Knowledge synthesis on biochar in Danish agriculture

Biochar is the term used for the residual product that remains when biomass such as straw or wood chips is thermally decomposed at high temperatures in the absence of oxygen in a pyrolysis plant. Biochar is a solid material with a high carbon content. This carbon is highly stable and, when applied to agricultural soils, decomposes only slowly into CO₂, unlike the original biomass.

“One of the benefits of adding biochar to soil is that it increases carbon storage. This helps reduce CO₂ emissions to the atmosphere,” explains Associate Professor Lars Elsgaard from the Department of Agroecology at Aarhus University.

However, increased carbon storage is not the only potential benefit of using biochar in agriculture. Research has shown, for example, that biochar can improve soil aeration and increase the soil’s ability to retain water and nutrients.

Growing interest in biochar

In recent years, interest in biochar has increased, particularly because of its potential to reduce CO₂ emissions, but also because biochar may, in some cases, be used as a soil amendment.

At the request of the Danish Agricultural Agency, researchers from Aarhus University have therefore prepared a knowledge synthesis on biochar. In the report, the researchers outline the necessary conditions for producing and applying biochar in agricultural systems. The report aims to ensure that biochar can be used as a cost-effective climate and environmental tool in Denmark without causing undesirable side effects in the soil ecosystem.

“The purpose of the knowledge synthesis was to compile current knowledge on the effects of biochar on the environment, climate, and soil properties, while also examining the economic aspects of biochar use in Danish agriculture. We placed particular emphasis on the pyrolysis of the most relevant biomass sources, including straw, digestate fibres from biogas plants, and sewage sludge, as all of these are highly relevant under Danish conditions,” explains Lars Elsgaard.

He adds that the report also identifies potential barriers to the use of biochar in Denmark and points out areas where knowledge is still lacking.

The importance of feedstock for climate impacts

Biochar is not a uniform product. It can be produced from many different types of biomass, and its properties depend heavily on both the feedstock used and the thermal conversion conditions, such as pyrolysis temperature and residence time.

“Biochar is an umbrella term covering products with widely varying characteristics. Therefore, it is difficult to generalise the effects of biochar when it is applied to agricultural soils,” explains Senior Researcher Anders Peter S. Adamsen from the Department of Biological and Chemical Engineering at Aarhus University.

The knowledge synthesis recommends that studies of biochar effects should always be accompanied by thorough characterisation of the biochar used and the conditions under which it was produced.

This is also being addressed at the European level. The European Biochar Certificate (EBC) includes a set of guidelines designed to encourage and ensure control of biochar production and quality based on scientifically sound, legally supported, economically viable, and practically applicable processes.

Biomass potentials

The type of biomass plays a crucial role in determining biochar properties. The researchers therefore focused on the most relevant biomass resources under Danish conditions: straw, digestate fibres, and sewage sludge. Among these, straw from cereals, oilseed rape, and grass seed production has the greatest potential.

“We estimate that under optimised scenarios for 2030, total straw resources available for bioenergy and biorefining will range between 3.09 and 3.85 million tonnes of dry matter. This estimate takes into account that a larger share of the straw currently left on fields may be collected in the future, while also accounting for an overall reduction in agricultural land due to urban development and the desire for more natural areas,” says Professor Uffe Jørgensen from the Department of Agroecology at Aarhus University.

The potential for using digestate fibres and sewage sludge was also evaluated.

“However, more research is required if the potentials of all three biomass types are to be fully realised. We need better knowledge of how to increase straw production and collection without compromising soil quality or crop yields,” says Uffe Jørgensen.

He also emphasises the need for research into whether all straw fractions, including stems, leaves, heads, and chaff, as well as different types of grass seed straw, can be used for pyrolysis while producing biochar with robust properties and consistent quality.

Soil properties matter

Soil physical properties describe the interactions between solid particles, water, and gases in the soil. These interactions play a major role in determining how well soil can support plant growth. Adding biochar alters these soil properties, which is why the researchers examined its effects across different soil types.

“We found that biochar generally reduces soil bulk density while increasing plant-available water content,” explains tenure-track researcher Emmanuel Arthur from the Department of Agroecology at Aarhus University.

