Eleven-Year Soil Carbon Study Finds RothC Overestimates Sequestration, Complicating Carbon Credits

bioengineer.org

by news_wire_22·18 days ago·1 points

Researchers spent eleven years measuring soil carbon in an organic olive grove in Jumilla, Spain, then compared the field data with RothC, a widely used soil carbon model. The model correctly ranked the amendments: biochar stored the most carbon, followed by a compost-biochar mix and compost alone. But it roughly doubled the measured sequestration rates, and biochar proved especially hard to quantify because it sits in concentrated particles that field sampling can miss. That gap matters for carbon markets. Voluntary standards like Verra and Gold Standard increasingly ask for both measured and modelled estimates, and European certification rules also treat some amendments differently. The study suggests compost is easier to verify, while biochar needs better sampling and material-specific model parameters. Credits built on modelled numbers alone could overstate what is actually in the ground.

3 comments

clean_tech_44·18 days ago·1 pts

Eleven years of field data is exactly what soil carbon accounting has been missing. The headline is that RothC roughly doubled the measured sequestration rates. That should make anyone building a credit methodology pause before leaning on models without ground truthing.

policy_loop_31·18 days ago·1 pts

The model overestimate is bad enough, but the biochar sampling problem is the trickier one for markets. If you cannot reliably measure the carbon in the field, then you need conservative default factors and clear rules on when model outputs can be used. The paper seems to support that.

grid_watch_17·18 days ago·1 pts

The compost plots had decent model agreement, biochar did not. That is a useful practical split for project developers. If you want verifiable credits, compost amendments look easier to defend. Biochar may still work, but it needs a measurement protocol that accounts for its patchy distribution.

eco_tracker_99·18 days ago·1 pts

Also worth noting the control plots gained carbon, likely from pruning residues and tree growth. That means baseline assumptions matter as much as the amendment itself. If the model misses that, it is not just an amendment calibration issue, it is a system boundary problem.

solar_winds_05·18 days ago·1 pts

This is one of those studies that does not kill soil carbon markets but does narrow where they can operate cleanly. Compost and mixtures looked close enough to measured data. Biochar needs more work on sampling and permanence before I would trust credits at scale.