Biochar in agriculture: Major climate potential - but a need for knowledge and regulation

Sometimes it looks like charred muesli. Other times like little black hare droppings. Biochar is as varied as the organic material you derive it from. 

What all biochar has in common is that it is a black, porous material with the potential to play an important role in the future of agriculture. 

Biochar is not a byproduct, but an active measure that can improve soil structure, sequester carbon and recycle nutrients. But what is biochar, how does it work, and why is it still surrounded by uncertainty? 

What is biochar? 

Biochar is a solid, carbon-rich material produced through pyrolysis - a process in which organic material (biomass) is heated to high temperatures (typically 400-800 °C) without access to oxygen. 

Under these conditions, the biomass is not broken down into ash, as in ordinary combustion, but is instead converted into a stable form of carbon. 

The biomass can be many things: straw, grass, biogas digestate, sewage sludge - in principle, any organic material that contains carbon. In Denmark, however, the focus is especially on side streams from agriculture and digestatefrom the production of biogas. 

Controlled combustion without oxygen 

Along with the type of biomass, the pyrolysis conditions determines how the finished biochar will appear - and what properties it contains. Temperature and residence time (i.e., the time spent in the reactor) both matter greatly. 

At lower temperatures (e.g. 400-500 °C), more nutrients are retained in the biochar, but the carbon is less stable. At higher temperatures (700-800 °C), the carbon becomes more resistant to degradation, but some nutrients - such as phosphorus - become tightly bound. 

"The more you heat the biomass, the more nutrients disappear, but the more stable the remaining carbon becomes," says Lars Elsgaard, professor at the Department of Agroecology at Aarhus University, who works with assessing the effects of biochar in agricultural soils. 

In addition to the biochar itself, a pyrolysis plant also produces pyrolysis gas and oil, which can be used to power the process or for energy purposes. At some plants, there is even a net surplus of energy. 

Biochar as a climate measure 

One of the most-discussed properties of biochar is its ability to store carbon in the soil - and thereby reduce the amount of CO₂ in the atmosphere. When plants grow, they take up CO₂ from the air. If the plant material is subsequently converted into biochar and incorporated into the soil, the carbon can remain there for many decades or centuries. 

This is in contrast with the original biomass, such as straw, which is quickly broken down into CO₂ when incorporated into the soil. 

Research shows that up to 90-97% of the carbon in biochar can remain in the soil for a very long time for the most stable types of biochar. Half-lives of many hundreds of years are not unusual. This makes biochar one of the few technologies that can deliver negative emissions - that is, remove CO₂ from the atmosphere and store it. 

But if biochar is so stable, why not just store it in silos or old coal mines? 

The answer is twofold: When biochar is spread on the fields, it can improve the soil's structure, water-holding capacity and pH - especially in sandy soils. At the same time, the biochar contains some nutrients, particularly phosphorus and potassium, which there is an agronomic interest in utilising. 

In order to document that the carbon is permanently stored, many climate registration systems also require that the biochar is mixed into the soil. If it is merely stored, it could potentially risk being burned off later, whereby the climate benefit would be lost. 

"Once you have mixed it into the soil, you no longer have that option," says Lars Elsgaard. 

Soil improvement and nutrients 

Beyond its potential to store CO₂, biochar also has agronomic effects, as mentioned. It can improve soil structure, especially in sandy soils, where it acts like a sponge and retainswater and nutrients. Biochar often has a high pH value, which can be beneficial in acidic soils, where optimum pH for crop production is normally maintained through liming. 

"Biochar has a very porous structure, so it can absorb and retain water - especially in periods of drought," Lars Elsgaard explains. 

The content of phosphorus and other nutrients in the biochar depends on the biomass and the temperature during pyrolysis. At very high temperatures, the phosphorus becomes so tightly bound that it may be unavailable as fertiliser. 

The effects also vary depending on soil type, climate, and the form of biochar that is applied. 

"Precisely how the phosphorus content in biochar is best utilised in agriculture is one of the great topics currently being researched," says Lars Elsgaard. 

Uncertainties and concerns 

Despite the many promising properties, there is also scepticism and concern. 

Biochar that is not produced correctly can contain undesirable substances such as tar and heavy metals. The heavy metals enter via the biomass pyrolysed and can therefore be avoided by controlling the biomass. 

The tar, on the other hand, can form during the pyrolysis itself. Producing biochar without undesirable tar substances therefore requires a well-controlled process - something that professional pyrolysis plants have a strong focus on and have technologies to achieve. 

"Biochar is not just biochar. Its properties and constituent substances depend greatly on how the biochar is produced. There is therefore still a concern about what biochar actually contains - and how it affects soil organisms," says Lars Elsgaard. 

There is especially uncertainty about how biochar affects soil microbiology and fauna over time. Many of the experiments carried out previously have been short-term studies, conducted under laboratory conditions or under field conditions that differ from Danish agricultural conditions. 

One possible point of concern is whether returning biomass to the soil in the form of non-degradable biochar could remove the food base for the microorganisms and thereby negatively affect food chains that are necessary to maintain good soil health. 

In order to gain better knowledge of the agronomic and environmental effects of biochar in Danish agriculture, Danish experiments examining the long-term effects of biochar have been initiated in recent years. 

Legislation and barriers 

The application of biochar has not yet become a fixed part of Danish agricultural practice. One of the biggest barriers to the use of biochar in agriculture is the lack of regulation. There is, so far, no specific statutory order defining quality requirements and permitted quantities for biochar. 

At present, it is up to the environmental authorities in the individual municipalities to grant permission for biochar to be spread on agricultural land, based on an overall assessment under section 19 of the Environmental Protection Act. 

"The municipalities have to grant environmental approval, but there are no clear rules - so it can be a challenge for the municipalities to process applications for biochar application," says Lars Elsgaard, noting that in November 2025 the Danish Environmental Protection Agency updated a guidance statement on the application of biochar for agricultural purposes. 

It is noted here that there is a lack of documentation of biochar's long-term effects on soil, the aquatic environment and ecosystems, including how the material degrades over time and whether any polluting substances may leach out. These effects must be investigated before specific legislation can be made in this area. 

Contact: 

Head of Communications 
Jesper Emborg 
61 22 02 91