the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jun 2026
O2:CO2 exchange ratio and isotopic composition of atmospheric O2 measured using soil chambers, and their application to evaluating enhanced rock weathering and the Dole–Morita effect
Abstract. Soil chamber measurements of δ(O2/N2), CO2 amount fractions, and the δ(18O) of O2 in air (δatm(18O)) were conducted at a forest site in Takayama (TKY), Japan, and at agricultural fields in Tsukuba (TKB) and Miyakojima (MYK), Japan. The latter fields included plots with and without crushed rock application for the evaluation of enhanced rock weathering (ERW) for carbon dioxide removal (CDR). The 7-year average O2:CO2 exchange ratios for soil–air O2 and CO2 fluxes were 1.10±0.01 at TKY, whereas the two-year averages at the agricultural fields varied from 0.38 to 4.32. Assuming soil respiration yields an O2:CO2 ratio of 1.1, we partitioned CO2 fluxes into soil respiration and abiotic reactions including CO2 dissolution and dissociation. At TKB, the low CO2 emissions from the basalt-applied plot were attributed to reduced soil respiration. In contrast, at MYK, both plots showed abiotic CO2 uptake, comparable to modeled CDR rates for ERW, regardless of basalt/olivine application. These results suggest that inherent soil processes such as alkalinity-driven CO2 absorption and leaching, rather than rock application, were responsible for the observed net CDR, questioning the impact of ERW at these sites. Our analytical approach effectively quantified CDR by accounting for both biotic and abiotic processes. The isotopic effect of soil respiration (εSR) from δ(O2/N2) and δatm(18O) was substantially lower at TKB and MYK than previously reported. Revising the plausible global average eSR to 14.6 ‰ would reduce the modeled Dole–Morita effect from 23.16 ‰ to 22.71 ‰.
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Status: open (until 26 Aug 2026)
- RC1: 'Comment on egusphere-2026-2751', Anonymous Referee #1, 27 Jul 2026 reply
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RC2: 'Comment on egusphere-2026-2751', Anonymous Referee #2, 06 Aug 2026
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General comments
This paper presents interesting data on CO2, O2 isotopes, and N2 isotopes exchange between soil and air from three different measurement sites with different treatments across several seasons. In addition, they measured at different soil depths in one location. The topic is of importance, to better understand impact of rock weathering and in general gas exchange of soils and this paper contributes substantially to it.
In general, it might be because I am not an expert in isotope measurements, but I found the introduction and results & discussion section hard to follow. I have made some suggestions on how and where to describe more clearly. A figure of the setup and a table summarizing the different measurement site, times, and conditions could be beneficial to the reader.
Specific comments
- Consider separating accurately the term inorganic, abiotic, and physical here. Inorganic refers to chemistry that generally lacks carbon-hydrogen (C–H) bonds, so CO2 processes in general are inorganic. Abiotic are non-living. While the dissolution of CO2 in soil water is an inorganic process when carbonic acid is formed, the initial step of the dissolution of CO2 gas molecules in water is a physical process. At least define what you mean by the term inorganic in your context.
How do you know FCO2_I as it is defined = 𝐹CO2 – 𝐹CO2R consists of an inorganic process? - Please report numbers in your result section 3.1-3.3 instead of saying “notably higher”, “substantially higher”, “much larger”, “lower” etc.
- Consider constraining the number of figure to the essential parts and putting additional figure into an Appendix/Supplementary. Maybe Figure 2 could be part of supplementary instead, as well as Fig. 3,4, and 5?
Introduction:
67 and 69 and 72 and again in line 221:
inorganic process and not physical or abiotic processes that are separated here from the respiration of soil biota (=biotic process)?69 to me it does not seem necessary to define “soil respiration” as respiration of soil biota
Methods:
How many replicate chambers per site did you have? What was their distance between each other?
Section 2.1: The method section would benefit from an overview table of measurement locations used, chamber type, flask type, dates, crops and manipulation with manure, basalt/olivine.
