Increased brick ratio in urban substrates has a marginal effect on tree saplings

From construction and demolition of buildings, brick wastes accumulate in large quantities and are difficult to recycle. Re-using bricks as planting substrate could partly substitute gravel or other raw materials, and would reduce deposition of bricks in landfills. High water-holding capacity and a high specific surface of such substrates are beneficial for plant growth, while high pH could be a disadvantage. This study focuses on potential effects of brick-based substrates on survival, growth and functional traits of two urban trees (Acer platanoides, Tilia cordata). We compared the effects of brick quantity (5 vs. 30%), pre-treatment with phosphoric acid, nutrient-poor vs. -rich soil, and mycorrhiza inoculation upon saplings in two greenhouse experiments. There were no effects on survival, while a high brick ratio slightly reduced growth of A. platanoides and its branching in nutrient-rich soil, and tend to increase the root-to-shoot ratio in both species. The acid pre-treatment caused negative effects on relative growth rate of A. platanoides. Mycorrhiza inoculation had a tendency for a positive effect on growth in T. cordata, but only with 5% brick ratio. Overall, the brick-based substrates have no clear effect on the study species. Thus, bricks can be recommended as a neutral component within constructed Technosols, and can be used to modify grain size distribution without negative effects on survival, growth and performance, while further studies are needed on bricks with cement and gypsum contaminations.