Mitigating Urban Pollution Stress in Trees: Biochar Effects on Norway Spruce ( Picea abies (L.) H. Karst.) and Norway Maple ( Acer platanoides L.) Seedlings

Urban trees are vital for air pollution mitigation, but their function is often compromised by exposure to particulate matter (PM), which impairs physiological processes and reduces growth. Enhancing tree resilience is therefore essential for maintaining their ecosystem services in polluted urban environments. This study examined the early growth and biochemical responses of Norway spruce ( Picea abies ) and Norway maple ( Acer platanoides ) seedlings to foliar PM exposure and assessed whether biochar (BC) soil amendment can alleviate PM-induced stress. Seedlings were cultivated outdoors under three treatments: Control (no treatment), PM (foliar exposure to particulate matter), and PM + BC (PM exposure with 10% biochar added to the substrate). Results revealed that Norway maple showed significant biochemical sensitivity to PM, including substantial reductions in chlorophyll and increases in antioxidant activity. However, Norway spruce showed more moderate pigment changes but reduced height growth. BC modulated oxidative and phenolic responses (TPC, TFC, MDA) and partially mitigated PM-induced stress, although its effectiveness varied by species. For Norway spruce, BC significantly enhanced resilience by restoring height growth, stabilizing pigments, and reducing oxidative stress compared with treatment using PM alone. In contrast, for Norway maple, BC failed to restore chlorophyll levels and increased oxidative and phenolic activity, yielding mixed outcomes. Despite physiological differences between the two species, multivariate PCA consistently showed that PM-treated seedlings diverged from the control cluster, whereas PM + BC-treated seedlings were closer to the controls, with mitigation substantially stronger in Norway spruce. These findings demonstrate that biochar can reduce PM-induced stress, but its successful implementation depends fundamentally on selecting appropriate species traits and understanding their specific metabolic response strategies.