Early pedogenesis of a young Technosol made from organic wastes

• Layered Technosols composed of organic and mineral urban wastes. • Over 3 years, early pedogenesis processes as in natural soils but much faster. • Biodegradation and shrinkage play a key role in this first phase of pedogenesis. • Important transfers of particles and solutes were observed down the profiles. • A trade-off between organic wastes biodegradation and stabilization. Constructed soils based on urban waste, i.e. Technosols appear as a promising solution to enhance circular economy while delivering ecosystem services. Their ability to provide such services depend on their constitution as well as on their temporal evolution and pedogenesis, which is poorly known. We studied during three years the changes in an isolatic Technosol made essentially of organic urban wastes. The study took place on a Parisian rooftop with an experimental trial of 24 plots. We compared four Technosols with similar upper layers (a mix of biowaste compost and spent mushroom substrate), but with either a mineral (shredded tiles and bricks) or an organic lower layer (shredded wood). Both types were run with and without plants. Technosols were monitored over three years and samples were taken at different times to study pedogenetic processes. We observed a rapid and sharp volume loss of the Technosols, responsible for an important mass loss, but not leading to any increase in bulk density. A clear physico-chemical differentiation of treatments from the parent materials appeared after one and three years. The Technosoil organic C stocks decreased over time. During three years, all Technosols were a net source of nitrogen and dissolved organic carbon, released in the drainage waters. Identified pedogenesis processes were the same as in natural soils, but the changes were very fast. The key equilibrium between mineralisation of organic matter and its stabilization that must be ensured when creating such Technosols. They show the necessity for long term research on the development of Technosols.