Anaerobic digestion removes the most readily decomposable carbon from a feedstock and converts it to methane. What comes out the other end is therefore not the same material that went in — it is enriched in the fractions microbes found hardest to break down. That changes how it behaves once it reaches the soil, in ways that are mostly favourable and one way that is not.
Why Digested Carbon Is More Stable
Raw manure applied to soil loses much of its carbon quickly; the easily-decomposed fraction is consumed within weeks.
Digestate has already had that fraction removed. The remaining carbon is more recalcitrant, so a greater proportion of what is applied persists.
| Raw feedstock | Digestate | |
|---|---|---|
| Total carbon applied per tonne | Higher | Lower |
| Share that persists in soil | Lower | Higher |
| Nitrogen availability | Slower release | More immediately available |
| Odour and pathogen load | Higher | Lower |
The trade in the first two rows is the crux: digestion reduces how much carbon there is while increasing the fraction that stays. Whether the net soil carbon outcome beats direct application depends on the feedstock and the digestion parameters, and is not a foregone conclusion either way.
The Nitrogen Problem
Digestion mineralises organic nitrogen into ammonium — which is agronomically useful and a direct nitrous oxide risk.
N₂O has roughly 273 times the warming effect of CO₂ per unit mass. A digestate application that raises soil carbon by a modest amount while triggering an N₂O pulse can be net-negative for climate even though the soil carbon measurement shows a gain.
Any credible accounting for digestate application must include N₂O from the applied nitrogen. Projects that measure the soil carbon change and omit the emission are reporting half a ledger.
The mitigations are known and practical: incorporate rather than surface-apply, time applications to crop demand rather than to storage convenience, and avoid application to saturated soil where denitrification peaks.
The Counterfactual Question Again
The carbon in digestate came from somewhere. If the feedstock would otherwise have been applied to land anyway, applying it post-digestion is a change in form, not an addition of carbon to the system.
The genuine additional cases are where the feedstock would otherwise have decomposed aerobically with no soil benefit, or been discharged as waste. In Indonesian palm oil, effluent that would otherwise sit in open lagoons emitting methane is the clearest example — and there the primary climate benefit is the avoided methane, with soil carbon a secondary effect.
What This Means for Project Design
Digestate projects usually have a strong climate case that runs through methane avoidance rather than soil carbon. Leading with the soil carbon claim tends to invite scrutiny of a modest, hard-to-measure quantity while underselling the large, well-characterised one.
Measure the soil carbon if the design supports it. Build the claim on the emissions avoided.
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.
