the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Sep 2025
Enhanced weathering leads to substantial C accrual on crop macrocosms
Abstract. Enhanced weathering (EW) is proposed as a key strategy for climate change mitigation. It involves the application of silicate rock powder to soils, where it is expected to react with CO₂ released from soil respiration, forming stable carbonate ions and thereby sequestering carbon. Here, we evaluated the effects of EW on a crop ecosystem within a macro-scale ecotron – an enclosed facility enabling complete quantification of carbon fluxes among the atmosphere, vegetation, soil, and leachates. EW treatment resulted in an almost three-fold enhancement of measured carbon flux into the soil, achieving rates up to 1.5 tons per hectare. Furthermore, the magnitude of carbon sequestration exceeded what could solely be attributed to electrochemical transformations. Therefore, we conclude that EW facilitated significant carbon accrual in our simulated ecosystems via not only carbonate precipitation but also enhanced biogeochemical activities promoting additional carbon storage. Based on these findings, we speculate on the underlying pathways responsible for such outcomes.
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Status: open (until 15 Oct 2025)
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RC1: 'Comment on egusphere-2025-4188', Anonymous Referee #1, 08 Sep 2025
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AC1: 'Reply on RC1', Francois Rineau, 15 Sep 2025
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Please see our replies below in bold:
This is an interesting experimental study on the effect of ERW using basalt, especially since it employed macrocosms that were monitored over time. This enabled the authors to determine the inorganic and organic budgets directly from the measurements, as opposed to modelling. It is also interesting that you found a significant CO₂ intake leading to the same extent of C storage.
My main comments concern the discussion section, which I found to be very poor. Firstly, you dismissed the possibility of carbonate precipitation being responsible for C storage after ERW (due to the lack of an increase in dissolved inorganic carbon). See 4.2. Why should carbonate genesis occur at the surface? The localisation of carbonate precipitation depends on the ratio between the precipitation rate and the mass transport rate. How can carbonate precipitation in June–July explain C storage in August and September? I think there has been carbonate precipitation since the beginning, certainly driven by relatively high temperatures and dry conditions. However, you cannot prove this due to the lack of chemical data (e.g. Ca, Mg) and geochemical modelling.
We would like to clarify that we did not intend to dismiss the possibility of carbonate precipitation. Rather, we stated that our dataset provides no direct evidence for it, while acknowledging that our experimental design may not have been suited to detect it. For instance, carbonate formation could have occurred below 20 cm depth, as the reviewer correctly points out; but this message is indeed poorly convened in the discussion. Concerning the second part of the comment, our point was that carbonate formation rates may simply have been below the detection limit of our sampling strategy. Even small concentration changes over a three-week interval—the frequency of our sampling—could translate into significant cumulative storage at the macrocosm scale over an entire growing season. In addition, the number of samples collected during the driest period was limited, which could also explain why carbonate formation was not captured. In this sense, we are in agreement with the reviewer’s interpretation, though we acknowledge that our original discussion did not convey this clearly. We will revise Sections 4.2 and 4.3 to better reflect this.
Secondly, although microbial activity is the only consistent variation, and roots are severely depleted after ERW, you emphasised the role of root exudates in explaining the increase in C storage as organic C in section 4.3. What about the increase in microbial biomass and dead cells? Wouldn't increase in microbial activity be likely to increase exudate degradation and thereby CO₂ emission?
We are not sure where in the paper the reviewer found evidence that roots were severely depleted after ERW. We stated that root biomass was generally low, but this was independent of the treatment. Regarding microbial activity, this is indeed an interesting point. We would speculate that increased activity would lead to higher respiration rates and thus greater CO₂ emissions from the basalt-amended soil. However, this could be offset by an increase in microbial biomass, of which a significant fraction is recalcitrant, ultimately contributing to net carbon sequestration. We will revise Section 4.3 to clarify this point in the discussion.
I also have several moderate comments:
- a) You state in the abstract that C storage is beyond the effect of electrochemical transformation (I assume you mean Ammann et al., 2022), but I cannot find any convincing demonstration of this in the main text.
