Urban Soils as Multifunctional Systems: Carbon Persistence, Vegetation Interactions, and Contaminant Dynamics

Urban and peri-urban soils are increasingly recognized as key components of urban ecosystems, providing essential ecosystem services while simultaneously hosting legacies of anthropogenic disturbance and contamination. Shaped by complex interactions among land-use change, vegetation management, atmospheric deposition, and soil heterogeneity, these soils function as dynamic and stratified systems in which carbon persistence, biological activity, and contaminant behaviour are tightly interconnected. Understanding these processes requires integrated approaches that explicitly consider soil depth, carbon quality, and spatial variability. This PhD thesis, developed within the framework of the PNRR project Biodiversity Future Center - Spoke 5: Urban Biodiversity, investigates the functioning of urban and peri-urban soils in the Metropolitan City of Milan, with a particular focus on soil organic carbon (SOC) dynamics, soil-vegetation interactions, pyrogenic carbon (PyC) persistence, and heavy metal mobility. By combining field-based soil sampling, laboratory analyses, and process-oriented approaches, the thesis aims to elucidate how carbon pools of different origin and stability interact with vegetation development and contaminant processes in highly heterogeneous urban environments. The first part of the thesis examines SOC dynamics and soil-vegetation interactions in urban and peri-urban forests established on former agricultural land. Results show that afforestation leads to rapid changes in SOC stocks, humus forms, and understory functional composition, with pronounced depth-dependent patterns and strong links to soil properties and forest development stage. These findings highlight that SOC accumulation in urban forests is governed by multiple, coexisting stabilization pathways rather than by a single dominant process. The second part focuses on PyC as a chemically distinct and persistent component of urban soil organic matter. At the city scale, PyC is shown to be a ubiquitous fraction of SOC, exhibiting relatively stable PyC:SOC ratios across soil depths and land-use contexts. PyC distribution reflects long-term atmospheric deposition and stabilization mechanisms rather than short-term ecological dynamics, emphasizing its role as a long-term carbon reservoir in urban soils. The third part addresses methodological uncertainty in PyC quantification by comparing chemo-thermal oxidation (CTO-360) and chemical oxidation (CO) approaches applied to the same soil samples. Methodological differences are most pronounced in surface soils, where CO captures a broader and less refractory aromatic fraction, while PyC estimates obtained by the two methods progressively converge with depth. This depth-dependent convergence indicates the dominance of highly condensed and persistent PyC forms in subsoil horizons and underscores the importance of method-aware interpretation of PyC data. The final part extends carbon-focused insights to the assessment of heavy metal mobility in unsaturated urban soils. Through laboratory experiments and stochastic modelling, the study demonstrates that contaminant transport is strongly influenced by soil heterogeneity, hydraulic properties, and parameter uncertainty. The results further show that non-pyrogenic organic carbon represents the primary reactive pool controlling metal retention, while PyC plays a limited direct role in metal binding. Overall, this thesis demonstrates that urban soils function as stratified, multifunctional systems in which carbon quality, soil depth, and heterogeneity govern both ecosystem services and environmental risks. By integrating pedological, biogeochemical, ecological, and process-based perspectives, the work provides a comprehensive framework for understanding urban soil functioning and supports the development of soil-aware strategies for sustainable management of urban and peri-urban landscapes.