Sweden's capital has set an example, and Trondheim is now following it. More than 10 years ago, Stockholm has started to test the effect of biochar on urban and suburban trees. Encouraged by positive results, the city's landscaping department has investigated various different soil amendments, biochar and stone mixtures. (Read more on Stockholm's experience here).
By now, biochar has found its way into the city's standard planting method for their urban trees. Inspired by this success, cities all over the world are starting to adopt the Stockholm method.
The ABC4soil project is bringing the biochar method to Norwegian city grounds. As part of the project, the first trees in Norwas growing on biochar-based structural soil will be planted in Trondheim in June 2019. The project team has been very lucky to have the support and advice of Björn Embrén, who has been Stockholm's tree officer for many years.
More and more people live in cities - despite (or because of) this, we like to surround us with nature. Parks and trees along the streets make our cities green and attractive. However, growing trees in urban surroundings is not without its challenges.
In order for a tree to grow and thrive, it needs light, oxygen, water and nutrients. In the city, many trees are planted alongside streets. The concrete and asphalt under which they grow, carry countless people, bicycles, cars and busses every day. As a result, the soil that surrounds the roots, becomes compacted. Over time, it is so dense that neither oxygen nor water can penetrate and reach the roots. The trees slowly starve and die.
In order to give urban trees space for their roots to grow and absorb water, oxygen and nutrients and to prevent compaction over time, the trees should be planted in a so-called structural soil.
Structural soil is built from one or several layers of crushed stones. The gaps between the stones are filled with soil. The stones prevent compaction and provide stability while the tree roots can grow into the soil-filled gaps.
Structural soil has been used since the 1970s and is a common method for growing urban trees.
Stockholm's approach is to build a structural soil, but instead of regular soil between the gaps of the stones, a nutrient-containing biochar is used. Just like regular soil, the biochar provides water and nutrients to the tree roots. Due to its porous structure however, it can store much more of these and release them in a more controlled manner. In addition, the carbon-content in the char is very stable. It doesn not oxidize into carbon dioxide, but remains bound in the soil. It therefore acts as a negative carbon emission technology. Every kg of biochar used avoids about 3 kg of carbon dioxde.
Part of the project ABC4Soil is to introduce biochar-based structural soil as a planting method for urban trees to Norway. In order to plant the country's first city trees on biochar, the project has teamed up with the city of Trondheim and TAG arkitekter. As part of a larger residential building project (Magnus den Godes gate in Trondheim), we will be planting seven urban trees in total. Three of these trees will stand on standard structural soil and act as a reference. Four of the trees will be planted on a biochar-containing strucural soil.
The trees will be planted in June 2019 and the tree growth and nutrient content monitored throughout the following years.
The project ABC4Soil is made possible through financial support from FACCE SURPLUS (Sustainable and Resilient agriculture for food and non-food systems), an ERA-net co-fund. We are thankful for this opportunity and appreciate the support from our funding agencies, the German Federal Ministry of Education and Research, the Research Council of Norway and PTJ Jülich.