Nutrient Interaction in the Soil-Plant System and Tree Physiological Functional Traits in an Urban Green Infrastructure

Soil-plant indicators are useful to select tree species suitable for the urban conditions and to maximize the benefits provided by green infrastructures (GE). To identify effective indicators for GE, soil-plant nutrient interaction and related physiological responses were assessed in evergreen (Cupressus sempervirens L.) and deciduous (Acer opalus Mill., Acer rubrum L., Tilia platyphyllos Scop., Ulmus ‘Plinio’) tree species, in a novel urban GE (Florence, Italy). Soil and leaf nutrient contents and the soil enzyme stoichiometry were applied as indicators of plant nutrient status and bioavailability. Gas exchange and stable isotopes of carbon (C) and nitrogen (N) were used as indicators of tree physiological status and resource-use strategies, respectively. The soil was suitable for tree growth, however, the enzyme activities estimated N limited condition. Trees differed in leaf nutrient composition and stoichiometry. Acer rubrum and A. opalus leaves had manganese concentration below and above the plant optimal range, respectively, leading to alteration in the nutrient uptake and on the leaf stoichiometry between C, N and phosphorus (C: N:P), with consequence for tree health status. Tilia platyphyllos and Ulmus ‘Plinio’ had the best photosynthetic performance, while photosynthesis in A. rubrum was severely impaired. Interspecific differences in N- and water-use strategies were observed. Tilia platyphyllos showed the highest water-use efficiency, leaf C: P and N: P compared to the other species. Tree nutritional and physiological traits gave insights into soil-plant nutrient interaction and may be proposed as useful indicators for choosing the most suitable species to improve GE management in urban environments.