Natural organic matter, including humic substances (HS), comprises complex secondary structures with no defined covalent chemical bonds and stabilized by inter- and intra-molecular interactions, such as hydrogen bonding, Van der Waal’s forces, and pi-pi interactions. The latest view describes HS aggregates as a hydrogel-like structure comprised by a hydrophobic core of aromatic residues surrounded by polar and amphiphilic molecules akin a self-assembled soft material. A different view is based on the classification of this material as either mass or surface fractals. The former is intended as made by the clustering of macromolecules generating dendritic networks, while the latter have been modelled in terms of a solvent-impenetrable core surrounded by a layer of lyophilic material. This study reviews the evolution of the increasingly refined models that appeared in the literature, all capable to describing the physicochemical properties of HS. All the models are critically examined and revisited in terms of their ability to provide key information on the structural organization of HS. Understanding how the molecular association pathway influences aggregation of HS also provides a key acknowledgment of their role in the environment.

The assessment of Technosols quality in urban environments is pivotal for the maintenance of ecosystems impacted by human activities. The study was performed on Technosols constructed in experimental mesocosms in the suburban area of Naples (Southern Italy) to highlight changes in the main soil properties over eight years and to identify the most suitable indices at quality monitoring. In this study, several chemical, biological, and integrated indices were analysed to evaluate the mineral accumulation, potential ecological risk, edaphon activity, fertility, and the overall soil quality. The Technosols showed alkaline pH, nitrogen ranged from 24.5 to 39.5 g kg⁻¹, high organic matter contents above 40 g kg⁻¹, and there were no evident processes of soil compaction. Heavy metals (Cr, Cu, Fe, Mg, Mn, Ni, Pb, and Zn) did not exceed the thresholds defined by the Italian law for urban soils, despite their volcanic components. During eight years, the chemical indices depicted changes in the elements balance and increase in ecological risk; the biological indices indicated a reduction in the fungal fraction (fivefold) and in the resources utilisation and carbon storage. The soil quality index with all parameters highlighted the reduction in the soil quality (from 0.78 to 0.65) due to the decrease of the chemical quality, the increase of microbial stress conditions, and changes of the microbial composition, underlining the importance of integrating chemical and biological information for monitoring Technosols.