"Digestate" covers material ranging from a thick solid fraction to something barely distinguishable from wastewater. Two projects applying digestate can be doing almost unrelated things. Four measurements separate them, and they should be in the project file before any carbon claim is made.
The Four Parameters
| Parameter | What it tells you | Why it matters for the claim |
|---|---|---|
| Dry matter content (%) | How much material is actually there | Carbon per tonne applied scales with it |
| Total organic carbon | The carbon being added | The direct input to any SOC claim |
| C:N ratio | Balance of carbon to nitrogen | Predicts N₂O risk and nitrogen immobilisation |
| Stability index | How far decomposition has progressed | Predicts how much persists |
Dry matter is the one that catches people. A liquid digestate at 4% dry matter delivers a fraction of the carbon per tonne that a solid fraction at 25% does. Reporting application in tonnes without stating dry matter makes the carbon input unquantifiable.
C:N Is the Operational Number
A low C:N ratio — nitrogen-rich relative to carbon — means rapid nitrogen release, high crop availability, and elevated N₂O risk. A high ratio means nitrogen gets immobilised by microbes decomposing the carbon, temporarily reducing crop availability.
For carbon purposes, a higher C:N is generally more favourable: more carbon delivered per unit of nitrogen risk. For agronomic purposes the preference often runs the other way. Projects that optimise purely for one usually create a problem in the other, and the sensible position is to know the number and manage around it rather than to target an ideal.
Stability Determines Persistence
A well-stabilised digestate has had its labile carbon consumed during digestion; what remains decomposes slowly in soil. Under-digested material still contains readily decomposable carbon that will largely respire away within a season.
This maps directly onto what can be claimed. Applying unstable material adds carbon that will not be there at the next monitoring event. Measuring stability — respiration rate under standard conditions is the usual approach — tells the project what proportion of the application is a real input rather than a temporary one.
Contaminants Are a Separate Gate
Heavy metals, pathogens and plastic fragments do not affect the carbon accounting and can stop the project entirely.
Application limits are typically set by agricultural and environmental regulation rather than by the carbon standard, and they bind regardless of what the methodology permits. A project designed around an application rate that exceeds the local nitrogen or metals limit has a compliance problem, not an accounting one.
Sampling Is Not Once
Digestate composition varies with feedstock, season and process conditions. A single characterisation at project start describes one batch.
Methodologies working seriously with amendments require repeated characterisation at a stated frequency, with the applied carbon calculated from the batch actually applied. Using a two-year-old analysis to quantify this year's application is a traceability gap that verification will find.
Turn climate goals into a verified portfolio
TREEO connects the full carbon cycle — eligibility, simulation, real-time monitoring and registry-ready reporting — combining expert consulting with dMRV technology so the evidence exists before anyone asks for it.
