Transforming excavation waste into functional soil: Effects of compost ratios and plant diversity on initial pedogenesis

Soil engineering is gaining increasing attention due to its potential to address soil scarcity while promoting waste recycling. However, functional soils do not arise from simply mixing waste materials. Instead, fundamental pedogenetic processes must be activated and supported, alongside the stabilization of organic carbon through complexation with mineral surfaces. Evidence suggests that interactions between plants and minerals could accelerate these early pedogenic processes—including carbon fixation and mineral–organic associations—while limiting carbon mineralization. This study reports the results of a field experiment conducted in Geneva to investigate these effects.

Excavated geological layers (DSH) from Geneva’s fluvio-glacial deposits were mixed with six levels of green waste compost (GWC) (10–90 %). Each plot was sown with a standardized indigenous plant mixture of 44 species, and plant diversity was maximized under the assumption that higher diversity would enhance the formation of a soil-like structure in the parent material. Treatments were replicated four times and monitored monthly for the first six months, with a final assessment at 12 months. Organic carbon forms were analyzed using Rock Eval® pyrolysis, and soil hydrostructural properties were evaluated through soil shrinkage analysis.

Results showed that a 25 % compost ratio promoted carbon stabilization, while the 10 % mixture demonstrated potential for carbon fixation and mineral–organic associations after 12 months, likely due to slower plant establishment in dry grassland. The 50 % compost mixture supported higher plant species richness, including ruderal and dry grassland species. Additionally, adding 10 % DSH to a 90 % GWC mixture reduced carbon mineralization compared with 100 % GWC, indicating potential for soilless applications. Overall, these findings suggest that pedogenic processes in engineered soils can be optimized by carefully selecting parent material-to-organic carbon ratios and plant combinations.