Restoration of alkaline spoil deposits at a highway construction site: woody plant responses to liming and organic soil amendments

This study aimed to assess the most effective experimental design for the successful restoration of a highway construction site using spoil material (SM) stabilized with lime (LSM). The design involved using LSM as the bottom filler of natural land depressions and the topsoil for the top layer. For this purpose, a 3-year experiment was conducted using five native wood species (Corylus avellana, Fraxinus ornus, Olea europaea, Ostrya carpinifolia, and Quercus pubescens) grown on various mesocosms partially filled with LSM or LSM amended with compost or with anaerobic digestate, or with olive pomace. A control (without LSM) was also set up. Survival, height, and stem diameter of plants were monitored. Fraxinus ornus and Corylus avellana, showed the highest growth performances across treatments even under stressful conditions. Quercus pubescens exhibited high mortality and minimal growth, indicating the lowest suitability on such substrates. The application of organic amendments did not result in a substantial increase in plant growth or survival; the dosages were likely too low for such nutrientpoor soil. However, amendments helped to first reduce the pH at the interface between the topsoil and the spoil material. Results highlight that species selection and long-term strategies are critical for restoring alkaline substrates. The use of tolerant species such as Fraxinus ornus represents a viable initial approach, while more sensitive taxa may require phased introduction or substrate preconditioning. This study addresses plant establishment for restoration purposes on lime-stabilized spoil to develop practical, cost-effective protocols for infrastructure landscapes.