This is very likely a vocabulary mistake from our side, we mean “weathering rate” instead of electrochemical transformation. We will then correct lines 374-377: " Remarkably, this figure slightly exceeds the theoretically achievable limit of converting applied basalt into entirely carbonates (144 kg/ha for a 10 t/ha amendment). Hence, either complete transformation of the amendment occurred exceptionally swiftly within three months, which appears improbable based on existing literature (Kelland et al., 2020), or alternative mechanisms contributing to soil carbon sequestration must be considered.”
- b) I think all the equations should be numbered, and all the units should be given.
Indeed. We will correct that.
- c) Why should basalt at the surface lead to greater soil disturbance (and corresponding CO₂ emission)?
You are probably referring to the sentence: “Immediately after applying basalt, elevated CO₂ emissions occurred likely due to soil disturbance (Figure 2).” We understand your point that since all plots were tilled during seeding, and basalt was applied at that time, there is no obvious reason why the basalt-amended plots should have experienced greater disturbance and CO₂ release. This is a valid observation. We think that the presence of basalt may have stimulated microbial activity, leading to higher CO₂ emissions a few days after application. We propose to revise this discussion point accordingly, if you agree with this interpretation.
I also have plenty of minor comments, and these are only a few of them. I will give you the rest if they persist in the second round.
Table S1 : which mineral is 2:1 Al clay?
We understand this to refer to layer silicates with an Al-dominated octahedral sheet. This describes a group of minerals with a characteristic structure, rather than a single specific mineral.
Table S2 : special character editing issue in the headings
Indeed, we will correct that.
Figure S6 : what are the R-squared and the number of samples? Typo: "top" instead of "sop"? p-value = 3.8.10-9
We will add the R2 and the sample size in the legend, and correct the scientific units. We don’t see the typo, however?
Citation: https://doi.org/10.5194/egusphere-2025-4188-AC1
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AC1: 'Reply on RC1', Francois Rineau, 15 Sep 2025
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RC2: 'Comment on egusphere-2025-4188', Anonymous Referee #1, 22 Sep 2025
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It's good that you've suggested revising the discussion section. Yes, it is my mistake regarding the effect of ERW on roots. However, I am not convinced that LMWOAs are preferentially released to enhance microbial activity, as it is insensitive. Nevertheless, I meant that more microbes may be related to more dead cells, which, along with secondary minerals, are an important factor in stabilising organic carbon (hence more CO₂ intake by the soil?).
One minor comment: it looks like there is a missing read character in Figure 1. Can you see all the fluxes other than plants and soils?
Citation: https://doi.org/10.5194/egusphere-2025-4188-RC2 -
AC2: 'Reply on RC2', Francois Rineau, 24 Sep 2025
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It's good that you've suggested revising the discussion section. Yes, it is my mistake regarding the effect of ERW on roots. However, I am not convinced that LMWOAs are preferentially released to enhance microbial activity, as it is insensitive. Nevertheless, I meant that more microbes may be related to more dead cells, which, along with secondary minerals, are an important factor in stabilising organic carbon (hence more CO₂ intake by the soil?).
It's true that we focus a lot in the discussion on root exudates while we do not explore the route where C is eventually entombed in microbial biomass. This is another very plausible explanation for the sequestration we observed. We will add a point in the discussion to address this.
One minor comment: it looks like there is a missing read character in Figure 1. Can you see all the fluxes other than plants and soils?
Thank you for pointing this out. The flux values for the basalt treatment are indeed not displayed in red, except for "plant" and "soil". We will correct figure 1 accordingly.
Citation: https://doi.org/10.5194/egusphere-2025-4188-AC2
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AC2: 'Reply on RC2', Francois Rineau, 24 Sep 2025
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RC3: 'Comment on egusphere-2025-4188', Anonymous Referee #2, 24 Sep 2025
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In their manuscript “Enhanced weathering leads to substantial C accrual on crop macrocosms” the authors present a mesocosm experiment (called “ecotron”) on carbon sequestration in soil cores upon application of finely ground basalt (<1mm). The soil cores were chosen and prepared to mimic the conversion of marginal lands to agricultural lands. The experiment was carefully set up. We particularly appreciate the measurement of all three major greenhouse gases (CO₂, CH₄, and N₂O). The full control of the atmospheric gas composition in the ecotron facility makes it possible to achieve a full carbon mass balance of the system. The inclusion of uncertainty and sensitivity analyses on plant roots and dissolved inorganic carbon (DIC) leaching via bootstrapping adds robustness to the results. In addition, the discussion around the potential stabilization of root exudates by weathering products and the subsequent formation of mineral-associated organic matter (MAOM) is valuable.
