Urban trees are indispensable for healthy air in cities. However, they are also exposed to special stress factors and therefore have significantly shorter lifespans and higher maintenance costs. For this reason, the Swedish capital Stockholm has been testing special soil substrates for city trees during the last 10 years. These substrates contain gravel, clay, sand, lava and, with increasing success: biochar.
Urban trees suffer above all from soil sealing and soil compaction in urban areas. This severely limits the availability of water and oxygen for the tree roots.
First measure: Stone layer
The first step in Stockholm was to replace the highly compacted soil around the trees with a layer of stone. Stones do not condense to the same extent as earth. This allows water to continuously penetrate between the tree roots, leaving enough space for the exchange of gases. In the early years, two separate stone layers with different stone sizes were stacked up. However, it turned out that the best results could be achieved with a uniform layer and a stone size of 32–63 mm.
Second measure: Biochar
In the beginning, the Stockholm team poured soil between the stones to provide the trees with an additional growth medium. The arborists increasingly replaced the soil with biochar, as plant charcoal resists compression and compaction just like the ground stones. Together with the ground stones, the biochar improves the void fraction, also called porosity. A higher porosity promotes gas exchange and water retention capacity, more permeability leads to improved root penetration. Biochar is not only much more porous than sand or clay, it is also not biodegraded or compacted as quickly as peat, for example.
1–10 mm proved to be the optimum particle size for biochar, and around 2.25 m³ of biochar per tree has also proven itself in Stockholm. This results in a mixing ratio of 85% gravel and 15% biochar per tree, while a mixing ratio of 75% gravel and 25% biochar proved to be optimal for shrubs.
Better rainwater management
Another additional benefit of the plant coal-stone substrate was improved rainwater management. Through a special design of the surrounding street pavement, the accumulated rainwater is directed from the roofs, pavements and streets to the trees. The high water storage capacity of biochar and its filtering effect not only reduce costs and efforts for tree care, but have also achieved cost savings in urban water management.
Storage of an additional 2,000 t CO2
If Stockholm were to use biochar for all its green areas and landscape projects, the author estimates the annual demand for biochar would be around 800 tons. This would have the additional benefit that an additional 2,000 tonnes of CO2 from the atmosphere could be bound in the soil each year.