the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jun 2025
Reviews and syntheses: Carbon vs. cation based MRV of Enhanced Rock Weathering and the issue of soil organic carbon
Abstract. We discuss the “monitoring, reporting & verification” (MRV) strategy of Enhanced Weathering (EW) based on carbon accounting and argue that in open systems such as arable land, this approach is ill-suited to close the balance of all carbon fluxes. We argue for total alkalinity (TA) as the central parameter for the carbon based MRV of EW. However, we also stress that tracking alkalinity fluxes using a systems-level approach is best done by focusing on charge balance maintenance through time. We start by explaining the concept and history of alkalinity conceptualization for the oceans. The same analytical method first proposed for the oceans – titration with a strong acid – is now commonly used for porewaters in agricultural soils. We explain why this is an accurate analysis for ocean water and why it is unsuitable to record TA for porewaters in agricultural soils. We then introduce an alternative MRV based on cation accounting. This requires translation of "carbon currency" into "cation currency" based on the concept of the "explicit conservative expression of total alkalinity" (Wolf-Gladrow et al., 2007). We finally discuss the fate of cations released from the weathering of basalt, soil cation dynamics and close by suggesting open research questions.
Competing interests: no competing interests; one of the co-authors is editor of the journal
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RC1: 'Comment on egusphere-2025-2740', Isabel Montañez, 09 Sep 2025
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Overview paragraph evaluating the overall quality
Bijma et al. address the critical issue of how society will abate the hard-to-abate residual (i.e., legacy) emissions, of which a quarter will come from the agricultural sector, using both marine and terrestrial sinks, specifically through enhanced rock-weathering (ERW). The authors successfully develop for the reader the connection of ERW to the oceans and how the transfer of alkalinity created by weathering in the near-field zone to the ocean contributes significantly to neutralize ocean acidification.
The focus of the paper is to address one of the primary challenges for ERW as a viable CDR method to be upscaled for CO2 drawdown of the magnitude needed to impact legacy emissions. That is, developing monitoring, reporting and verification (MRV) strategies for enhanced rock weathering that can constrain the uncertainty in current assessments of carbon removal. In agricultural lands, this is challenging given the complexities of soil and crop systems, which could make scaling ERW cost prohibitive despite its potential. Notably, the manuscript addresses organic-inorganic carbon cycle interactions in agricultural systems, which is a welcome component as most papers on ERW to date have under-addressed this sufficiently. They raise the issue of how soil organic carbon contributes to total alkalinity and stress how this potentially large, but difficult-to-constrain, contribution to alkalinity can yield erroneous estimates of soil porewater carbonate chemistry — on which tracking and validating CO2 drawdown depends. Coupled with the impact of temporary or permanent retention of weathering products in the soil profile due to cation sorption on exchangeable sites/functional groups and secondary mineral formation, this paper illustrates why using tradition aqueous-phase approaches have been shown to substantially underestimate weathering rates.
The paper offers a way forward that reconciles the inorganic carbon-based and cation-based approaches as well as accounts for all biogeochemical processes in agricultural studies, i.e., a cation-accounting approach. Specifically, Bijma et al. offer a modified version of the conservative expression for total alkalinity, which assesses the total concentrations of all potential major ions in the soil system and accounts for charge balance maintenance, as a primary parameter for MRV of ERW. This cation-accounting within the TA context approach is more practical and time-efficient and notably accommodating of biogeochemical processes in agricultural systems.
Additionally, the manuscript offers a good background on enhanced rock weathering (ERW or EW) – highlighting the co-benefits (neutralizing acidity; increased availability of micronutrients for plants) as well as the more viable approach to validating CDR. The accompanying appendices are excellent tutorials on controls on carbonate chemistry, including ocean and soil alkalinity, base cation fluxes and their fate in soil systems, etc for the broader community. The final closing section offers thoughtful follow-up research questions.
Overall, I find the manuscript suitable for publication in EGUsphere after straightforward revision. I emphasize that my suggested edits are primarily to further develop/elucidate on the geochemical and biogeochemical processes and reactions that are at the core of the manuscript, primarily for making the review and synthesis paper more accessible to a braoder readership.
Specific Comments: individual scientific questions/issues
- In Section 2: Enhanced Rock Weathering and agriculture (Lines 90-91) — I would add additional potential co-benefits of soil health such as increased water infiltration and holding capacity.