The manuscript is well structured overall; we have a few suggestions for restructuring the text to enhance the story line. But several sections should be strengthened before publication. For example, the introduction section is very short and contains sloppy wording on core concepts (see detailed comments below). The discussion section lacks detail on the potential changes in plant metabolism that could explain observations.
This study definitely merits publication but the comments below need careful consideration.
Bring introduction section in line with state of the art
The introduction section does not capture the state of the art of ERW research For instance, the absence of DIC leaching to deeper water layers, the complications introduced by interactions between the organic and inorganic carbon dynamics in natural soils (e.g. impact of weathering on SOM stabilization, soil organic carbon decomposition and plant C inputs), the difficult to quantify role of secondary mineral formation (PIC, clays, Fe/Al (hydr)oxides) are all well established and merit consideration. In this sense it lacks some recent references (e.g. Vicca et al (2022) Vienne et al. (2024), Steinwidder et al. (2025), …). A more complete and up to date description of the state of the art will strengthen the introduction, with as additional benefit to further support the relevance of the underlying study.
Context of marginal land application:
It should be stressed that basalt was applied on artificially marginalized land that has been subjected to fertilization and cropping in the mesocosm setup. This is crucial to contextualize the results, as the implications may differ from intensively managed agricultural systems. Positioning the work in this context, in line with and related literature (e.g. https://doi.org/10.3389/fclim.2022.928403), could highlight the potential of enhanced weathering (EW) to increase food production and carbon sequestration on marginal soils, particularly in the Global South.Discuss importance of biological processes
The comprehensive mass balance results show that increases in soil carbon are the primary driver of carbon sequestration. However, the manuscript does not fully articulate the logical implication that this increase is likely due to enhanced CO₂ uptake through photosynthesis, followed by subsequent release as root exudates (that can be stabilized in MAOM) (see e.g. https://www.science.org/doi/10.1126/sciadv.abd3176). In our opinion the discussion should be elaborated, and emphasize that biological processes, rather than geochemical mechanisms (DIC + PIC increases), appear to be the dominant contributors to the observed CO₂ uptake. Specifically, the fertilization effect of basalt (enhanced nutrient availability) in driving the observed changes in net ecosystem exchange (NEE) and CO₂ fluxes after basalt addition deserves stronger emphasis.
Incomplete characterization of t0
Methodologically, the characterization of t0 is confusing. In our opinion, the experiment starts at the beginning of the basalt amendment. However, soil pH and soil organic carbon are given at the time the soil cores were taken from the field. Between this time and the actual start of the experiment, the mesocosms were subjected to cropping and fertilization. Concerning the soil characteristics after the transition period, the authors only briefly mention that “the pH was 7 in all units”. This is a high and atypical value for agricultural soil, and no information on vertical variation in pH is provided. No mention of soil carbon at the start of the experiment is given. It would strongly strengthen this paper if soil pH and organic carbon were available at t0 of the experiment. This is particularly relevant when interpreting the experimental results in terms of real agricultural systems. Including pH data after application would further strengthen the dataset and provide valuable context regarding soil chemistry changes. If soil samples were taken at the end of the experiment and are still present, this could be an interesting addition to the dataset.
We further have the following detailed comments and suggestions:
Abstract:
- Mention that you monitored all GHGs in deep mesocosms and emphasize that 10 ton basalt ha-1 was added to degraded soils first. Emphasize that this experiment was not done on a typical rich agricultural soil.
- L30 “It involves the application of silicate rock powder to soils, where it is expected to react with CO₂ released from soil respiration, forming stable carbonate Ions". While this is an intuitive way of understanding the weathering process, it is more correct to rephrase it along the lines “Dissolution of silicate minerals enhances the alkalinity of the pore water, resulting at a shift of the carbonate system towards carbonate and bicarbonate, leading to higher dissolved inorganic carbon when the water is equilibrated with the atmosphere”.
