The potential of processed mineral construction and demolition waste to increase the water capacity of urban tree substrates - A pilot scale study in Munich

·Construction and demolition waste (CDW) improve plant available water in tree substrates.·Improvement of water retention correlates positively with brick content of CDW.·During drought brick enhanced CDW reduces days with plant water stress.·Developed substrates improve urban tree habitats and reduces waste. Construction and demolition waste (CDW) represents one of the most significant waste streams worldwide. Due to a high brick content, CDW amended soil substrates are expected to improve water retention, benefitting urban trees. In this two-year field study evaluating the effects of CDW on the water holding capacities of urban tree substrates, six substrate mixtures are tested. Three test substrates contained a standard CDW mixture with 30 % and three 60 % bricks. Soil water contents were monitored and plant available water contents (PAWC) were analysed. From soil water contents and PAWC, the relative extractable water contents (REW), and subsequently the number of days with REW < 40 % and water stress intensity (WSI) are calculated. The substrates with enhanced brick content showed higher PAWC. However, the effect of brick on PAWC was only significant at a minimum of 30 % bricks in the substrate. Pore size distribution of the brick fraction confirmed the assumed relationship between addition of brick and increased PAWC. Substrates with an enhanced brick content showed reduced numbers of days with REW < 40 % and a reduction in WSI. Our findings demonstrate that CDW has potential to foster water holding capacities of urban tree substrates.