Valorisation of Invasive Alien Plant Species (IAPS) biomass: potential bioenergy uses and biochar-driven soil remediation

Invasive alien plant species (IAPS) are a global problem, threatening ecosystems, biodiversity, and human health. The legislation mandates IAPS proper management, yet it is costly and produces large amounts of waste biomass that need to be disposed of. Valorising IAPS biomasses from eradication practices may offer ecological and economic benefits, embracing circular and bio-economy principles, and creating synergies with restoration projects. During the Ph.D. activity, we evaluated i) the energy production potential of the IAPS biomass through the analysis of plant functional traits, ii) the use of these biomasses for the production of biochar, and iii) the efficiency of IAPS-derived biochar as a phytoremediation tool in a multi-heavy metal contaminated soil. In particular, we first used plant functional spaces (Global Spectrum of Plant Form and Function, and Grime’s CSR strategies) to identify the bioenergetic potential of IAPS’ biomasses. To this aim, we identified four IAPS groups representative of key trade-offs between plant economics and size variation, which correlated respectively with C:N (Carbon to Nitrogen ratio) and GHV (Gross heating value), indicating suitability for biochemical and thermal bioenergetic processes, respectively. Results demonstrated that the herbaceous IAPS were better suited to biochemical processes while woody IAPS were potentially better suited to thermal ones. As the second step, we selected five herbaceous and five woody IAPS for small-scale biochar production and a consequent chemical-physical characterization. The analysis revealed differences between woody and herbaceous IAPS, the latter being more suitable for soil amendment due to their greater pH and macronutrient content, and the former characterized by higher lead adsorption from aqueous solutions being more appropriate for heavy-metal remediation. Finally, based on these findings, IAPS-derived biochars (from Ailanthus altissima and Solidago gigantea) combined with Urtica dioica powder were assessed in their remediation potential of an Arsenic and Lead-contaminated technosol with Phaseolus vulgaris used as indicator species. The amendments improved soil pore water properties and lowered Pb concentration, promoting plant growth, enhancing fine root traits, and reducing Pb plant uptake. Also, combining biochar and U. dioica powder improved the soil's enzymatic activities. On the contrary, when As was analysed, both biochar types mixed with the U. dioica powder, increased its mobility in soil pore water, leading to a higher plant root uptake, but no translocation to the aboveground organs occurred.