A review of the scientific literature also showed that straw-derived biochar can increase soil pH. According to the researchers, biochar may therefore partially substitute for liming. This has both economic value and climate benefits because it can reduce CO₂ emissions associated with lime application.

Does biochar affect soil biology?

Biochar causes both physical and chemical changes in soil. It can increase water-holding capacity, alter pH levels, and create improved pore spaces for microorganisms. But how does it affect soil biology itself?

“The literature describing the effects of biochar on soil biology often reaches the same conclusion: the effects depend on the specific characteristics of the different biochar types as well as on the amount applied,” explains Professor Anne Winding from the Department of Environmental Science at Aarhus University.

Biochar is not a homogeneous product. For example, the proportion of degradable carbon, particle size distribution, and pore structure vary significantly between products. Consequently, impacts on soil biology also vary. Furthermore, soil characteristics, agricultural management practices, and climatic conditions all influence the outcome.

“For that reason, it is difficult to draw clear and unequivocal conclusions about how soil biology is affected,” says Anne Winding.

An international study found both positive and negative effects on earthworms, while a corresponding Danish study found no effect on earthworm abundance. Other studies have shown that biochar can alter microbial communities in the soil, although the implications for soil quality remain uncertain.

“We need more research into soil biology so that we can better understand both the short-term and long-term effects of biochar in agricultural soil ecosystems,” says Anne Winding.

Biochar may contain problematic substances such as polycyclic aromatic hydrocarbons (PAHs), heavy metals, dioxins, PFAS, and volatile organic compounds (VOCs). Their occurrence depends strongly on both the feedstock and the pyrolysis conditions, including temperature, oxygen levels, and processing time.

In particular, sewage sludge and household waste can lead to elevated concentrations of heavy metals, dioxins, and PFAS, while PAHs may form under suboptimal pyrolysis conditions. On the other hand, certain heavy metals and organic contaminants, including pharmaceutical residues, pesticides, and microplastics, may be reduced or removed during the pyrolysis process.

“Before biochar use becomes widespread in Danish agriculture, it must be ensured that these problematic substances do not accumulate in the soil. At the same time, biochar may also bind contaminants already present in the soil and reduce their biological availability,” says Anne Winding.

Carbon sequestration and greenhouse gas emissions

The knowledge synthesis concludes that biochar has considerable potential for carbon storage in soils.

“Biochar decomposes more slowly in soil than the original biomass used to produce it,” explains Postdoctoral Researcher Henrik Thers from the Department of Agroecology at Aarhus University.

However, decomposition rates depend strongly on both biochar properties and soil characteristics. Several studies show that a small fraction of the carbon in biochar consists of relatively labile compounds. The amount of easily decomposable carbon varies and should therefore be determined for each specific biochar type when estimating its carbon sequestration potential. The same applies to the stability of the more recalcitrant carbon fraction.

“The goal of using biochar for carbon sequestration is to achieve a long-term effect lasting more than 100 years. However, the experiments and results currently available are based primarily on short-term studies, typically lasting only one year,” explains Henrik Thers, emphasising the importance of conducting more long-term field experiments under realistic agricultural conditions.

The knowledge synthesis also concludes that long-term studies are needed to determine whether biochar can sustainably reduce nitrous oxide emissions from agricultural soils. Most existing studies have been short-term experiments. Even small reductions in nitrous oxide emissions are important because nitrous oxide is a greenhouse gas nearly 300 times more potent than CO₂.

Biochar and nutrients

“Biochar has the potential to improve agricultural soils, for example by enhancing water retention, improving aggregate stability, and increasing nutrient availability. However, it is difficult to generalise these effects because biochar properties depend on both the original biomass and the pyrolysis conditions. Furthermore, the effects depend on factors such as soil type, climate, and nutrient status,” says Senior Researcher Peter Sørensen from the Department of Agroecology at Aarhus University.

Although international studies show a statistically significant positive average effect of biochar on soil productivity and crop yields, clear positive effects on crop yields have generally not been demonstrated in temperate soils.

“Danish agricultural soils are often limed and contain high levels of nutrients. As a result, yields are already optimised, and the scientific literature typically finds no significant yield increases from biochar application under Danish field conditions,” says Peter Sørensen.

He adds, however, that very sandy soils may benefit from improved water management and root growth, which could potentially improve yields. These aspects are currently being explored in several research projects.