Evaluating the role of sewage sludge in improving hydrophysics and tree growth in urban structural soils

Urban tree growth faces significant challenges, primarily due to soil compaction, poor soil structure, and limited water retention capacity. This study examined the effectiveness of structural soils for tree plantations prepared using different growing media. Two composting approaches were used to develop the growing media: one incorporating sewage sludge with bark and peat (SSGM), and another using compost with biochar (CBGM). Four structural soil treatments were compared against a control (intact neighbouring soil): SSGM at 15 % and 30 %, and CBGM at 15 % with either crushed amphibolite or recycled concrete. The results showed that growing media significantly improved water-holding capacity compared to the control, with SSGM at 30 % exhibiting the highest available water capacity, being approximately double that of SSGM at 15 % and the control. During field monitoring, CBGM with recycled concrete experienced water stress for just four days, compared to 164 days for the control. While macroporosity increased in growing media treatments, the control exhibited highest aeration capacity, indicating a trade-off between aeration and moisture retention. Tree trunk circumference increase was significantly influenced by the structural soil treatment, with SSGM at 15 % showing greatest growth, followed by SSGM at 30 %, while the control showed lowest growth. These findings emphasise the importance of optimising the proportion of growing media in structural soils, in order to strike a balance between water retention and aeration, support tree development and promote sustainable urban forestry practices.