Urban soil phosphorus hotspot and its imprint on tree leaf phosphorus concentrations in the Beijing region

Purpose: Rapid urbanization has altered regional nutrient cycles and consequently affected soil nutrient availability for plant growth. This study aims to explore the way in which urbanization shapes the spatial patterns of soil phosphorus (P) concentrations as well as the imprint on tree leaf P concentrations across gradients of urban-rural forests. Methods: We collected soil samples of the surface (0-10 cm) and subsurface (10-20 cm) layers and leaf samples of three common tree species (Chinese pine, Pinus tabuliformis; Chinese scholar tree, Sophora japonica; golden rain tree, Koelreuteria paniculata) from forest patches in twenty parks across urban-rural transects in the Beijing metropolitan region, China. By measuring soil total P, soil available P, and leaf P concentrations, we tested the urban soil P hotspot hypothesis and assessed the consequent imprint on tree leaf P concentrations. Results: Soil total P and available P concentrations in the surface and subsurface layers both increased significantly with closer distance to the urban core. Spatially, leaf P concentrations showed an increase with soil available P concentrations only for the legume Chinese scholar tree, while leaf P concentrations of Chinese pine and broadleaf golden rain tree both increased significantly with the diameter at breast height. Conclusion: Our results confirmed the occurrence of an urban soil P hotspot and identified its significant imprint on leaf P nutrition of legume trees across transects of urban-rural forests. The findings improve the understanding of the way that urbanization alters regional P cycles and consequent P availability for plant growth.