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The effect of biochar on cultivated land and the environment

Biochar increases the soil carbon stock and may increase the yield, depending on soil status. In relation to the environment biochar seems to be able to reduce nitrous oxide emissions.

[Translate to English:] Biokul er ikke et ensartet produkt, men derimod en samlet betegnelse for kulstofrester af forskellige biomasser. Foto: Henning Thomsen

Biochar is the term used for the residual product left when biomasses such as stubble and wood chips are degassed at high temperatures in an oxygen-free atmosphere and thus transformed to gas and oil. The remaining residual product is solid material with high carbon content.  

In recent years scientists have demonstrated an increasing interest in biochar as this product may be able to provide some of the solutions needed in order to reduce the emission of greenhouse gases and, at the same time, may be used as a soil improvement tool.

- One of the advantages of adding biochar to the soil is that it increases the soil carbon stock and thus reduces the emission of CO2 to the atmosphere. Further, as biochar usually has a high pH value it may act as a lime source, says Lars Elsgaard. He further explains that biochar has a low density, which – after being added to the plow layer – may help reduce the density of the soil and improve the soil respiration and transport processes.

In addition, biochar may also increase the ability of the soil to retain water and nutrients in the root zone. This is primarily due to the porous structure of biochar and its huge surface area.

Half-life of 2700 years

One of the prerequisites for using biochar as a means of carbon sequestration is that biochar is a stable product and it is not degraded into CO2 by soil microorganisms, which is what happens in the case of mulching stubble. A longer-lasting German experiment (duration of 8 years) using radioactive 14C biochar demonstrated that it is possible to distinguish between CO2 originating from the decomposition of the soil’s own carbon stock and CO2 originating from the decomposition of added biochar.

- This experiment indicates that added biochar has a half-life of 2700 years in field conditions. The decomposition may be different for other biochar types, but it is generally recognized that biochar has a high stability in the soil, says Lars Elsgaard.

Biochar is not simply biochar

When comparing the available results relating to the use of biochar for agricultural purposes a general problem arises. Biochar is not a homogenous product, but rather a general term for carbon residues of various biomasses. This means that results achieved by means of one type of biochar may not be immediately transferable to another type of biochar.

- A comparison of more than 80 studies, in which the effect of biochar on plant yield was examined, demonstrated an average yield increase of 10 % when adding 3 tons of biochar per hectare. However, this result includes yield reductions as well as yield increases, says Lars Elsgaard.
He further adds that the application of biochar as a means of soil improvement will be most relevant in soils with a high level of leaching, a low capacity for cation exchange, low pH value and a low content of nutrients and organic carbon.

- Actually, it seems as if the soil characteristics are more important than those of biochar in relation to achieving a positive yield effect by means of biochar.

Environmental impact

Biochar also has a beneficial impact on the environment. Particularly in relation to loss of nitrous oxide some interesting and consistent results have been put forward. A recently published analysis of accomplished experiments demonstrated that biochar was able to reduce the loss of nitrous oxide from cultivated land by 49 % on average.

These results were achieved under controlled conditions in laboratories, greenhouses and in field conditions. If we consider the results from the fields alone the impact of biochar seems to be less significant, but still it corresponds to a reduction in nitrous oxide losses of 28 %.

- Concerning Danish loamy soils with added stubble biochar our own experiments demonstrated that biochar was able to reduce nitrous oxide losses by 24-54 % during a period of 21 days in soil samples to which either nitrate or pig slurry were added as nitrogen sources, says Lars Elsgaard and he adds:

 - Several attempts have been made to explain the impact of biochar in relation to nitrous oxide losses. Nitrous oxide is a strong greenhouse gas – 295 times stronger than CO2 – and therefore it will be of significant interest to be able to predict and provide advice on the impact. However, a general understanding of the mechanism has not yet been achieved.  

Further information

Associate professor Lars Elsgaard
Mail: lars.elsgaard@agro.au.dk
Phone: 87157674