The growth of urban trees is often inhibited by low soil quality and numerous disturbing factors such as (air) pollution or drought stress. Therefore, attempts are generally made to stimulate the growth of urban trees through synthetic fertilizers. In view of negative consequences such as greenhouse gas emissions, acidification, salinisation and losses of soil carbon, more and more cities are looking for alternative strategies. In this study 5 alternative fertiliser substrates were examined for their effect: Biochar (BC), dried sewage sludge (BS), wood chips (WC), compost (COM) and compost tea (ACT).
Dried sewage sludge and biochar had the greatest influence on tree growth and improved soil properties. In this study, the growth of urban trees seemed to be limited primarily by the supply of nitrogen (N). This was particularly evident in the treatment of the soil with dried sewage sludge, where the study showed the largest increase in tree growth and also the largest amount of nitrogen.
Typical urban soils & trees
Three soil types were selected for the study to represent a gradient in urban soil quality and possible restrictions on tree growth. On the one hand, the scientists selected an uncompacted sludge soil as an exemplary soil of the highest quality. In addition, a compacted clay soil was used for the experiments, as an example for the compacted soil with low organic matter that is typical for cityscapes. As a third variant, the scientists chose a sandy soil to represent a soil with low organic matter, well permeable and relatively dry, which can often be found in street tree sections. The scientists focused on two tree species: Sugar Maple and Gleditsia are common urban trees in the United States. Because of their faster growth reaction rates compared to old trees, seedlings of these species were used.
Biochar and dried sewage sludge are the better alternatives
Conventional nitrogen fertilizers, compost and wood chips are the most frequently used soil improvers in urban areas. However, the results of the study suggest that biochar and dried sewage sludge are preferable to these substrates. Firstly, the largest increase in tree growth was observed in the treatment with dried sewage sludge. The researchers explain this by the fact that dried sewage sludge reduces the pH value of the soil and thus improves the availability of nutrients (e.g. nitrogen) and also increases microbial activity. The total tree biomass of both tree species and all three soil types also increased by 44% compared to the control for the plant-charcoal variants. In contrast to the dried sewage sludge variants, the scientists were unable to detect any changes in the soil pH value of the plant-charcoal variants, but found an increased carbon value. In addition, the microbial activity only increased in the sandy soils, in all other soils treated with biochar there was no change here.
The researchers were surprised that no increase in the total tree biomass was observed for the variants compost, wood chips and compost tea. The desired effect therefore did not materialize. The nitrate losses, on the other hand, which can provide an indication of the nutrient losses, were the same and minimal for all variants – irrelevant which soil improver was used.
The authors of the study therefore concluded that the use of dried sewage sludge or biochar is also attractive for urban tree and soil management in view of the many potential environmental advantages. Both substrates are produced from waste products and recycle materials that would otherwise end up in landfills. An estimated 21 million tonnes of wood waste and 12 million tonnes of municipal green waste are produced annually in the USA, of which 54% have so far been simply disposed of. Biochar would also have the advantage that the carbon contained in the biomass would be bound in the long term and stored in the soil and thus would no longer have any climate-damaging effects.