the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Mar 2025
Triple oxygen isotope evidence for the pathway of nitrous oxide production in a forested soil with increased emission on rainy days
Abstract. Continuous increases in atmospheric nitrous oxide (N2O) concentrations are a global concern. Both nitrification and denitrification are the major pathways of N2O production in soil, one of the most important sources of tropospheric N2O. The 17O excess (Δ17O) of N2O can be a promising signature for identifying the main pathway of N2O production in soil. However, reports on Δ17O are limited. Thus, we determined temporal variations in the Δ17O of N2O emitted from forested soil for more than one year and that of soil nitrite (NO2−), which is a possible source of O atoms in N2O. We found that N2O emitted from the soil exhibited significantly higher Δ17O values on rainy days (+0.12±0.13 ‰) than on fine days (−0.30±0.09 ‰), and the emission flux of N2O was significantly higher on rainy days (38.8±28.0 μg N m−2 h−1) than on fine days (3.8±3.1 μg N m−2 h−1). Because the Δ17O values of N2O emitted on rainy and fine days were close to those of soil NO2− (+0.23±0.12 ‰) and O2 (−0.44 ‰), we concluded that although nitrification was the main pathway of N2O production in the soil on fine days, denitrification became active on rainy days, resulting in a significant increase in the emission flux of N2O. This study reveals that the main pathway of N2O production can be identified by precisely determining the Δ17O values of N2O emission from soil and by comparing the Δ17O values with those of NO2−, O2, and H2O in the soil.
Status: open (until 22 May 2025)
-
RC1: 'Comment on egusphere-2025-996', Anonymous Referee #1, 28 Apr 2025
reply
In the manuscript “Triple oxygen isotope evidence for the pathway of nitrous oxide production in a forested soil with increased emission on rainy days” by Ding et al., the authors present results from nitrous oxide Δ¹⁷O measurements at a forested site. Additionally, they conducted tracer addition experiments, including nitrate enriched in Δ¹⁷O, to complement the in situ measurements. The results show that nitrous oxide produced under high and low soil moisture conditions can have different Δ¹⁷O values within the limits of analytical precision. These variations fall within the range defined by the Δ¹⁷O of soil O₂, H₂O, and NO₂⁻, suggesting that N₂O production from nitrification and denitrification could be inferred by comparing the Δ¹⁷O values of N₂O and potential oxygen sources.
Overall, I find this manuscript well written, with the results clearly presented. I am particularly impressed by the analytical rigor demonstrated in this study, which is critical given the typically subtle variations in Δ¹⁷O. However, I do have several specific questions and comments that I hope the authors can address in their revision:
[1] Section 2.3: While extraction with 2 M KCl is standard for measuring extractable soil nitrate and ammonium, significant loss of soil nitrite can occur during the extraction. This issue is well recognized in the soil nitrogen cycling community (e.g., Homyak et al., 2015). Given that nitrite concentration and Δ¹⁷O measurements are critical to this study, could the authors discuss how potential nitrite loss and associated isotopic fractionation might impact their analysis?
[2] Section 2.4: The manuscript uses a β (triple oxygen proportionality factor) range of 0.525 to 0.5305 to quantify the potential impact on Δ¹⁷O. Although several references are cited, it is unclear why this specific range was chosen. Please clarify. In particular, earlier studies (e.g., Matsuhisa et al., 1978; summarized by Miller, 2002) reported lower β values (e.g., ~0.5164). How would using lower β values affect the results?
[3] Lines 362–363 and throughout the analysis: A constant Δ¹⁷O value was assumed for O₂ (-0.44‰) and soil H₂O (0.03‰). Please clarify whether these values could vary due to hydrological and biogeochemical cycling. For instance, could O₂ diffusion and heterotrophic consumption affect O₂ Δ¹⁷O, or could evaporation significantly alter soil H₂O Δ¹⁷O?