Section 2.2: The section would benefit from a schematic figure of the chamber air sampling set-up.
dimensions of the used chambers are missing: Area, Height of the collars
What flow rate did you use to collect/circulate air into flasks?
What were the suppliers or manufacturing process for the two types of used flasks and the used diaphragm pump?144: Please clarify: you re-introduced the air exhausted from the back-pressure valve of the flasks into the chamber but also state that you had and influx from ambient air? What was the flow rate and volume for the influx of ambient air compared to the circulated air?
155: define minuscule amount
161: of dry air by volume
162: what was the type of the used mass spectrometer
163: please add supplier of the cylinder
194: How long before measurements was the stainless-steel air-sampling tube installed into the soil?
Section 2.5:
249-252: Clarify that you adopted the values of Dd and Dr as in Angert and Luz (2001) and do not only report here what they have used. Like: “Dd of 0 ‰ was adopeted from Angert and Luz (2001), because….” Instead of “Angert and Luz (2001) adopted a Dd of 0 ‰ because”
Results and Discussion
Please consider separating results and discussion.
Section 3.1:
258: Maybe rephrase to decreased and increased with during each chamber closure event or similar to make it easier to understand this sentence.
259: What does it mean for each observation? Please specify what you mean by observation, I guess winter, spring, summer and autumn?
266-269: Here explanation of the values meaning is before reporting the value. Consider switching it. Please in general first report your findings and then discuss them. As mentioned above seriously consider splitting into results and discussion.
Section 3.2:
Consider splitting this section into vertical soil pore air distribution and the relationship with SWC.
302: What do you mean with a high of SWC in summer? It seems to me in your plot that it is lowest in summer? As you defined summer as June-August and the lowest values of around 30-36% are in August/September, while the highest values are in April-March with 36-41%? Also please make the SWC data available. I think it is not part of the supplemented data yet. I also don’t understand how the values shown are different means if the number of shown values in the plot are the same. Please explain better what it shows.
305: Please put the equation in the text before referring to it. And this should probably be placed in the method section (?)
307: This sounds as if it is continuous data. Please rephrase to measurement dates in the period from …
308: Could you please add a reference and explanation here on why you are multiplying by 0.2094 μmol mol−1. Also consider putting this into the method section.
311: Add a reference for the average atmospheric fractions.
315: If possible, give a number for the difference.
318-323: Possibly put into method section
331-347: In my opinion this part should be in the introduction.
Consider splitting Figure 11a and 11b and calling the different panels within each subfigure a,b,c to be able to better refer to them in the text.
362: Where is this continuous y(O2) and y(CO2) in soil pore air you are showing. Please add to supplement or make otherwise available or reference the data.
370: Maybe briefly add what it means if ERSA is exceeding zero. Positive means there is a flux from the air to the soil? I thought both examples for ERSA are above zero? Just one below 1.1 and the other above?
375 and 379 are repetitive statements. Condense
380: Show this exceptional data. Possibly in an Appendix or Supplementary.
Section 3.3:
417: How is it clearly apparent, if the difference is in the range of the error bar (standard error)? Did you test this with an appropriate statistical approach?
419: How did you test that there was no significant differences?
429-433 Explain better how the estimation of F_CO2_I helping with MRV of CDR is the same as the estimation from cement plants.
Section 3.4:
435: Maybe here similar as in the Figure 13 caption “Relative changes in the isotopic composition of O2 as a function of the natural logarithm of the remaining O2 fraction (Eq.12)” could be used to make understanding this phrase easier. Can you add the removed data in Figure 13 but in a different color and shape?
456: Add citation for Dd and Dt values as done before.
Conclusion:
To me the conclusion sounds more like a condensed discussion rather than concluding the main results.
496: Consider also adding numbers for ERSA from TKY.
Technical corrections
Text inside Figures and legends overall are too small.
70: replace” due only to” to “stemming from” or similar, sentence structure is difficult to understand
200: Maybe don’t use an abbreviation for TU but write the full name here
219: Different variable formating was used: FCO2_R or FCO2R
395: Fig.
