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Biochar as a planting soil component for urban trees: a short-term study from Hyväntoivonpuisto, Helsinki

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Title: Biochar as a planting soil component for urban trees: a short-term study from Hyväntoivonpuisto, Helsinki
Author(s): Kopakkala, Topi
Contributor: University of Helsinki, Faculty of Agriculture and Forestry
Degree program: Master 's Programme in Agricultural Sciences
Specialisation: Plant production sciences
Language: English
Acceptance year: 2022
In order to achieve carbon neutrality and slow down climate change, it is necessary not only to curb greenhouse gas emissions but also to remove carbon from the atmosphere. In agricultural sector, adding biochars to soils has proven to be one of the most effective methods to sequester carbon. Using biochars in urban planting soils could be simple and effective way to create carbon sinks also in urban environment. Wood based biochars are already available in the market and their viability as soil amendment is supported by an extensive body of research. Despite large evidence from agricultural and greenhouse sectors, research focused on biochars in planting soils for urban trees is scarce. To assess and demonstrate viability of biochars as component of urban planting soils, an experiment was established in 2019-2020 in Hyväntoivonpuisto-park in Helsinki. The experiment consists of four tree species and nine different planting soils, seven of which include biochars. Three of the planting soils were structural soils installed below a sealed surface. Tree growth in planting soils was followed and biomass accumulation was estimated with allometric equations. Planting soil nutrient composition was analysed at the time of soil application and nine months later after first growing season. Pyrogenic carbon fractions were analysed by BPCA analysis. Planting soil physical and hydrological properties were analysed by water retention curves with pF range of 0 – 4.2. After two growing seasons, biochars had increased tree growth in two planting soils compared to the control while in other planting soils with biochar, the growth was similar to control. In structural soils biochars had improved growth, but there were no statistically significant pairwise differences between treatments. Biochars increased the macroporosity of planting soils, indicating they could improve aeration and water conductivity in planting soils. Effects to total porosity and water retention capacity were mixed, but highest total porosity and water retention capacity was observed in planting soil with the highest amount of biochar. Planting soil nutrient composition varied a lot due to different raw materials, limiting the possibilities of making mechanistic analysis of effects of biochars. These results indicate that biochars are viable and safe constituent for planting soils which may increase tree growth by improving soil physical properties and improve carbon sinks in urban infrastructure.
Keyword(s): Avainsanat  Nyckelord  Keywords Biochars carbon sequestration growing media planting soil urban tree structural soil planting soil

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