Enhanced weathering sounds simple in theory: spread crushed rock on farmland, let it dissolve in soil water, and the chemical reactions pull CO₂ out of the atmosphere. In practice, the soil has opinions.

New research presented at EGU 2026 — the European Geosciences Union General Assembly in Vienna — digs into one of the field’s most important open questions: what happens when you combine enhanced weathering with organic carbon amendments? The answer appears to be “it depends” — which in soil science means “it’s complicated.”

The Experiment

The researchers tested how different rock materials — specifically metal slags and inorganic fertilizer minerals — react when combined with various organic carbon sources, including compost and biochar. They measured how these combinations affected the dissolution rate of the minerals and the dynamics of inorganic carbon in the soil.

This matters because in real-world farming, enhanced weathering rarely happens in isolation. Farmers already add compost, manure, and sometimes biochar to their soils. If a CDR company comes along and says “also spread this crushed basalt,” the rock doesn’t land on a blank canvas. It lands in a complex soup of organic matter, microbes, root exudates, and existing minerals.

Why Organic Carbon Changes the Game

Organic carbon in soil breaks down into organic acids. Those acids can accelerate mineral dissolution — which is good for carbon removal, because faster dissolution means faster CO₂ drawdown. But they can also change the soil pH, alter microbial communities, and shift the balance between dissolved inorganic carbon (the stuff you want to measure as CDR) and dissolved organic carbon (which complicates the accounting).

The EGU research found that the interaction between rock type and organic carbon source matters more than either factor alone. Metal slags — industrial byproducts that some companies are testing as enhanced weathering feedstocks — reacted differently depending on whether compost or biochar was the organic amendment. The dissolution rates, the carbon dynamics, and the downstream chemistry all shifted.

This isn’t surprising if you think about it. Compost is biologically active and decomposes rapidly, releasing acids and nutrients. Biochar is relatively inert, changes soil structure and water retention, and persists for centuries. They create fundamentally different chemical environments for mineral weathering to occur in.

Implications for CDR

For enhanced weathering companies and researchers, this has several practical consequences:

MRV gets harder. If the rate and pathway of mineral dissolution depend on what else is in the soil, then measuring carbon removal requires understanding the full soil system — not just how much rock you spread. Cation-based and total alkalinity-based MRV approaches may be better suited to capture these interactions than simple soil carbon measurements.

Feedstock choice matters more than we thought. Metal slags, basalt, olivine, and wollastonite don’t just differ in their inherent reactivity. They differ in how they interact with real farm soils that already contain organic amendments. Testing a mineral in a lab is one thing. Predicting its behavior in a field that gets annual compost applications is another.

Biochar + enhanced weathering could be powerful — or confounding. Some companies are already exploring combined biochar and crushed rock applications, reasoning that both methods sequester carbon through different mechanisms. This research suggests the interaction between the two may not be simply additive. They change each other’s behavior.

Multi-year, multi-pool monitoring is non-negotiable. A short-term experiment measuring one type of carbon in one type of soil with one mineral amendment will miss the interactions that drive real-world outcomes. The field needs longer experiments with more variables tracked simultaneously.

The Bigger Picture

Enhanced weathering is one of the most researched CDR pathways globally, and for good reason — it’s potentially cheap, scalable, and produces co-benefits for agriculture. But the gap between “this works in a controlled pot study” and “we can reliably measure how much carbon this removed from a real farm field” remains wide.

Research like this narrows that gap — not by making the answer simpler, but by showing us where the complexity actually lies. The soil doesn’t care about our models. It reacts to what’s actually in it.

🔗 Related Reading