Growth, physiological and yield responses of olive tree to biochar-enriched substrates irrigated with domestic and urban treated wastewaters

This study assessed the combined effects of biochar substrate amendment (BC0: 0%, BC25: 25%, and BC50: 50% v/v) and irrigation with treated wastewaters (TWWs) on three-year-old potted olive trees (Olea europaea L., cv. ‘Arbosana’) over a full growing season. Two types of treated wastewater were used: domestic (TWW1) and urban (TWW2), both of which underwent phytoremediation. Additionally, tap water (TaW) served as control. Plant growth, physiological, and yield parameters were evaluated. Biochar addition improved substrate physical properties, decreasing bulk density (up to 56%) and increasing water-holding capacity (20–25%). Irrigation with TWWs promoted vegetative growth. TWW1 increased fruit yield and weight by ∼30–35% due to high nitrate and phosphorus loads. Conversely, TWW2 reduced fruit number and yield by ∼24%, likely due to lower phosphorus availability during reproductive development. Despite the elevated salinity of TWWs, olive trees exhibited strong osmotic tolerance. However, TWW1 induced ionic imbalance, characterized by increased Na accumulation and reduced K and P concentrations in leaves. This response may be associated with the high nitrate content of TWW1, which can promote Na uptake. Biochar application decreased heavy metal uptake, particularly Mn, which was reduced in BC50 olive leaves by 56%. Moreover, BC50 also reduced fruit yield by 27% due to a trade-off favoring vegetative biomass over fruit set. Overall, a moderate 25% biochar rate combined with domestic TWW1 represents a viable strategy to substitute freshwater and peat in sustainable olive nurseries and cultivation, but it requires precise substrate formulation and proper fertilization management.