800: remove “in (a)” or state “ as the values in (a). Also consider prhasing text different to Relationship between ERSA and SWC as 30 min, 24 h, and 1 week mean values. Solid line… and so on
838: missing (b)
841: missing (c) ? What does this line tell me?
844: missing a legend for marker size for SWC
Citation: https://doi.org/10.5194/egusphere-2026-2751-RC2 - Consider separating accurately the term inorganic, abiotic, and physical here. Inorganic refers to chemistry that generally lacks carbon-hydrogen (C–H) bonds, so CO2 processes in general are inorganic. Abiotic are non-living. While the dissolution of CO2 in soil water is an inorganic process when carbonic acid is formed, the initial step of the dissolution of CO2 gas molecules in water is a physical process. At least define what you mean by the term inorganic in your context.
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RC3: 'Comment on egusphere-2026-2751', Anonymous Referee #3, 10 Aug 2026
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General comments
This manuscript applies precise measurements of soil–air O2, CO2, and δatm(18O) in chamber experiments to separate biotic and abiotic CO2 fluxes, with the broader goal of evaluating enhanced weathering (EW). The chamber design is thoughtful, the measurement approach is carefully described, and the long-term observations across forest and agricultural sites provide a valuable dataset.
One general question I had while reading the manuscript is about the overall strategy for identifying the EW signal. I discuss this concern further in the Specific Comments (Section 3.2, Lines 358–388).
Overall, I think this manuscript provides useful insight into soil–air gas exchange and has the potential to make a valuable contribution to the development and evaluation of enhanced weathering monitoring approches. I hope the comments below are helpful in further strengthening the manuscript.
Specific comments
I have two general questions regarding the methodology and interpretation, followed by several more specific comments in the order they appear in the manuscript.
(Methods/Figures): I could not find a clear description of the experimental replication. It would be helpful to clarify the experimental design, including how many independent plots, biological replicates, or duplicate measurements were included for each treatment. I also noticed that the figures provide relatively little information about the uncertainty or variability of the measurements. If replicate or duplicate measurements are available, it would be helpful to either show the individual measurements or summarize their variability in the figures. In addition, when averaged values are presented, it would be helpful to include appropriate error bars in the plots.
(Methods/Discussion): One general question I had throughout the manuscript concerns the assumption of a fixed respiration ER (OR) of 1.1. I understand that this value is commonly used and appears to work well for the TKY forest site, where the observed ERSA is close to 1.1. However, the manuscript also shows that the three sites differ substantially in soil type, vegetation, climate, management, and even oxygen conditions inferred from the isotope analysis. This made me wonder how robust the assumption of a constant respiration ER is across all three sites. Since much of the partitioning between FCO2, R and FCO2, I, as well as many of the subsequent interpretations, rely on this assumption, I think it would be helpful to discuss its uncertainty a little more. A simple sensitivity analysis using a reasonable range of respiration ER values might also help show whether the main conclusions remain robust.
(Section 2.5, Lines 239–241): I like the chamber design, especially the pressure-equalization tube. Since the Rayleigh analysis assumes an approximately closed system, it might be helpful to briefly mention here that any isotopic effect caused by the small amount of gas exchange with the atmosphere is expected to be negligible.
(Section 3.2, Lines 340–344): I was a little confused by this discussion. My understanding is that the chamber only represents a relatively small soil volume over a short period of time. Even if the complete carbonate-weathering cycle is carbon neutral, weathering could still reduce gas-phase CO2 within the chamber if dissolved products leave the sampled volume before the reverse reaction occurs. I was wondering if there is previous work showing that this effect is negligible at the chamber scale. If so, adding a reference here would make this interpretation much easier to follow. Otherwise, it might be worth briefly acknowledging this limitation.
