the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jul 2025
In situ production of hybrid N2O in dust-rich Antarctic ice
Abstract. Nitrous oxide (N2O) is a potent greenhouse gas involved in the destruction of stratospheric ozone. Past atmospheric mixing ratios of N2O are archived in ice cores; however, the presence of in situ N2O production in dust-rich Antarctic ice complicates their accurate reconstruction, especially during glacial periods. This production occurs in extremely cold ice and without sunlight. This study aims to understand the reaction producing N2O in Antarctic ice by identifying the precursors and the reaction pathway. We compared the oxygen and nitrogen bulk and position-specific isotope composition of in situ N2O in ice cores to the isotopic composition of nitrate (NO3-), a possible precursor of N2O. The 15N signature of NO3- is fully transferred into the central N atom (Nα) of in situ N2O, but it is not transferred into the terminal N atom (Nβ), resulting in a 50 % transfer of the 15N signature of NO3- into the bulk 15N isotopic composition. These findings suggest that the in situ N2O production involves two different nitrogen precursors present in ice: the central N atom (Nα) originates from NO3- and the terminal N atom (Nβ) from a different precursor not yet identified. Oxygen isotope analysis shows that NO3- cannot be the only reservoir for the O atom of in situ N2O. Temperature, pH, and absence of sunlight in Antarctic ice point to an abiotic N-nitrosation reaction. The limiting factor of the reaction is probably associated with mineral dust and might be Fe2+, reducing NO3- to NO2- or the precursor of the Nβ atom. The site preference (SP) values of in situ N2O are highly variable between different ice cores and depend on the bulk 15N isotopic composition of N2O, itself depending on the 15N isotopic composition of the NO3- precursor. This finding is unexpected because SP is usually determined by the production pathway through symmetric reaction intermediates that mix the N atoms in α and β positions and average out their isotopic difference. In contrast, our results provide the first evidence of a hybrid N2O production pathway involving an asymmetric intermediate that preserves the distinct 15N signatures of two different precursors – one contributing to the Nα atom and the other to the Nβ atom. This finding has important implications: in this pathway, SP reflects the isotopic difference between the two precursors rather than the pathway itself, challenging how SP is commonly interpreted in environmental studies.
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CC1: 'Comment on egusphere-2025-3108', Reinhard Well, 16 Jul 2025
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The paper supplies very interesting data on N2O isotopic values in antarctic ice cores, congratulation. Authors interpret the isotopic fingerprint consisting of d18O , d15Na, d15Nb, SP and d15Nbulk in conjunction with d15N and d18O of NO3. They concluded that there must have been abiotic N2O formation in dust rich sections by a process yielding hybrid N2O, ie were N2O molecules carry N from different precursors in the alpha and beta N position.
Although the previous assumption of biotic production of N2O in ice cores is considered less probable according to the introduction, authors might test whether biotic reduction of N2O to N2 could also explain isotopic patterns (see eg (Yu et al., 2020)), since high SP and d18O values observed in the ice cores (up to or even higher than 100 per mil) have also been found previously from microbial N2O reduction, e.g in groundwater. But a quick look at Fig 3 and Fig. 6d suggests that the data could probably not be explained by microbial N2O reduction, since high SP values were associated with lower d18O and vice versa. In contrast, microbial N2O reduction to N2 would lead to parallel increase in d18O and SP of residual N2O.
Using the measured data authors could evaluate whether biotic reduction can be excluded as a relevant pathway, which would further support their assumption that biotic processes were not relevant. From my view one possible way to address this could be easily done by adding a figure plotting SP against d18O and compare the distribution of the data points with the typical N2O reduction line (see eg, Yu et al 2020). Absence of N2O reduction would not be a strict proof for absence of microbial N2O production, but in terrestrial and aquatic ecosystems, N2O production by bacterial denitrification is almost always associated with N2O reduction to a certain extent.
Moreover, in view of the high relevance of N2O consumption in the stratosphere in in terrestrial and aquatic systems, it might be adequate to address N2O consumption processes in the paper, which has not been done as far as I can see. Thus, even without showing a SP/d18O plot, it might be useful to explain why consumption processes in the ice cores are not probable.
Reinhard Well
Yu, L., Harris, E., Lewicka-Szczebak, D., Barthel, M., Blomberg, M.R.A., Harris, S.J., Johnson, M.S., Lehmann, M.F., Liisberg, J., Müller, C., Ostrom, N.E., Six, J., Toyoda, S., Yoshida, N., Mohn, J., 2020. What can we learn from N2O isotope data? – Analytics, processes and modelling. Rapid Communications in Mass Spectrometry 34, e8858.
Yu, L., Harris, E., Lewicka-Szczebak, D., Barthel, M., Blomberg, M.R.A., Harris, S.J., Johnson, M.S., Lehmann, M.F., Liisberg, J., Müller, C., Ostrom, N.E., Six, J., Toyoda, S., Yoshida, N., Mohn, J., 2020. What can we learn from N2O isotope data? – Analytics, processes and modelling. Rapid Communications in Mass Spectrometry 34, e8858.
