Protecting the Climate with Biochar

Agroscope Science 112, 1-71, 2021

An Agroscope study commissioned by the Federal Office for Agriculture shows that the use of biochar clearly has potential, yielding benefits for the climate and ecosystems.

Biochar as a carbon sink

Around half of the dry matter in plants consists of carbon drawn from the atmosphere via photosynthesis during their growth. If the plant decomposes after it dies, the carbon is released back into the atmosphere in the form of CO₂. To prevent this, the biomass can be pyrolysed, i.e. thermally treated at a minimum temperature of 400 °C in the absence of oxygen. This process converts around 30−50% of the plant carbon into stable molecular structures. Biochar is also produced naturally during vegetation fires. Its mean residence time in soils ranges from 1440 to 14,500 years.

Generating biochar from agriculturally produced biomass removes CO₂ from the atmosphere, and can thus help mitigate climate change. Consistent processing of the residues from agriculture and forestry into biochar could sequester up to 4 million t CO₂eq annually by 2050, thus offsetting 80% of Switzerland’s expected remaining greenhouse-gas emissions.

Higher soil fertility, lower emissions

The traditional terra preta soils of the Amazon region, notable for their high yields, are known for the positive qualities of biochar. Here in Switzerland too, biochar can be used in agriculture as a soil amendment and carrier matrix for fertilisers as well as a feed supplement, stable litter and slurry additive. Numerous international research studies confirm that biochar increases yield, root mass and microbial activity builds up soil organic matter and improves water-use efficiency, and can reduce greenhouse gas emissions per hectare by around 30%. Through these emission reductions alone, biochar could save around 8.6% of agriculture emissions, and hence around 1.2% of all greenhouse gas emissions in Switzerland.

In Switzerland, biochar’s main function is climate protection

The highest yield increases using pure biochar can be achieved in acidic tropical soils that are poor in soil organic matter. For Switzerland, however, the use of biochar-based fertilisation is recommended. Here, biochar is loaded with organic or mineral fertilisers and introduced into the root zone of plants – a particularly promising approach for achieving yield increases in special crops. The potentially greatest benefits of biochar use in Switzerland are therefore climate protection, the improvement of soil microbiological activity and water availability for the plants, and reduced nitrate leaching.

At present, biochar is mainly used in animal husbandry as a cost-efficient feed additive. Input is therefore for the most part via farmyard manures that are spread primarily on pasture, temporary leys and cereals.

Production from residues must be approved

To date, only untreated wood has been approved in Switzerland as feedstock for biochar. From a climate-protection and bioeconomic perspective, however, high-quality wood from forests should first be utilised as building material in order to replace fossil or energy-intensive materials such as steel, concrete or plastics. Priority should be given to the pyrolysis of secondary materials and non-recyclable biomass. Biochars from non-woody biomass such as straw would also have agronomic benefits, e.g. a higher mineral content and larger macropores, which improve microbial colonisation. The possibility of authorising residues from agricultural and forestry production as feedstock for biochar production should therefore be examined.

Use only certified biochars

During pyrolysis, organic pollutants are generated in the form of polycyclic aromatic hydrocarbons (PAHs). If pyrolysis is not carried out properly, these PAHs can condense on and in the biochar. The current Swiss certification standards according to the European Biochar Certificate (EBC) must therefore be upheld.