Soil–Plant Indicators for Assessing Nutrient Cycling and Ecosystem Functionality in Urban Forestry

Nature-based solutions (NbS) are multidimensional, resource-efficient, and sustainable growth approaches to cope with current challenges, including biodiversity and carbon loss, pollution, climate change and land degradation. Amongst NbS, urban forestry is an important tool to enhance environmental resilience and sustainability, providing useful ecosystem services for human well-being. In this context, using suitable soil and plant indicators allows us to evaluate the efficiency of urban forestry in sustaining ecosystem functionality. Effective indicators should be sensitive to environmental changes and representative of ecological processes. Many studies focus on the selection of soil or plant indicators. The prior investigations considered soil–plant interaction and the related complex heterarchical and bidirectional effects involving plant strategy and soil biota. The choice and the use of indicators related to the soil–plant system could be an innovative strategy to better assess the following: (1) the ability of soil to support healthy plants and their ability to improve air quality; (2) the effect of urban forestry on ecological processes, in particular carbon and nutrient cycles. This review investigates the suitability of soil–plant system indicators related to nutrient cycles, e.g., ecological stoichiometry, enzyme activity and stoichiometry, and carbon and nitrogen stable isotopes, as valuable tools for planning and evaluating the effectiveness of urban forestry interventions.