Mixing biochar into your enhanced weathering plot doesn’t just add carbon — it changes the entire soil chemistry playing field. That’s the headline from a new Wageningen University study in Geoderma, and it has real implications for how we think about stacking CDR approaches.
The researchers deployed dunite (an ultramafic rock, rich in olivine) with and without biochar on two different soil types. What they found: biochar co-deployment boosted reactive (hydr)oxide mineral formation and raised soil pH — both factors that accelerate silicate weathering. In clayey soils specifically, biochar slightly enhanced dunite dissolution rates. Two CDR methods working together, each making the other more effective. Sounds great, right?
Here’s where it gets uncomfortable. Soil respiration — the CO₂ coming back out of the ground — increased significantly on both soil types. In these short-term experiments, that respiratory CO₂ release actually exceeded the inorganic carbon sequestered through weathering. The biochar stimulated microbial activity, which chewed through organic carbon faster than the mineral weathering could lock up inorganic carbon.
Short-term pain, long-term gain? Probably. Biochar’s own carbon is highly recalcitrant (centuries-scale persistence), and weathering rates should compound over time as mineral surfaces activate. But the study is a sharp reminder: you can’t just add interventions together and assume the CDR math is additive. Biology has opinions.
The soil type finding is particularly striking — and it rhymes with what CDI has been seeing in our own research. CDI runs the world’s largest enhanced weathering greenhouse experiment, with over 400 lysimeters tracked across 2+ years. Our Paper 1 revealed that cation retention in soil is massive: 10–50x more cations stay retained in the soil matrix than get exported as alkalinity in drainage water during Year 1. Acidic soils weather fastest and retain the most. High-pH soils hit chemical saturation and stall out.
The Wageningen results reinforce this. Soil type isn’t a minor variable — it determines whether your enhanced weathering deployment sequesters carbon efficiently or barely moves the needle. This is why CDI advocates for cation-based MRV that tracks total weathering products, not just the fraction that makes it into runoff as dissolved inorganic carbon. If you only measure alkalinity export, you’re missing the majority of the action in Year 1.
What this means for the field: Co-deploying biochar and crushed rock is genuinely promising. The synergies in soil chemistry are real. But anyone building MRV protocols for stacked deployments needs to account for the respiratory CO₂ spike, the soil-type dependency, and the massive cation retention that masks true weathering rates in early years.
The era of single-pathway CDR thinking is ending. Stacking works — but only if your measurement framework can handle the complexity.