Untreated Versus Forest-Conditioned Biochar: Early Growth and Rhizosphere Communities of Pinus halepensis in Degraded Soil

Soil degradation can strongly constrain plant establishment and ecosystem restoration, highlighting the need for amendments able to improve both soil functionality and plant–microbiome interactions. This study evaluated whether biochar preconditioned for six months in healthy pine forest soil could enhance the early growth of Pinus halepensis Mill. compared with untreated biochar in degraded agricultural soil. Seedlings were grown for 85 days under controlled conditions in five substrates, including pine forest soil, non-intensively cultivated agricultural soil, degraded agricultural soil, and degraded soil amended with either untreated or bioactivated biochar. Above- and below-ground plant morphological traits, soil chemistry, and rhizosphere microorganism communities were assessed. Untreated biochar markedly promoted root development, increasing absorptive fine-root length, root biomass, and stem diameter compared with degraded soil and forest-conditioned biochar. In contrast, bioactivated biochar resulted in more limited early seedling growth. Microbial community composition varied among treatments, whereas Shannon diversity did not differ significantly. Bacterial communities showed partial overlap among treatments, while fungal communities displayed clearer treatment-associated separation. Overall, forest-soil conditioning did not improve biochar effectiveness during early seedling development. Further drought and field trials are required to determine whether these contrasting growth and rhizosphere responses influence long-term establishment and restoration performance.