(Section 3.2, Lines 358–388): My understanding is that the O2–CO2 chamber approach is intended to separate biotic and abiotic CO2 fluxes, and then use this information to evaluate the contribution of enhanced weathering. I think the method itself is convincing for distinguishing biotic and abiotic processes. However, once the abiotic component is estimated, the interpretation of the underlying mechanism seems to rely mainly on the chamber observations and the comparison between the treated and control plots. I wonder whether some additional evidence that enhanced weathering was actually occurring in the treated plots (for example, changes in soil or pore-water chemistry, weathering indicators, or results from related studies) would strengthen this interpretation. Otherwise, it seems difficult to distinguish between a situation where enhanced weathering occurred but contributed little to the observed CO2 signal and one where EW itself was limited by factors such as the experimental duration, soil conditions, hydrology, or mineral reactivity, and therefore produced little measurable response. In the latter case, the lack of a treatment effect would not necessarily indicate that enhanced weathering had only a minor contribution to the CO2 signal. I think some additional supporting evidence would make the interpretation much more convincing.
(Section 3.3, Lines 403–406): The explanation here seems to rely on the soil being rich in carbonate/bicarbonate and on rapid water percolation. I don’t think I saw much supporting information earlier in the manuscript. If these are well-established characteristics of the site, it might help to include a few measurements or point readers to a previous study.
(Section 3.3, Lines 409–412): The statement that “soil respiration was suppressed by spreading crushed basalt” is a little stronger here. It might be safer to describe this as one possible interpretation unless there is additional supporting evidence.
(Section 3.4, Lines 475–484): I also wondered about the extrapolation to the global average εSR and the revised Dole–Morita effect. Since this conclusion is mainly based on one forest site and two agricultural sites, I think it would help to discuss a little more why these sites are considered representative of their broader ecosystem types or simply acknowledge this limitation.
Technical corrections
Line 84: Subject–verb agreement. Consider “The isotopic effect of soil respiration (εSR) was assumed…” or “The isotopic effects of soil respiration were assumed…”
Line 85: “on Bender et al. (1994)” should be “by Bender et al. (1994).”
Line 94: Probably a typo, “εDR” should be “εSR”.
Line 171: “Sugawara et a., 2018” should be “Sugawara et al., 2018.”
Line 195: “ventilation halls” should probably be “ventilation holes.”
Line 287: “TBK” should be “TKB.”
Line 369-370: Please check “The contribution of inorganic processes was negative when the ERSA exceeded zero.” Based on the surrounding discussion, did the authors intend “the ERSA exceeded 1.1”?
Line 445: “temporal deciduous forest” should probably be “temperate deciduous forest.”
Line 491: “the thermally diffusive fractionation … was collected” should probably be “was corrected.”
Figure 6 caption: “calculated from the data shown in Figs. 2–4” should probably be “Figs. 3–5.” Figures 3, 4, and 5 contain the TKY, TKB, and MYK chamber data, respectively.
Citation: https://doi.org/10.5194/egusphere-2026-2751-RC3
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The reviewed manuscript deals with O2 consumption and CO2 emission in soil respiration. This is an important contribution, since the vast majority of papers address only the latter. Moreover, in addition to accurately measuring the O2:CO2 exchange ratio, the isotopic fractionation of O2-18O by soil respiration was measured in the field using a soil chamber. Such measurements are extremely rare. Hence, this manuscript is an important contribution to science.
My main comment is that the data were not corrected for water vapor dilution. Since this has a much stronger effect on O2 variations (since it is not a trace gas), all the data must first be corrected.
I also found the discussion on the O2:CO2 exchange ratio results too long and maybe repetitive. Maybe separation into “results” and “discussion” will make this more readable. The maximum possible effect of CO2 dissolution under these conditions should be quantified. Also, I found that the authors did not consider and did not explain enough other mechanisms that may affect the O2:CO2 ratio.
The Dole – Morita effect section is well written and very clear.
Minor comments:
The abstract should say something about the different (or not) ratio in ERW plots.
Defining the “net CO2 flux between soil and the overlying atmosphere (hereafter referred to
as “soil–air”)” is very confusing. Since soil-air is used even in this sentence to refer to the air in the soil.