Citation: https://doi.org/10.5194/egusphere-2025-3108-CC1 -
RC1: 'Comment on egusphere-2025-3108', Dominika Lewicka-Szczebak, 05 Sep 2025
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Dear authors, this is a very important and interesting study and I’ve read your manuscript with high interest. It is written precisely and clearly with very well-prepared figures. All the data are easily to follow, and your interpretations are clear. You present a very interesting new theory on the asymmetric intermediate in hybrid N2O production, which is well documented with your data and very well presented in your Figure 8.
I have only one major concern about your data, because you have calculated the in-situ N2O isotopic signatures from very low N2O increase in concentration, as low as 20ppb. From my experience such low increases are not possible to be precisely determined for their isotopic signatures because of accumulation of all the uncertainties in the calculations, associated with isotopic signature of the background N2O and analytical errors. While analysing carefully all your data presented for me it is clear, that these lowest N2O samples may be biased, which I point out in below specific comments. This makes your interpretations less robust. I think if you disregard these lowest N2O samples (20ppb) you would get much clearer and reliable picture (see in the comments below).
L 74 (δ15Nα) (δ15Nβ) alpha and beta should be in uppercase, not lowercase
L133-135 – what are the products of the NO3 photolysis, can N2O be also produced in this process? Can the gaseous products be trapped in snow layers? Or is NO2- one of the photolysis products? NO2- could be then the precursor for further chemodenitrification.
L221 add basic details on the instruments used for N2O preparation and isotope measurements, also for SP analysis L227 and for NO3 analysis L305
L310 Fig.3 You calculated and showed isotope values for N2O in situ as low as 20 ppb. These values are probably very unsure due to very small difference between atmospheric background N2O concentration and measured N2O concentration. Taking into account all the uncertainties associated with background N2O values (which are major, as you discuss a lot in the manuscript, which assumptions should be accepted for these!) and measurement errors, these values are associated with large uncertainties. For soil N2O emissions we accept at least 65ppb increase in N2O concentration to calculate the isotopic signatures for in-situ production (for lower increases the calculated propagated error was too large).
(e.g. Buchen et al.,2018, https://doi.org/10.1002/rcm.8132)Please check such calculations of error propagation for your data and decide which minimal N2O in situ makes sense in your case. I expect not lower than 50 ppb. It is also visible on your graph 3B that the lowest in situ N2O concentrations are showing most extreme unrealistic isotope values.
L344 “N2O analyzed in the extracted air” or the calculated in situ N2O isotopic signature? – Fig. 4 shows “N2O in situ” – ok – but be precise in distinguishing these terms in the text
L375, Fig 6b and 6d clearly supports my assumption that for low in-situ N2O you get biased results, your SP values below 0 for very low N2O amount of 20ppm are just calculation artefacts
L385-387 SP values are independent of bulk 15N for microbial N2O production processes, but such relation is typical for N2O reduction where preferentially N-O bonds of light isotopes are broken which results in enrichment in bulk 15N and on alpha position (=increase in SP). The reported slope (Fig. 6d) is within the typical slopes observed for N2O reduction (see Yu et al., 2020, Lewicka-Szczebak et al., 2020).
L391, Fig. 7a – OK – this Figure clearly indicates that N2O reduction plays no role, because due the partial N2O reduction the alpha position would be strongly affected and the slope would be much higher than 1. But I think it would be worth to add a short discussion on N2O reduction in the manuscript, you may indicate that you disregarded this process based on this strong relationship, Fig 7a.
L395 ‘This correlation is robust when excluding the TG data.’ – because TG data are calculated for very low N2O concentration and are simply biased. I believe they should be excluded from any further interpretations.
L430 The d18O values are not really explained, for EDML exchange with water could explain the observed low d18O-N2O values, but for Taylor Glacier the very high d18O-N2O values, largely exceeding the d18O-NO3 values are not plausible – again in my opinion these values are biased and should be excluded from any further interpretations.
L460, Table2: I do not see the point of referring here all the isotope effects for the processes which you do not deal here with (single precursor N2O) after previous review papers, I would only give the data for hybrid N2O.
L465 you mean Table 2 here, I think your description of values for the different pathways in text is enough with respective citations, as suggested above I would shorten the table 2 only to values directly found in your study
L467-468 – a citation for this theory of the common intermediate and the mechanism of N2O formation is missing
L470 – as above – a citation for denitrification N2O formation mechanism is missing
L492 – citation needed
L505 – very good and helpful Figure, your explanation and visualisation of the idea of asymmetric intermediate in hybrid N2O production is very convincing
L542 – I agree that pH probably has the large impact on the extend of O-exchange, do you have data of pH for your samples? If we deal with very high pH it could have completely blocked the O-exchange and would support your theory. The lack of pH values is an important drawback, should be added if available.
Otherwise, without the known differences in pH, I do not believe that the mechanism could be so different to give totally opposite effects, from strongly negative to extremely positive. The story of O-exchange between nitrite and water is very convincing and the reported values are very plausible. However, the very high values in TK are suspicious, even with maximal branching effect they are a bit too high. Importantly, these values are based on 3 points calculated with in-situ N2O production of 20ppb, which is in my opinion to low to calculate representative and true isotope values. I strongly encourage you to critically evaluate this data. I would reduce the calculated in-situ N2O to the higher production only and present and further discuss only the proper values, not associated with possibly large errors. This would vastly improve your manuscript because you wouldn’t need to find theories for the values which are very unsure.
Citation: https://doi.org/10.5194/egusphere-2025-3108-RC1
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