Ecological condition and functioning of urban soils in public green spaces: a multi-dimensional assessment in Milan, Italy

This thesis investigates the ecological condition and functioning of urban soils within public green spaces of the city of Milan, Italy, using an integrated, multi-dimensional approach. Soil physical and chemical properties, soil fauna communities (earthworms and microarthropods), and water infiltration capacity were jointly assessed to evaluate how urban pressures affect multiple components of the soil ecosystem. The study was conducted across five categories of publicly accessible green areas differing in vegetation type, management intensity, land-use history, human use, and spatial location within the urban context. An extensive survey of soil physical and chemical properties was carried out at 60 sites across the city, with particular attention to soil compaction. Measured parameters included bulk density, penetration resistance, texture, pH, organic carbon, total nitrogen, and available phosphorus. Overall, the investigated soils exhibited favourable conditions for soil ecological functioning, with relatively high organic matter content, pH values suitable for biological activity, and generally low levels of compaction. However, urban pressure left a clear imprint on several soil properties (e.g., pH, bulk density), consistently reflecting an urban–peri-urban gradient. Based on these results, a subset of 15 sites was selected for the investigation of soil fauna and water infiltration. Earthworm communities were characterised in terms of abundance, biomass, and taxonomic and functional composition, and their relationships with soil and environmental variables were examined. Earthworm abundance, biomass, and taxonomic richness were high, indicating the absence of widespread ecological degradation. Pb concentration emerged as a major driver negatively affecting multiple attributes of earthworm communities. Urban pressures shaped the functional composition of earthworm communities, with marked differences between urban and peri-urban green spaces. Soil microarthropod communities were assessed using the QBS-ar index to evaluate soil biological quality and to examine the ability of the index to capture the effects of urban pressures. QBS-ar values indicated generally modest levels of biological quality, with peri-urban forests supporting disturbance-sensitive taxa and central urban parks showing the lowest values. While capturing broad ecological contrasts, the index showed limited responsiveness to fine-scale differences in management intensity and soil properties. Water infiltration rates were quantified and compared across green area categories. Soils in Milan’s public green spaces were generally able to infiltrate typical rainfall events, whereas the highest rainfall intensities recorded over time may exceed infiltration capacity. Infiltration rates were influenced by the combined contribution of soil physical and biological properties, with bulk density, sand content, and earthworm biomass. Overall, despite anthropogenic pressures, soils in Milan’s public green spaces retain favourable ecological conditions and the capacity to sustain multiple ecosystem services. Clear differences between urban and peri-urban contexts emerged, reflecting contrasts in management intensity, land-use history, and levels of human use. These findings challenge the assumption of pervasive urban soil degradation and underline the importance of preserving and sustainably managing urban soils to maintain their ecological functioning.