There is a great deal of attention on climate and greenhouse gas emissions from agriculture, and in recent years it has increased its focus on ways of binding carbon. One option might be to use so-called biochar, which can increase the carbon storage in soil. And that's far from the only benefit of biochar. According to researchers from Aarhus University, biochar can return a number of important nutrients to the soil, so that future crops can benefit from them. Senior Researcher Peter Sørensen and Associate Professor Gitte Rubæk from the Department of Agroecology at Aarhus University, together with researchers from the University of Copenhagen and the Technical University of Denmark, have investigated to the plant availability of phosphorus in biochars.

“Phosphorus is an important nutrient for plants, which is why we found it relevant to investigate whether plants will be able to absorb phosphorus from biochar,” says Peter Sørensen.

From residue to nutrients in the soil

Biochar is charred residue from plant or animal biomass from pyrolysis or thermal gasification. Unlike carbon from plant material that has not undergone so-called gasification, carbon in biochar is very stable and does not convert to CO₂ until many years after being returned back into the soil. It simply binds the carbon and thereby gives a significant climate effect. One way of reducing climate emissions from agriculture could be to use, among other things, surplus straw in pyrolysis plants, thereby converting it into gas, energy and biochar for fertiliser and soil improvement in the fields.

“The interesting thing for us in this project was to find out whether phosphorus in biochar is available to the plants, so it can also act as fertiliser”, explains Peter Sørensen.

Four different biomasses

Biochar can be made from a wide variety of biomasses, and there may be differences from one biomass to another. The researchers used biochars from five different types of biomass to investigate whether there is a difference in the availability of phosphorus depending on the origin of the biochar. The five biomasses were:

• Wheat straw
• Two different sludge and straw mixes
• Sheanut shells
• Chicken Manure

“We have worked with five different biochars from different biomasses in this project to test the availability of phosphorus, and to see if there is a difference in availability depending on which biomass biochar comes from. And it turns out that phosphorus is actually available to the plants, but in varying amounts,” says Peter Sørensen.

pH in the soil plays a role

A laboratory incubation study showed that all five biochar types significantly increased the pH of soil and that biochars from wheat straw, sheanuts and chicken manure had a higher phosphorus availability than biochars from different types of sludge.

“We saw that the soil pH increased significantly when we added biochar in our experiments depending on the dose. The highest soil pH gave the highest phosphorus release. It shows that soil pH plays an important role in the availability of phosphorus, and contrary to what we thought, a high pH value means more available phosphorus,” says Peter Sørensen.

Behind the research