Evaluation of Biochar from Agricultural and Forestry Residues: A Comparative Study of Olive and Poplar Prunings

The sustainable valorization of agroforestry residues through thermochemical conversion can support circular bioeconomy and renewable energy production. This study evaluated olive pruning (Olea europaea L.) and poplar (Populus alba L.) residues from Central Italy as feedstocks for biochar production through a pilot-scale fixed-bed pyrolysis process. The feedstocks and resulting biochar were characterized in terms of their physicochemical properties, while their agronomic potential was assessed through germination assays and lettuce (Lactuca sativa L.) pot experiments. The carbon content of the biochar produced from poplar (BFOR) was statistically higher than that produced from olive pruning (BAGR): 92.0% and 79.9%, respectively. BFOR consistently showed a germination index above 100%, indicating phytostimulant potential within the tested conditions, whereas the BAGR produced species-dependent responses. In lettuce pot experiments, biochar alone resulted in limited effects on plant growth. However, when combined with organic fertilization, fresh biomass increased by approximately 10% compared with fertilization alone, reaching 107.88 g with BAGR and 107.08 g with BFOR, compared with 97.53 g for fertilization alone. The combined treatments also showed improved photosynthetic performance. These results highlight the potential of pilot-scale biochar production from locally available agroforestry residues as an integrated strategy for renewable energy production and sustainable soil management.