Nature-based solutions using organic amendments for biorestoration of alkaline spoil material

There are several tunnelling projects underway in Europe and worldwide to facilitate connections between distant areas or ones with geographical constraints (e.g. mountains). As a result, the tunnel industry has been producing huge amounts of excavated material (spoil material: SM), which can be re-used for different purposes (including green areas). Adding lime can be a practical procedure at a construction site for the stabilization of SM and makes it possible to manage it better at its final destination site. This work reports a nature-based solution (NBS) for decreasing the highly basic pH (12.53 ± 0.01) of the SM after adding lime, using three different organic fertilizers which are by-products of local olive oil (pomace), compost and digestate production. For this purpose, microcosm experiments were performed with SM amended with compost, pomace or digestate. Half of the microcosms were also seeded with Medicago sativa to evaluate its growth in terms of biomass. The changes in pH and in the structure and functional potential of the microbial community were evaluated in the amended SM at the start and end (4 months) of the experiment. Aerial and root plant biomass and concentrations of leaf chlorophyll and total phenolic compounds were also measured. The microbial biodiversity was evaluated using amplicon sequencing (16S rRNA gene) and a predictive functional analysis was performed by processing sequencing data with the PICRUSt2 tool. Adding the amendments to SM significantly decreased the pH values by 4 units and introduced organic carbon, nitrogen, and active microbial populations. Although pomace promoted the lowest pH decrease (7.98 ± 0.18) and the highest microbial abundance and activity, it did not favour the highest plant growth. Adding compost and digestate to SM can be considered the best NBS for decreasing pH and supporting plant development. M. sativa did not have any direct effect on lowering pH, demonstrating in this study the key role of microorganisms in responding efficiently to this environmental stress and mitigating it. In accordance with these results, predicted bacterial genes coding for membrane proteins (nuo, mrp) and for indoleacetic acid metabolism (ami, nthA, ipdC) involved in pH regulation were identified among the microbial populations.