Ecosystem services-based soil quality index tailored to the metropolitan environment for soil assessment and management

The soils in urban greenery provide essential ecosystem services. However, only a few studies have assessed urban soil quality based on a comprehensive view of ecosystem services and soil multi-functionality. In this study, we suggest an urban soil quality index (uSQI) to evaluate soil status in various spatial types of urban greenery. Our objectives are 1) to develop an uSQI incorporating a range of urban soil ecosystem services in metropolitan environments and 2) to test the efficacy of the developed uSQI by applying it to nine different sites. To fully consider ecosystem services provided by the urban soil, a DPSC (drivers and pressures, state, and changes) framework was constructed. Drivers and pressures are influencing factors that continuously alter the state of the urban greenery, eventually leading to changes in ecosystem services and soil functions. The six soil functions considered were physical stability and support, water storage and infiltration, habitat provision, organic matter stabilization, nutrient supply and retention, and pollutant immobilization and decomposition. These functions were measured using ten soil indicators which can be quantified: bulk density, saturated hydraulic conductivity, litter-layer depth, mineral-associated organic matter, clay+silt content, fluorescein diacetate hydrolytic activity, cation exchange capacity, inorganic nitrogen concentration, pH, and concentrations of potentially toxic elements. The uSQI was calculated as the arithmetic mean of the scores of the six soil functions, obtained through the fuzzy logic functions. The uSQI successfully identified the low soil quality sites among nine urban greeneries with different spatial types (point, line, and polygon). In addition, we could examine the degraded soil function of each site and suggest a management guideline using our uSQI. Our novel index can help urban stakeholders evaluate and monitor the soil quality of urban greenery.