Incorporating reclaimed asphalt pavement into constructed urban soils: Effects on plant growth and nitrogen-cycling bacteria

Purpose: This study explores the feasibility of using removed and crushed asphalt pavement mixed with compost from household organic waste to construct soil for green spaces, aiming to promote the recycling of wastes in urban contexts. Methods: We conducted a pot experiment testing different treatments with varying proportions of asphalt pavement (10%, 25%, or 50% v/v), a fixed amount of compost (5% v/v), and excavated urban soil making up the remainder. A mixture of grasses commonly used in urban green infrastructure was sown and monitored for vegetative growth. Potential phytotoxicity of the constructed soils was assessed through a germination test, while the release of heavy metals and polycyclic aromatic hydrocarbons (PAHs) was evaluated by analysing both the solid phase and its leachate. The abundance of functional nitrogen-cycling bacterial communities in the constructed soils was characterised using quantitative PCR. Results: The combined addition of asphalt and compost did not impair soil properties, and the treatment with an intermediate proportion of asphalt exhibited satisfactory plant growth without exceeding legal limits for heavy metals and PAHs. Furthermore, substrates containing asphalt effectively supported the development of key bacterial populations involved in the nitrogen cycle. Conclusions: This study demonstrates the potential for disused asphalt to be repurposed on-site as a component in constructed urban soils for greening rather than being discarded as landfill waste, thereby aligning with the UN Sustainable Development Goal 11 (Sustainable Cities and Communities) of the 2030 Agenda. However, further studies in other contexts and over a longer period are needed to fully demonstrate that this practice does not have significant negative consequences on herbaceous vegetation growth, soil health, or public safety.