Can Digestate Increase Soil Organic Carbon? Evidence from Long-Term Field Studies

Anaerobic digestion can contribute to soil-carbon management by producing materials that return organic matter to farmland. Long-term research provides encouraging evidence, including results for compost produced after a biogas-production stage. However, the material applied, application rate and soil conditions all affect the outcome.

When this post first appeared in 2018, it considered the potential contribution of anaerobic digestion to sustainable land carbon storage (sequestration) in the context of the then current reports from the UK’s Committee on Climate Change. Carbon storage (often referred to as CCS) is becoming established as a way to ameliorate global warming.

There is now more field evidence to examine. One particularly useful source is Jakob Magid’s report, Recycling, Fertility and Resilience in Soil, based on the University of Copenhagen’s CRUCIAL experiment.

Its findings support a positive but specific conclusion: some organic amendments associated with anaerobic digestion can increase soil organic carbon. That in itself is a big long-term finding. They do not establish that every digestate stores the same proportion of carbon, or that the carbon remains permanently.

How Anaerobic Digestion Connects to Soil Carbon

Anaerobic digestion converts part of the organic material entering a digester into biogas. Some carbon leaves in methane and carbon dioxide, while some remains in the digestate.

After land application, the remaining organic material continues to decompose and interact with the soil. Consequently, carbon applied to a field is not the same quantity as carbon retained there.

Research on carbon sequestration following application of digestates from one- and two-step anaerobic digestion examines these distinctions directly.

For an introduction to the material and its liquid and fibrous fractions, see our guide to what digestate is.

What the CRUCIAL Experiment Actually Tested

CRUCIAL was established in its current form in 2003 on sandy-loam soil near Copenhagen. Its original design included 11 fertilisation treatments, each replicated three times.

The treatments included composted household and garden waste, sewage sludge, livestock manures, human urine, mineral fertiliser and unfertilised plots. Some treatments received deliberately accelerated applications.

The report confirms two connections with anaerobic digestion:

• The household-waste compost underwent an AIKAN biogas-production stage before further composting to maturity.
• The wastewater sludge underwent anaerobic digestion for methane production before application.

These are relevant examples of returning organic material to land after energy recovery. Nevertheless, mature compost and digested sewage sludge must remain distinguishable from ordinary agricultural or food-waste digestate.

The report’s methods appear on pages 18–19. The university’s CRUCIAL project page also describes the experimental treatments.

Our separate review of what 20 years of soil data show about biosolids, digestate and manure discusses the wider soil-health and contamination findings.

What the Report Says About Carbon Retention

On page 34, Magid summarises research examining soil organic matter after 12 years of repeated applications.

The report identifies compost as the most effective treatment for increasing soil carbon content and states that approximately 45% of the added carbon was retained in soil.

That is an encouraging finding for an amendment produced through a sequence involving biogas recovery and composting.

However, the percentage refers to carbon added in the compost treatment. It does not mean that 45% of the original incoming waste carbon survived digestion, composting and land application. Nor does it establish a permanent-storage fraction.

The underlying study also found that different amendments produced different types of accumulated soil organic matter. Read Peltre and colleagues’ research on CRUCIAL soil-carbon composition.

Why the High Application Rates Matter

The experiment deliberately tested conditions beyond normal farming practice.

Table 1 of the report estimates the following cumulative inputs during 2003–2022. These figures describe carbon and phosphorus inputs, not the wet weight of material spread.

The accelerated compost and sludge treatments received phosphorus equivalent to approximately 211 and 249 years respectively at the report’s reference rate of 30 kilograms per hectare annually.

Those are phosphorus-loading equivalents, not centuries of elapsed observation. The author also describes the cumulative inputs as estimates because precise load weights were unavailable in the early years.

The results therefore cannot be used to recommend comparable application rates on farms. For practical land-application considerations, see our guide to using digestate fertiliser in agriculture.

What the Long-Term Carbon Graphs Show

Figure 5, on page 23, shows increasing soil-carbon concentrations under compost and several other organic treatments. The accelerated compost treatment shows the largest increase.

Other treatments largely maintained carbon or showed declines. The report notes that cattle slurry produced no clear improvement over mineral fertiliser or human urine despite additional carbon inputs.

This is a useful reminder that adding organic carbon does not automatically produce a matching increase in soil carbon.

There is also an explicit analytical limit. On page 24, the author says comprehensive analysis of the carbon time series has not been completed and discusses substantial variation between sampling years.

The graph reports carbon concentration, rather than a complete carbon-stock calculation. Its plotted changes should therefore not be converted directly into tonnes of carbon dioxide removed per hectare.

Direct Field Evidence for Conventional Digestate

CRUCIAL is complemented by research examining digestate applied directly to commercial farmland.

A 2025 Iowa study analysed 14 fields receiving anaerobic digestate over 5–12 years. Across 421 soil samples from the upper 15 centimetres, most sampled locations showed increased soil organic carbon stocks.

The reported average accumulation rate was approximately 0.8 tonnes of carbon per hectare annually. However, phosphorus also accumulated, eventually exceeding crop requirements in the system studied.

This supports digestate’s potential under the conditions examined while demonstrating why carbon and nutrient management must be assessed together. It is not a standard sequestration rate for other farms. See Adding anaerobic digestate to commercial farm fields increases soil organic carbon.

Soil Carbon Storage and CO2 Recovery from Biogas Are Different

Returning organic matter to farmland differs from recovering gaseous carbon dioxide during biogas upgrading.

• Digestate application: returns organic material to soil, where some carbon may contribute to soil organic matter.
• CO2 recovery: separates and captures carbon dioxide from a gas stream for subsequent use or storage.
• Permanent CO2 storage: requires a durable storage route and assessment of the wider carbon balance.

Our separate article explains food-grade CO2 recovery from biogas.

Recovering CO2 for a product does not automatically establish permanent carbon removal. That requires consideration of its eventual fate and the wider lifecycle, as discussed in research on carbon capture and storage in biogas systems.

What Farmers and AD Operators Should Take from the Evidence

The findings support assessing the actual material and receiving soil rather than assuming a standard carbon benefit from every tonne of digestate.

• Identify whether the material is whole digestate, separated fibre, liquid digestate, digested sludge or a composted amendment.
• Measure carbon and nutrient inputs alongside the application history.
• Use consistent soil sampling and account for bulk density when estimating carbon stocks.
• Distinguish maintaining existing soil carbon from increasing stocks above their starting level.
• Match applications to nutrient requirements rather than applying extra material solely to add carbon.

Material quality also matters. Our article on digestate composition, contaminants and uses explains the importance of understanding what reaches the field.

The Advantage of Anaerobic Digestion for Soil Carbon

The evidence gives the AD industry a credible soil-carbon story.

CRUCIAL demonstrates benefits from specific recycled amendments, including materials that underwent anaerobic processing. Direct digestate research provides additional evidence from commercial fields.

These findings strengthen the case for viewing anaerobic digestion as a system that can combine renewable energy production, nutrient recycling and contributions to soil management.

The strongest claims identify which material was tested, how much was applied and what remained in soil over the measured period. They keep those findings separate from gaseous CO2 recovery and permanent carbon storage.

Sources and Further Reading

• Jakob Magid, Recycling, Fertility and Resilience in Soil, published by Genanvend Biomasse. See particularly pages 18–24 and 34.
• University of Copenhagen: CRUCIAL long-term field experiment.
• Peltre and colleagues, Repeated application of organic waste affects soil organic matter composition, 2017.
• Villarino and colleagues, Adding anaerobic digestate to commercial farm fields increases soil organic carbon, 2025.