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

Desealing and soil renaturation have recently gained attention in European cities as strategies to enhance the sustainability of urban ecosystem services. To reduce both economic and environmental costs associated with urban greening, waste materials generated from desealing operations could potentially be repurposed for soil construction. However, the impact of incorporating asphalt millings into constructed Technosols on soil properties and plant growth has not yet been evaluated.

An 8-week laboratory experiment was conducted in which Lolium perenne was grown in 1 L pots (V/V) with the range of constructed Technosols containing 0 to 30% milled asphalt (MA; particle size < 3.15 mm) combined with 0 to 30% green waste compost (GWC). Soil physicochemical properties were assessed, including pH, cation exchange capacity (CEC), exchangeable cations, and water-holding capacity (WHC). Microbial community abundances were analyzed using qPCR, while microbial activities were evaluated with Biolog and MicroResp techniques. Plant root biomass and shoot growth were measured, and their chemical composition was determined using X-ray fluorescence (XRF).

The chemical properties of the constructed Technosols were significantly influenced by the addition of compost, with milled asphalt (MA) having a lesser impact, except for a slight increase in pH. MA addition led to a notable decrease in bulk density and water-holding capacity (WHC), particularly in the absence of green waste compost (GWC). MA also negatively affected the abundance of both bacteria and fungi, though this effect was mitigated by the inclusion of GWC. A higher proportion of asphalt increased the metabolic quotient of microorganisms, indicating potential stress. Similarly, plant biomass was negatively affected by MA, likely due to the reduced WHC. MA increased the proportion of macroelements such as P, K, Mn, and Cl.

This initial study on recycling asphalt millings into constructed Technosols supports our hypothesis that green waste compost can offset the potential loss of fertility associated with asphalt, which is generally considered an inert material. The addition of organic matter, particularly green waste compost, mitigated most of the negative impacts on soil properties, such as reduced water-holding capacity (WHC) and nutrient availability, which in turn affected both microbial communities and plant biomass. These preliminary findings encourage further research into the use of desealing waste for constructing fertile soils and advancing a circular economy.