Plant Functional Traits and Soil Nutrients Drive Divergent Symbiotic Fungal Strategies in Three Urban Street Tree Species

Understanding species-specific mechanisms governing symbiotic fungal responses to plant traits and soil factors is critical for optimizing urban tree “plant-soil-fungus” systems under pollution stress. To address this gap, we combined δ¹³C/δ¹⁵N isotope analysis and ITS sequencing for three common street trees in Beijing: <i>Sophora japonica</i>, <i>Ginkgo biloba</i>, and <i>Populus tomentosa</i>. In <i>S. japonica</i>, symbiotic fungal abundance was positively associated with leaf δ¹⁵N, indicating root exudate-mediated “plant-microbe” interactions during atmospheric NO_x assimilation. <i>G. biloba</i>, with weak NO_x assimilation, exhibited a negative correlation between fungal abundance and soil available N/P, suggesting mycorrhizal nutrient compensation under low fertility. <i>P. tomentosa</i> showed decreased fungal abundance with increasing soil N/P ratios and specific leaf area, reflecting carbon allocation trade-offs that limit mycorrhizal investment. These results demonstrate that symbiotic fungi respond to atmospheric and edaphic drivers in a tree species-dependent manner. Urban greening strategies should prioritize <i>S. japonica</i> for its NO_x mitigation potential and optimize fertilization for <i>G. biloba</i> (nutrient-sensitive fungi) and <i>P. tomentosa</i> (nutrient balance sensitivity). Strategic mixed planting of <i>P. tomentosa</i> with <i>S. japonica</i> could synergistically enhance ecosystem services through complementary resource acquisition patterns. This study provides mechanism-based strategies for optimizing urban tree management under atmospheric pollution stress.