New paper: Carbon vs. cation-based MRV for Enhanced Rock Weathering (and why we should check how much soil carbon matters)

A new and interesting paper from Jelle Bijma et al. was published today (and we helped a bit over the last 5 years to do the science behind it).

What the paper says

Enhanced Rock Weathering (ERW) sounds simple on paper: spread crushed rock on fields, the rock slowly dissolves, and part of the CO₂ ends up stored as dissolved bicarbonate in water that eventually reaches the ocean. The hard part is MRV—Monitoring, Reporting, and Verification—meaning: how do you prove how much CO₂ was removed, in a way that is credible enough for carbon credits? Bijma et al. argue that in real farmland (an “open system” with plants, microbes, fertilizer, harvest, rain, and drainage), trying to “close” a full carbon budget is usually not practical. They propose using total alkalinity (TA) as a central bookkeeping variable—but only if you treat it as a charge-balance problem over time, not as a simple “count bicarbonate and you’re done” shortcut.

A big part of the paper is a warning label on a method that many people (understandably, due to a lack of alternatives) borrowed from ocean chemistry: measuring alkalinity by acid titration and then converting it into bicarbonate. In seawater, this works well because alkalinity is dominated by the carbonate system and can be standardized. In soil porewater, as the paper points out, it often does not work the same way, because soils contain lots of organic acids (from decomposing plant material and microbial activity) that also contribute to alkalinity in messy, hard-to-parameterize ways. The authors explain that this “organic alkalinity” can distort both titration alkalinity and pH-based calculations—so using TA + pH (and sometimes DIC) to compute bicarbonate in agricultural leachate can easily produce numbers that are not reliable enough for robust MRV.

So what do they propose instead? Think of alkalinity in two “currencies” that are equivalent in the ocean but not easy to track on land: carbon currency (bicarbonate/carbonate) and related cation currency (positively charged ions like Ca²⁺, Mg²⁺, K⁺, Na⁺ released when rock dissolves). The authors recommend a cation-based MRV approach grounded in an explicitly conservative charge-balance expression for TA (TAec), combined with (bi)annual soil sampling—because tracking cations in the solid phase can be more practical than continuously monitoring all carbon flows in and out of a field. Crucially, they also argue you can’t ignore soil organic carbon (SOC): ERW can shift carbon into mineral-associated forms (e.g., via clay formation and cation-bridging) on climate-relevant timescales, so “only count bicarbonate” is the wrong mental model.

How this connects to our latest work

This paper reads like a theory-and-MRV backbone for what we have been seeing experimentally. In our 2023/2024 greenhouse lysimeter dataset (which Bijma et al. directly references in their paper), we tracked soil CO₂ efflux (upward gas flux) and leachate TA (downward aqueous signal) over 24 months and found that they often behave like two different worlds: different time patterns, different phases, and no simple correlation in year-to-year deltas—especially in the first months where disturbance effects dominate. Our conclusion there was already pointing toward the same implication Bijma et al. make: short experiments and single-channel measurements are a weak foundation for long-term claims, and “carbon-budget closure” in an open, biologically active system is not something you get for free.

And the paper’s focus on “where the cations go” maps directly onto our biggest open loop. In our multi-year greenhouse monitoring write-up, we saw strong soil–rock interactions and unexpectedly high variability, including cases where basalt looked “procrastinating” and soils looked like cation sinks—suggesting that a large share of weathering products may be retained in the soil system rather than showing up quickly in leachate. That is exactly why our own conclusion was: we need far more soil–rock combinations and solid-phase measurements to understand cation fate and build data-based MRV models.

Our September 2025 call for proposals is the operational next step: we have 1,000+ soil and biomass samples in storage from the 2023/2024 wave that are largely not yet analyzed, and we’re inviting collaborators (with grant support) to help quantify what happened inside the soil columns—precisely the kind of evidence a cation/SOC-aware MRV approach needs. We will hold a symposium in June 2026 where results from these samples will be announced.

Why we recommend reading this paper

If you work in ERW (research, MRV, project development, procurement, policy), this is one of those papers that you should not ignore.

Key reasons:

• It clearly explains the theory why ocean-style alkalinity thinking breaks in soils (organic alkalinity and sampling realities matter).

• It suggests to explore new ways for MRV: charge balance and cation accounting in the solid phase, complemented by SOC monitoring—because it will remain a challenge to close every carbon flux in living farmland.

• It calls for more science on SOC as a potentially meaningful (and MRV-relevant) pathway.

An author list full of ERW MVPs

The author team spans major nodes of the enhanced weathering ecosystem—ocean/alkalinity chemistry, isotope geochemistry, soil carbon science, and MRV-heavy applied work—across institutions like Alfred Wegener Institute, Wageningen, Yale, UCL, Newcastle, Georgia Tech, Heriot-Watt, and the University of Antwerp, with direct connection to implementation through Carbon Drawdown Initiative involvement.

Read the paper!

Here is the link: Biogeosciences (2026), “Carbon vs. cation based MRV of Enhanced Rock Weathering and the issue of soil organic carbon” (doi: 10.5194/bg-23-53-2026).