They then started experimenting with structural soils, a planting holes with a layer of big, 3-7 cm diameter rocks in the bottom and soil washed between the rocks.
This was a success, and as the next step, in 2009, they switched to using biochar instead of soil for all of their urban tree planting projects. They have found that the main advantage of biochar for urban tree projects over other soil amendments is that it resists compression and compaction. The crushed stones and biochar mix results in an improved porosity in the soil (roughly 40%). Increased porosity facilitates better gas exchange, permeability leading to improved root penetration.
|Schematics from Björn Embrén.|
A further improvement on their methods has been to have the biochar and stones delivered pre-mixed. They have found that this saves up to 25% in time and labour costs. The preferred biochar particle size of the biochar is between 1 – 10 mm and currently they recommend approximately 2.25 cubic meters of biochar for each urban tree (likely mixed with rocks already, but still quite a lot!). This explains why loads of biochar from Europe have been shipped to Stockholm, mostly from Germany but also from RPK Hiili OY (Mikkeli, Finland).
In order to be truly sustainable, however, a goal of using locally produced biochar from underutilized biomass was set. To attain this goal the City of Stockholm initiated the Stockholm Biochar Project with the objective of producing both biochar and renewable energy using urban green waste collected from municipal parks as well as city residents. This particular biomass is often difficult to dispose of and very much underutilized.
The Stockholm Biochar Project team succeeded in winning one of the five coveted prizes of the 2014 Mayors Challenge funded by Bloomberg Philanthropies and the EUROCITIES organization which netted them 1 million euros to be used to set up their initial pilot plant.
We visited the plant last summer and according to latest news in the Biochar journal, it will produce its first tons of biochar within a few months from now, in summer 2016! The pilot plant will start from producing 300 tonnes of biochar per year and at full scale, will reach 1,500 tonnes per year. The heat generated during production will be added to a local or district heating network. Biochar will be used both by city residents and by city landscapers in public parks and urban tree beds. Probably we will soon see many other cities following the example and getting their green waste recycled, carbon fixed and trees happy!
Read more and see images at: https://www.biochar-journal.org/en/ct/77-Planting-Urban-Trees-with-Biochar