Tree species and urban green space types jointly drive rhizosphere SOC fractionation and stability via microbial-enzymatic pathways

Abstract Urban soil organic carbon (SOC) dynamics are governed by complex vegetation-microbe interactions, but the synergistic effects of tree functional types and green space management on rhizosphere SOC fractionation and stability remain unclear. This study elucidates the distinct mechanisms by which two native tree species—Populus tomentosa (deciduous broadleaf) and Pinus tabuliformis (evergreen conifer)—regulate SOC stabilization across four urban green space types (urban parks, residential areas, roadside shelterbelts, and suburban forests). Combining extracellular enzyme stoichiometry, microbial community assembly, and SOC fractionation analyses, we demonstrate that carbon (C) and phosphorus (P) availability are the primary constraints on microbial metabolism in urban soils. P. tomentosa rhizospheres exhibited significantly higher SOC content (up to 77.0% in residential areas) and stability (MAOC/SOC ratio, peaking at 73% in roadside shelterbelts) than P. tabuliformis, driven by fungal-dominated C/P limitation and elevated hydrolytic enzyme activity (β-1,4-glucosidas, β-1,4-N-acetylglucosaminidase, and alkaline phosphatase; 1.3-2.1-fold higher). In contrast, P. tabuliformis favored oxidative enzyme activity (13% greater phenol oxidase), leading to diminished SOC accumulation despite greater stability in suburban forests (63% MAOC/SOC ratio). Critically, green space type mediated this trade-off: intensive management (e.g. urban parks) boosted absolute SOC storage via nutrient enrichment, whereas low-disturbance sites (e.g. suburban forests) enhanced stabilization efficiency (MAOC/SOC) through microbial niche partitioning. Our results provide a novel mechanistic framework linking tree functional traits, microbial metabolic constraints, and enzymatic trade-offs to SOC dynamics, offering science-based strategies for urban green space design to optimize carbon sequestration.