- In Section 2: Enhanced Rock Weathering and agriculture (Lines 107-117) — for clarity for the ‘less-well-initiated in ERW’ readers, consider providing links between the role of organic matter breakdown leading to soil acidity and impact on soil porewater carbonate chemistry, CO2 degassing, and the influence of basic cations on pH via charge balance as it will tie better into the readers understanding of the fate of all weathering products in the soil and how it relates to delays in CDR. Addressing this briefly in this section will also make a clearer link for some readers as to why cation- rather than carbon-accounting is preferable for estimating/verifying CDR. I appreciate this is addressed subsequently in this section and in Appendix 3, but a couple of additional sentences that makes these mechanistic linkages here rather than later (Lines 144-150) in Section 2 would be useful.
- Section 4 — Impact of ERW on organic carbon cycling: what is missing here is a counterpoint regarding the role of MAOM on SOC stabilization. Mesocosm studies are beginning to show that the response of microbial ecosystems to the addition of silicate (and even more so with organic) amendments may have as large, if not larger impact on the fate of SOC stabilization. One paper by Sohng et al. in review at Global Change Biology addresses this – but I believe there are others that have come out recently. Or perhaps adding a statement acknowledging that recent/upcoming research papers document that impacts on MAOM creation and changes in microbial ecosystem responses (initially increased SOC mineralization as an alternative energy source due to stress of the silicate amendment; on the annual scale, a shift to increased microbial biomass (SOC)) are creating the SOC stabilization effects. If the authors would like a preprint of the Sohng et al. paper, then we can share it, although other related studies have been published.
- In Section 5 MRV in agricultural soils (primarily referring to Lines 235-240) — a further developed summary discussion here of how cation accounting is used in the context of the modified (again, stating ‘how’ it is modified) conservative total alkalinity (as shown in detail in Appendix 2), and a reference to the preferred methodology(ies) for measuring cations (in solution? in the soil, extractable fraction? all?) with citations to existing papers (summarizing in words what is presented as equations in Appendix 2) will make the main text more mechanistically linked to subsequent sections and in turn reinforce why typically used TA approaches won’t work in agricultural fields — and therefore more useful to a broader range of readers.
- Section 6 — This is a well-presented and comprehensive discussion of the role of sorption and cation exchange in soils on CDR. That said, I think it would be additionally useful for the broader readership if a bit more background was provided to define soil characteristics such as base saturation, CEC and indicate the mechanistic linkages to each other and to soil pH and formation of secondary clays — especially in the context of the last paragraph in the section. Also, can the potential for the relative impact of CO2 sequestration by secondary clay-organic interaction in the ocean vs. the negative impact of cation sequestration by secondary clay formation in the soil system be addressed – even semi-quantitatively? I also question whether the impact of neo-formed clays is not the primary one in terms of secondary silicates in such systems. There is literature that addresses this. Adding a couple of soil-science based papers on this would make this discussion more robust.
- Robust overview of the different soil processes that can influence pathways of cations but based largely on the ERW community’s literature. perhaps could expand to include more soil-based literature.
Technical Corrections:
- Lines 137 to 143: perhaps this can be cut and possibly integrated into Appendix 2– this ocean-focused discussion is a bit distracting from the arable lands focus at this point.
- Line 232: consider replacing ‘derive’ with ‘disentangle’.
- Section 6 — Fate of the cations: Line 252: consider adding ‘primarily due to pH-dependent negatively charged exchange sites provided by ‘cation exchange capacity’. Again, for clarification for a broader readership.
- Line 282: I believe ‘ions’ should be ‘ion’. And I would add that the preferential ion sorption is also dependent on charge density (lyotropic (Hofmeister) series)
- Lines 290 to 295: Discussion of influence of carboxylate and phenolate groups on cation mobility through the soil system — consider adding to a statement about how deprotonation of these functional groups can increase negatively charged exchangeable sites for basic cations as well as SOC bioavailability.
- Footnote on page 13: I’d add a reference or two for the reader as this is a review.
- Appendix 2. Line 49. I’d add Brantley et al. 2023 to the reference list. Ditto for the references listed in Line 464.
- Appendix 4: refs for role of SOC on cation sorption?
Citation: https://doi.org/10.5194/egusphere-2025-2740-RC1
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