the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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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Status: open (until 25 Oct 2025)
- RC1: 'Comment on egusphere-2025-2740', Isabel Montañez, 09 Sep 2025 reply
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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
Technical Corrections: