Biochar, known in some circles as ‘black gold,’ has significant potential. Carboliva, a biochar production company based in Puente del Obispo (Jaén), explains a use that is little known in Spain but is being implemented in numerous cities: planting urban trees with biochar through the “Stockholm Model.”
The use of biochar in city or town trees offers significant benefits. One of these, which is increasingly interesting to administrations, is that it prevents tree roots from damaging sidewalks. With the “Stockholm Model” system, which mixes stones and biochar, soil compaction is reduced, maximizing the circulation of oxygen and water. This facilitates optimal root growth toward the depths and, therefore, prevents pavements from suffering damage. Furthermore, at these depths, roots are more resistant to drought because the subsoil, rich in pores, provides them with water and oxygen. For this reason, and due to the inherent benefits of biochar—which improve soil fertility by favoring the development of microorganisms and increasing water retention capacity—urban trees improve their health and, consequently, their survival rate.
The study conducted by Bjorn Embrén, director of the Landscaping Department for the city of Stockholm, which resulted in this system, concluded, for example, that 6-year-old trees planted in structural soils with biochar were five times larger than 30-year-old trees planted using more traditional urban tree planting techniques. The Swedish National Road and Transport Research Institute (VTI), an independent and internationally renowned research institute in the transport sector, gave its approval to this technique, which has subsequently been used in many cities around the world and is attracting interest from many others.
Another advantage of using biochar in urban trees is that it functions as a carbon sink by sequestering a significant amount of CO2 in the soil, which therefore means reducing pollution in that locality. Additionally, biochar reduces nutrient leaching from urban soils and fixes heavy metals, something equally valuable for municipalities.
Finally, it is a fully sustainable practice, as biochar is produced from biomass—in Carboliva’s case, from olive pulp and pits.