- L33: 1.5 tons ha-1 sequestration; please specify that this is ton C ha-1 and not ton CO2 ha-1. Also add the timescale after which this C sequestration took place, 150 days = 1 growing season I suppose?
- Change electrochemical transformations into “geochemical C sequestration” or specify that DIC or PIC increases could not explain the observed C sequestration. Throughout the MS: change electrochemical into geochemical.
Intro:
- L39 – The first sentence does not make sense. The relative contribution of agriculture to GHG emissions, does not “highlight the urgent need for scalable mitigation strategies.” If the authors want to stress the urgent need for mitigation, they can support and strengthen that argument with recent reports and publications on risks involved with the current emission pathways, literature on tipping points, etc ...
- L41 Please be more specific. ERW should not be framed as a major mitigation strategy, but as a negative emission technology and a CDR method, specifically proposed to achieve a net negative GHG balance in the near future.
- L42 “where it reacts with CO2 etc...” -> see comment on abstract. Please rephrase accordingly.
- L42 – L50 This core part of the introduction section does not capture the state of the art of ERW research. (see general remark)
- As mentioned above, the authors should introduce and motivate the relevance of specifically studying ERW on marginal soils that are converted to agriculture and clearly mention the artificial “marginalization” and conversion in the mesocosm setting.
Methods:
2.1 Ecotron facility
- Remove “(as Heading 1)” in the section title
- L61 adapt to “[...] in gas-tight enclosure. High-frequency measurements allow [...]”
- L62-L64 -> Move to introduction section
- L62 “this enables to study the actual dynamics of the process of EW ...” this oversells the system bit. Please be more precise, e.g. along the following lines: “This enables us to close the carbon mass balance in the system and to study the fate of released cations, while the large mesocosms [...]”
- Lacking from the “ecotron”-part of the methods section is the way CO2 is regulated in the mesocosm -> L126-127 can be moved here.
2.2 Macrocosm
- L83 “Six large macrocosms” -> Six large soil cores. In our opinion, it becomes a macrocosm once those cores are incorporated in the full setup.
- L90: Agricultural treatments to mimic the conversion of heathland to crop fields. Please mention which treatments these were? Only the foliar Si, and NPK fertilization? Can you add details on how much NPK was applied?
- L92: How deep was the basalt plowed in?
- L93: Is there a more detailed basalt particle size distribution than just <1 mm particle size? More specific information on the particle size and the specific surface area (e.g. BET surface area) would greatly benefit comparison to other experimental studies.
2.3 Climate simulations
- L101-106 Move to Ecotron section. The current section should focus on the climate scenarios that were simulated, and how.
- L100-101 and L109-110 convey are identical in message – remove one of both.
2.4 Plant biomass
- L115: How were roots exactly measured? How much cores/m2 … for the entire 1.5m depth?
2.5 CO2-C net flux
- L126-127 move to the Ecotron section of the methods.
- L133 “Data gaps were filled using a moving average function (ALMA function from the TTR package in R)” Please be more precise on the gap filling. Perhaps data was (linearly?) interpolated before running the moving average filter?
2.7 Rainwater C flux
- L150: Rainwater C flux: DOC in rainwater was measured, is inorganic C in rain water negligible? Or what are typical values please mention why this is not measured.
Non-purgeable C or DOC, how was this measured, device, precision …?
Results:
L259: The largest input flux, by far, came from net CO2 exchange (366 to 457 gC/m2 depending on the treatment) ==> not clear which one is basalt and which one the control from the text, please rephrase.
L261: Not clear: were there significant differences in CH4 and N2O emitted after basalt amendment. Please add details.L264, Figure 1: add the amount of days the growing season lasted. Add red / black everywhere for basalt/control to improve interpretation of this figure.
Figure 2: please add error bars for the cumulative NEE differences around the red and black line.
L320: Table 1: should be a figure for visual interpretation or just move to the supplement.
Colors are not explained.
L342: I think the evidence for microbial activity is a strong result and may be included in the main text, potentially as an alternative for Table 1.Discussion:
- see general remark about discussing role of photosynthesis, root exudates and MAOM.
L374: You’re mentioning that the C sequestration is higher than the theoretically achievable inroganic C sequestration. Based on the EW potential of the rock, we assume the Renforth (2019) formula was used to calculate this. If not already cited, please cite this work in the methods somewhere.
