Engineered Soils from excavated molasse materials: Evaluating Plant and Compost Interactions at the OpenSkyLab

Engineered soils (human-made soils) can provide solutions for the recovery of excavated materials; however, these innovative approaches remain limited and require careful development.

The international Future Circular Collider (FCC) study hosted by CERN develops processes with the goal to use excavated materials from the construction of a particle-collider based research infrastructure in the frame of an R&D project called "OpenSkyLab". Part of the project is a platform located on 1 ha of terrain made available by CERN in France to develop standard operating procedures for excavated materials re-use.

The project addresses several research questions, including how to recycle low-clay molassic materials while managing the complexity of mixing processes and enhancing microbial activity in cost effective manner; how to promote plant growth while improving pedogenesis; and identify plant species can enhance soil pedogenesis processes and ecosystem functioning.

The project includes demonstrative, replicated plots and elevated hedgerows plots. Engineered soils were constructed from molassic materials, heterogenous sedimentary rocks typical of Geneva basin, and amended with 0%, 15%, or 30% compost by volume. These substrates were tested under different vegetation types, including Miscanthus giganteus, Kernza (perennial wheat), pasture mixtures, and annual cover crop mixtures. Innovative mixing techniques incorporating inert clay were evaluated to improve substrate aggregation and homogeneity.

After one year of installation, primary results showed that all plant species established successfully except Kernza, which failed to grow. A mixture containing 30% compost and 70% molasse provided good soil cover and good adaptability. Notably, pasture mixtures with 15% compost (~1.5 % organic matter) exhibited strong development, better macrofauna integration, and improved soil structure compared with other plots. These findings contribute to the development of processes for the engineered soil-based recovery of molasse excavated from the FCC and other large-scale construction projects.