[4] Lines 378–397: The characterization of δ¹⁸O offsets between O₂ and N₂O, and between NO₂⁻ and N₂O, does not necessarily represent true isotope effects between N₂O and its oxygen precursors because field-measured N₂O is a mixture of multiple sources. For example, in Fig. 6a, the actual δ¹⁸O difference between NO₂⁻ and N₂O may be larger than calculated if O₂-derived N₂O has δ¹⁸O values similar to that of O₂. Similarly, the O₂-N₂O difference may be smaller than estimated. This mixing effect could confound the use of δ¹⁸O differences to estimate Δ¹⁷O variations and warrants further clarification.
[5] Fig. 7 and related discussion: I commend the authors for conducting a sensitivity analysis to assess how much Δ¹⁷O variation may stem from biogeochemical processes versus purely geochemical processes (i.e., β variability). However, applying the β range to the net δ¹⁸O difference between N₂O and oxygen sources treats the N₂O-producing processes as a single step. In reality, processes like nitrite reduction involve multiple sub-steps (e.g., NO₂⁻ to NO, NO to N₂O, isotope exchange with H₂O), each potentially associated with different β values. This could lead to larger Δ¹⁷O variations than those estimated from a single-step approach. This limitation should be discussed.
[6] Lines 434–438: It is unclear how the 24% contribution of soil H₂O was derived. Additionally, Fig. 6b shows that the Δ¹⁷O of N₂O in the CS plot was significantly lower than that of NO₂⁻ six days after tracer addition. This suggests that soil H₂O may have played a significant role during nitrite reduction to N₂O.
[7] Lines 444–449 and Fig. 6b: Apparent differences in Δ¹⁷O between soil H₂O and N₂O cannot be used to conclusively rule out H₂O contributions during N₂O production. In the NF and U plots, the Δ¹⁷O of soil H₂O lies between that of NO₂⁻ and N₂O, and both soil H₂O and NO₂⁻ have higher Δ¹⁷O than O₂. Could significant H₂O exchange during N₂O production explain these observations, leading to a mixed Δ¹⁷O signal from both H₂O- and O₂-derived N₂O?
[8] Section 4.5: Early in the manuscript, the authors argue that bulk isotopic and SP-based techniques for N₂O source apportionment are limited due to isotopic fractionations during cycling (lines 56-61), whereas Δ¹⁷O measurements may be more robust. After presenting the results, I would encourage the authors to revisit this point with more specificity. Given potential complications such as H₂O exchange and multiple contributing sources (H₂O, O₂, NO₂⁻), can Δ¹⁷O measurements realistically achieve quantitative source apportionment? If so, what would the total uncertainty be, considering analytical precision, β variability, and uncertainties from the Keeling approach? Under what conditions would Δ¹⁷O approaches be preferable to conventional methods, and when might they be less effective?
References:
Homyak, P.M., Vasquez, K.T., Sickman, J.O., Parker, D.R. and Schimel, J.P., 2015. Improving nitrite analysis in soils: Drawbacks of the conventional 2 M KCl extraction. Soil Science Society of America Journal, 79(4), pp.1237-1242.
Miller, M.F., 2002. Isotopic fractionation and the quantification of 17O anomalies in the oxygen three-isotope system: an appraisal and geochemical significance. Geochimica et Cosmochimica Acta, 66(11), pp.1881-1889.
Matsuhisa, Y., Goldsmith, J.R. and Clayton, R.N., 1978. Mechanisms of hydrothermal crystallization of quartz at 250 C and 15 kbar. Geochimica et Cosmochimica Acta, 42(2), pp.173-182.
Citation: https://doi.org/10.5194/egusphere-2025-996-RC1
Viewed
Since the preprint corresponding to this journal article was posted outside of Copernicus Publications, the preprint-related metrics are limited to HTML views.
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 70 | 0 | 0 | 70 | 0 | 0 |
- HTML: 70
- PDF: 0
- XML: 0
- Total: 70
- BibTeX: 0
- EndNote: 0
Viewed (geographical distribution)
Since the preprint corresponding to this journal article was posted outside of Copernicus Publications, the preprint-related metrics are limited to HTML views.
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1