L363: Enhanced CO₂ levels are known to stimulate both weathering rates and CO₂ uptake via electrochemically-driven reactions (Amann et al., 2022). ==> change to geochemically.
L357: Alternatively, carbon sequestration rates nearing 2 tons per hectare were realized within a single year using equal amendment quantities (10 tons per hectare), though this involved utilizing a more chemically reactive substance—olivine—as opposed to basalt (Dietzen et al., 2019).
==> We believe Dietzen et al. 2018, measured an increase in soil respiration in the incubation experiment (Effectiveness of enhanced mineral weathering as a carbon sequestration tool and alternative to agricultural lime: An incubation experiment). A “CDR potential” is estimated if all Mg of the exchangeable pool were to leach out as Mg and charge balance with HCO3-. I think this is the value you refer to with “C-sequestration”. Please nuance that no proof of inorganic C sequestration was found in the referenced Dietzen et al, 2018 study.
L366: Secondly, we also measured significantly higher microbial activity in basalt-amended soils, mirroring findings by Li et al. (2020), who reported a 33% increase in microbial activity following enriched rock-dust applications.
Please add some recent literature to emphasize this point. The simulation work of Klemme et al. (2022) also suggests increases in SOM decomposition after basalt amendment and pH increase. Steinwidder et al. (2025) also makes this point. Mention that for C sequestration in Kelland et al. (2020), only inorganic C was considered and that the reported sequestration was a modelled value.L379: On the lack of substantial DIC / Ca / Mg leaching, others are finding similar results (e.g. Steinwidder et al. (2025), Vienne et al. (2024), Amann et al. (2020)). It is possible that processes such as cation exchange produce protons and degas DIC, please comment on this as a possible mechanism for the non-observation of a DIC increase in leachates.
L405: Also mention the work of Noah Sokol on MAOM and EW, there is at least 1 field study that discusses this. Other relevant recent work: https://doi.org/10.1016/j.scitotenv.2025.180179.
Citation: https://doi.org/10.5194/egusphere-2025-4188-RC3 -
AC3: 'Reply on RC3', Francois Rineau, 25 Sep 2025
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We thank the reviewer for the very constructive review. We agree with the suggestions made and will prepare a revised document accordingly.
Citation: https://doi.org/10.5194/egusphere-2025-4188-AC3
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This is an interesting experimental study on the effect of ERW using basalt, especially since it employed macrocosms that were monitored over time. This enabled the authors to determine the inorganic and organic budgets directly from the measurements, as opposed to modelling. It is also interesting that you found a significant CO₂ intake leading to the same extent of C storage.
My main comments concern the discussion section, which I found to be very poor. Firstly, you dismissed the possibility of carbonate precipitation being responsible for C storage after ERW (due to the lack of an increase in dissolved inorganic carbon). See 4.2. Why should carbonate genesis occur at the surface? The localisation of carbonate precipitation depends on the ratio between the precipitation rate and the mass transport rate. How can carbonate precipitation in June–July explain C storage in August and September? I think there has been carbonate precipitation since the beginning, certainly driven by relatively high temperatures and dry conditions. However, you cannot prove this due to the lack of chemical data (e.g. Ca, Mg) and geochemical modelling. Secondly, although microbial activity is the only consistent variation, and roots are severely depleted after ERW, you emphasised the role of root exudates in explaining the increase in C storage as organic C in section 4.3. What about the increase in microbial biomass and dead cells? Wouldn't increased microbial activity be likely to increase exudate degradation and thereby CO₂ emission?
I have also several moderate comments:
I also have several moderate comments:
a) You state in the abstract that C storage is beyond the effect of electrochemical transformation (I assume you mean Ammann et al., 2022), but I cannot find any convincing demonstration of this in the main text.
b) I think all the equations should be numbered, and all the units should be given.
c) Why should basalt at the surface lead to greater soil disturbance (and corresponding CO₂ emission)?
I also have plenty of minor comments, and these are only a few of them. I will give you the rest if they persist in the second round.
Table S1 : which mineral is 2:1 Al clay?
Table S2 : special character editing issue in the headings
Figure S6 : what are the R-squared and the number of samples? Typo: "top" instead of "sop"? p-value = 3.8.10-9