, and additionally impact of potential carbon limitation on the magnitude of emissions. The manuscript is well written, using clear and precise language, and contains sufficient detail to easily follow described work. In particular, methodology section is well developed and would be useful to early career researchers seeking to improve their understanding of these methodologies.
The topic itself is of importance due to large uncertainty associated with N2O emissions from non-agricultural soils and in response to NH3 deposition. While tree belts are suggested as a potential measure to mitigate impacts of high NH3 producing industries (i.e. pig and poultry farms) by capturing NH3 plume, little is known of the impact on N2O. This manuscript provides experimental data to address this knowledge gap and clearly shows that environmental conditions (soil moisture and temperature) as well as carbon availability can be the main drivers behind N2O emissions rather than N availability.
I only have few minor comments below:
L167, please correct information around analytical uncertainties-one number for N2O and one for CH4
Some of the highest observed N2O fluxes occurred in July and August 2021, prior to the start of NH3 release. I cannot find any narrative or explanation for this in the discussion section. Do you have any idea why this happened? What were the environmental conditions on the experimental site during this time?
In L507-508 you say that observations in this study are consistent with the IPCC EF for non-agricultural soils. However, there is no mention anywhere in text what an indicative EF from the current experiment could be. Could you stipulate based on data from the control and impact chambers?