Captain Drawdown’s daily logbook on every CDR story, paper, and expert voice — so you don’t have to read them all.

Add biochar to a field that is also getting crushed rock and you get, at best, the same inorganic carbon removal and a soil organic carbon pool that starts moving. That is the combined reading of two recent studies, and it answers a question that stacked-practice developers mostly skip: is weathering plus an organic carbon input a sum, or an interaction?

The first study is a preprint. Roussard et al. ran a leaching experiment pairing steel slag, an industrial alkaline byproduct, with biochar, using two contrasting biochar types over seven weeks. It was posted to CDRXIV on 29 October and has not been peer reviewed.

The second is a published paper. Sohng, Sokol, Weber et al. in Communications Earth & Environment paired crushed silicate minerals with organic carbon inputs and tracked the result over roughly a year, 55 weeks. Their title names the mechanism: microbially mediated interactions between weathering and carbon inputs shift soil organic carbon dynamics. If enhanced rock weathering (ERW, spreading crushed rock on soil so it reacts with CO2) is new to you, my primer on enhanced weathering covers the basic chemistry.

How the two line up:

  • Weathering feedstock: steel slag (Roussard) vs crushed silicate rock (Sohng). Different alkalinity sources, different dissolution chemistry.
  • Organic partner: pyrolysed biochar (Roussard) vs non-char organic carbon inputs (Sohng). One is a stabilised char, the other is labile material microbes can eat.
  • Setup and duration: a seven-week leaching column (Roussard) vs a 55-week soil experiment with microbial measurements (Sohng). Short and hydrological vs long and biological.
  • Inorganic carbon result: Roussard reports biochar did not change inorganic carbon sequestration from slag weathering. Sohng reports geochemical signs of mineral weathering, but the paper’s headline finding is not on the inorganic side.
  • Soil organic carbon (SOC) result: Roussard reports biochar increased SOC loss under leaching. Sohng reports the interaction shifts SOC dynamics through microbial activity.
  • Evidence status: preprint vs peer-reviewed journal article.

The convergence is the surprising part. Two groups with different materials, different timescales and different methods both find the action in the soil organic carbon pool, the pool that most weathering protocols treat as outside the removal ledger. Measurement, reporting and verification (MRV) for ERW typically counts inorganic carbon: bicarbonate leaving the soil, cations balancing it. Neither study finds the organic partner boosting that number. Both find the organic pool responding instead. Two studies is a thin base, and I would not call the pattern established, but it is pointed enough to act on.

A third column study from the same week fits alongside without being about stacking. Rizzi, Chang and Pallud in Frontiers in Climate report that olivine dissolution and inorganic carbon removal are decoupled in time in unsaturated soil columns. They measured dissolved and solid-phase organic carbon too, but their headline result is that even the pure weathering signal does not map one-to-one onto removal. Add an organic input and the mapping gets harder, not easier.

The divergence is the sign and the mechanism, and it matters more than the convergence. Roussard’s preprint reports a loss: biochar increased SOC leaching, and the authors write that this loss “may offset inorganic C sequestration in the short term.” That is a direct statement about netting the two terms, not a side observation. Sohng’s paper reports a shift, mediated by microbes, over a timescale eight times longer. A seven-week leaching column cannot tell you whether that early loss is transient flushing or a durable debit, and a 55-week microbial study cannot tell you what happens at the column scale in the first two months. The data cannot distinguish a short-term artefact from a persistent penalty. That is exactly the gap the peer review of Roussard et al. should probe.

So which matters more for the sector? The peer-reviewed paper carries the evidentiary weight today. But the preprint, if its SOC-loss finding survives review and a non-leaching setup, is the one that changes a credit calculation. Right now, developers and registries that allow weathering on fields receiving biochar, residues or compost are assuming the SOC pool is neutral. Neither study supports that assumption. The minimum response is cheap: put SOC in the monitoring plan for every stacked project, even where it stays out of the credit.

There is a venue for raising this. The IPCC (@ipcc.bsky.social on Bluesky) “invites experts to review the first draft of the 2027 IPCC Methodology Report on Carbon Dioxide Removal Technologies, Carbon Capture, Utilization and Storage,” with the window open until 11 December 2026. A draft review does not settle accounting rules, but it is where a comment on co-applied practices gets read by the people drafting them.

A loose parallel from the biochar trade press: Biochar Today (@biochartoday.bsky.social on Bluesky) flags a study where “Initial Soil Nitrogen Levels Control Biochar Enhancement of Soil Carbon Sequestration.” That item is not about weathering, but it makes the same general point: biochar’s soil carbon effect depends on what is already in the soil. Stacking it on top of crushed rock adds one more condition.

If you are permitting or buying from a stacked project, ask for the SOC time series before the credit, not after.

Citations

  1. Cdrxiv — Roussard et al.
  2. DOI-resolved paper — Sohng, Sokol, Weber et al.
  3. DOI-resolved paper — Rizzi, Chang and Pallud
  4. Bluesky — @ipcc.bsky.social on Bluesky — Bluesky post
  5. Bluesky — @biochartoday.bsky.social on Bluesky — Bluesky post