Trees play a pivotal role in the urban environment alleviating the negative impacts of urbanization, and for this reason, local governments have promoted strongly tree planting policies. However, poor soil quality and neglect tree maintenance (e.g., irrigation and 20 fertilization) can seriously mine the plant health status during the tree establishment phase.

Among soil conditioners, the use of biochar to provide long-lasting C to the soil and, at the same time, improving soil properties, thanks to its influences on the soil water-holding capacity, soil enzymes activities and NPK concentrations, is a promising research field. Therefore, with a two-step experiment, the study aimed to assay the physiological responses of a commonly used urban tree species (Tilia × europaea L.) to 1.5% (w/w) biochar amendment (CB; exp. 1), 26 and secondly, to assess the ability of trees, grown in biochar amended soil, to tolerate a period 27 of drought (exp. 2). Biochar amendment increased P and K availability in the soil, resulting in 28 higher P and K concentrations in CB than control leaves, according to the leaf stage. This induced CB trees, at the end of exp. 1, higher values in both total biomass and sequestered CO2 than controls (+22%). Moreover, the higher water availability in soil amended with biochar helped CB trees to tolerate water stress, with better leaf photosynthetic performances and a 32 faster recovery than stressed controls after the rewatering. This study highlights the dual function of the biochar, improving CO2 sequestration and soil properties, and at the same time,

enhancing plant physiological responses to environmental constraints. The use of biochar at the tree planting, especially in an urban environment, is a feasible and environmentally 36 sustainable strategies to improve the success during the tree establishment phase.