Reusing asphalt millings with excavated materials and compost to construct Technosols: effects on plant and soil properties

Desealing and soil renaturation is a major concern in European cities to rehabilitate degraded ecosystem services in urban areas. In order to minimize both the economic and environmental costs of urban greening, wastes generated by desealing operations could be reused for soil construction. To our knowledge, the effect of asphalt milling incorporation into constructed Technosols on soil physical, chemical and biological properties and plant growth has never been evaluated yet. In this study, we tested different compositions of Technosols including excavated deep horizons of soils (EDH) mixed with milled asphalt and with different ratios (0-10-20-30%) of compost. The same combinations were made with coarse sand instead of milled asphalt as a reference. The experiment was undertaken for 3 months in a phytotron, with planted ryegrass (Lolium perenne). Thereafter, plant root and shoot biomass were collected and elemental composition analysis was performed using X-ray fluorescence (XRF). The soil physicochemical properties (pH, water retention, CEC…) were measured as well as the microbial characterization (Biolog® Ecoplates, DNA extraction and qPCR). Results obtained showed that the milled asphalt addition had a small negative or no influence on soil properties depending on the dose of compost added. Plant biomass tends to decrease for the Technosol with the highest asphalt content but, again, these effects were mitigated by compost addition. The optimal substrate was with 10% of milled asphalt for low organic matter content. The highest compost content (30%) evened the shoot biomass differences among the treatments. The root biomass followed the shoot biomass, although the highest root/shoot ratio was observed for the modalities with low compost content (0-10%). A significant effect of asphalt millings was also detected in plant elemental composition with relative enrichment of more macro-elements (P, K, Cl, S), certain oligo-elements (Mn, Zn, Cu) and trace metals (Cr). Overall, these results suggest that asphalt millings could be used as parent material for constructed Technosols.