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The iron (Fe) oxide particles affect a wide range of important environmental processes related to soil fertility and plant growth. They often occur as aggregates, mineral nanoparticles or as nanoscale coatings on other grains in the soil, where they may also control the fate and transport of nutrients such as phosphate, sulfate, molybdate and pollutants (arsenic or chromium). Solubilization of Fe from minerals control iron availability for plant growth; this process is regulated by chemical mechanisms (pH and dissolution-precipitation), sometimes mediated by biological interactions at rhizosphere scale, of both Fe crystalline and poorly ordered Fe oxide minerals. In cold and temperate climate the organic carbon storage and the Fe minerals are highly correlated, possibly because the interactions of nano-size iron oxide particles with humic substances. The aims of this review are to summarize the main findings and the classical iron soil chemistry and to propose chemical and biological relationships with the hope of stimulating new model in soil fertility.

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.

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