the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Sep 2026
Technical note: Cavity ring-down spectroscopy is a precise and accurate method for measuring 15N-NO3– in soil extracts from 15N tracer experiments
Abstract. Cavity ring-down spectroscopy (CRDS) provides a low-cost alternative to isotope ratio mass spectrometry (IRMS) for the analysis of 15N-NO3⁻ in nitrogen cycle assays, but its performance has not been tested systematically for highly enriched samples. Here we report results from 271 parallel analyses of 15N-NO3⁻ in soil extracts after chemical conversion to N2O, including low (0.36–1.45 at%) and high labelled samples (0.46–8.49 at%). We demonstrate that CRDS measurements deliver comparable accuracy and precision to continuous-flow Gas Bench-IRMS analysis (slope: 0.982 and 0.972, RMSE: 0.0018 and 0.254 at% for low- and high-labelled samples, respectively), thereby offering a robust and affordable alternative for classic 15N-NO3⁻ analysis.
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RC1: 'Comment on egusphere-2026-5050', Anonymous Referee #1, 18 Sep 2026
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AC1: 'Reply on RC1', Wasi Hashmi, 20 Oct 2026
We thank the reviewer for their enthusiastic endorsement of our manuscript.
Citation: https://doi.org/10.5194/egusphere-2026-5050-AC1
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AC1: 'Reply on RC1', Wasi Hashmi, 20 Oct 2026
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RC2: 'Comment on egusphere-2026-5050', Anonymous Referee #2, 25 Sep 2026
This is a short, but concise paper comparing 15N analysis of enriched samples conducted via laser spectroscopy and isotope ratio mass spectrometry (IRMS). The focus lies on the analysis of 15N-NO3- from 15N labelling experiments, after conversion to nitrous oxide (N2O).
While the paper is concise, I think that the discussion could be broadened. In this paper the 15N analysis in NO3- is discussed, after conversion to N2O. Therefore, the analysis actually conducted is for N2O, and the implications for 15N-N2O analysis in tracer experiments could be included in the discussion.
It seems that for the laser spectroscopy, the N2O concentration was about 25-times lower compared to the IRMS analysis (Line 78-82). This is only very briefly stated again in the last paragraph, but could be highlighted and discussed more.
Specific comments:
Line 25-29: When introducing 15N pool dilution and tracer approaches, I am not sure that the best reference is used, as the cited paper focusses on 15N analysis, not experimentation.
Line 41: Could the “lower capital and running costs” quantified, to support this statement?
Line 57: This is strictly speaking not true, as the NO3- pool also contain natural abundance 15N.
Line 98: The individual samples (grey circles) are not visible in the figure. Either, if it is important to show individual samples, they need to be made more visible, or the authors might only show means and uncertainties. I also noted that the colours mentioned in the caption do not match the actual colours in the figure.
Line 124: From my perspective, using a 68 hour time step is not common in isotope pool dilution experiments, but rather long, with the risk of e.g. remobilization. This might not be crucial for the scope of the paper, but I would still recommend rephrasing.
Citation: https://doi.org/10.5194/egusphere-2026-5050-RC2 -
AC2: 'Reply on RC2', Wasi Hashmi, 20 Oct 2026
We thank the reviewer for carefully reading the manuscript and providing helpful comments.
1. While the paper is concise, I think that the discussion could be broadened. In this paper the 15N analysis in NO3- is discussed, after conversion to N2O. Therefore, the analysis actually conducted is for N2O, and the implications for 15N-N2O analysis in tracer experiments could be included in the discussion.
The reviewer is correct; this analysis can also serve to detect 15N in N2O samples. However, our calibration was based on nitrate: we did not measure isotope ratios traceable to N2O reference materials, but rather N2O isotope values calibrated against nitrate standards. Furthermore, we diluted samples to a set nitrate concentration (and therefore N2O concentration), something that is not as easily possible for N2O samples. Therefore, we would like to avoid strong claims about the suitability of this method for N2O based on our study but instead encourage further studies on this important topic.
2. It seems that for the laser spectroscopy, the N2O concentration was about 25-times lower compared to the IRMS analysis (Line 78-82). This is only very briefly stated again in the last paragraph, but could be highlighted and discussed more.
This was because samples were diluted in synthetic air prior to CRDS analysis to achieve an optimal concentration range (1:25). The lower detection limit of CRDS is an advantage of this analysis over the classic IRMS analysis. We will discuss this in the revised manuscript.
Specific comments:
3. Line 25-29: When introducing 15N pool dilution and tracer approaches, I am not sure that the best reference is used, as the cited paper focusses on 15N analysis, not experimentation.We agree with the reviewer and will update the citation at Lines 25–29 in the revised manuscript to a comprehensive review of 15N experimental methodology:
Murphy, D. V., Recous, S., Stockdale, E. A., Fillery, I. R. P., Jensen, L. S., Hatch, D. J., & Goulding, K. W. T. (2003). Gross nitrogen fluxes in soil: theory, measurement and application of 15N pool dilution techniques. Advances in Agronomy, 79, 69–118. https://doi.org/10.1016/S0065-2113(02)79002-04. Line 41: Could the “lower capital and running costs” quantified, to support this statement?
We are currently in communication with the instrument manufacturers and will provide these numbers in the revised manuscript (CRDS + autosampler: ca. 282 000 EUR with almost no running costs and no dedicated technician needed; IRMS + GB + PreCon ca. 350 000 EUR + multiple EUR per sample running costs, technician needed).
5. Line 57: This is strictly speaking not true, as the NO3- pool also contains natural abundance 15N.
Thanks for noting this, we will fix this sentence in the revised manuscript.
6. Line 98: The individual samples (grey circles) are not visible in the figure. Either, if it is important to show individual samples, they need to be made more visible, or the authors might only show means and uncertainties. I also noted that the colours mentioned in the caption do not match the actual colours in the figure.
We will adjust the figure to show the mean + SD of calibration standards and also adjust the colours.
7. Line 124: From my perspective, using a 68-hour time step is not common in isotope pool dilution experiments, but rather long, with the risk of e.g. remobilization. This might not be crucial for the scope of the paper, but I would still recommend rephrasing.
We appreciate this feedback. The 68-hour interval was chosen due to the low incubation temperature, which mimics the field context of these Arctic soils. However, we agree that this poses a risk of isotopic remobilization in pool dilution experiments. In our experiment, we have analyzed samples after different time points (4, 24, 48, and 72 h), so we can provide data for either of these intervals (all of which show strong agreement between CRDS and IRMS). However, for clarity, we chose to present only one time period.
In the revised manuscript, we will replace the 68-hour data with calculations based on a 24-hour time interval. This shorter step will minimize the risk of remobilization while still effectively capturing the isotopic dilution dynamics. We will update the text, methods, and corresponding figures (including Figure 3) to reflect the 24-hour interval
Citation: https://doi.org/10.5194/egusphere-2026-5050-AC2
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AC2: 'Reply on RC2', Wasi Hashmi, 20 Oct 2026
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This paper is a timely contribution to the measurements of 15N in inorganic substances in nature. I cannot see any flaw in it.