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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. [ABSTRACT FROM AUTHOR]

• Soil N properties were investigated across urban-rural-natural forests in the Beijing metropolitan region. • Soil N concentrations and δ15N changed nonlinearly along with the distance to the urban core. • The spatial variations of total and alkali-hydrolyzable N concentrations were mainly explained by park age. • The spatial variation of soil δ15N was mainly explained by cropland share. • Surface and subsurface soil layers show distinct soil N properties with different drivers. Global urbanization has profoundly altered regional biogeochemical cycles across urban-rural-natural continuums. A better insight in the changes in soil nitrogen (N) properties across urban-rural-natural forests sheds lights on the impacts of urbanization on regional N cycles and ecological consequences. Based on a systematic survey in forest patches of twenty parks across the urban-rural-natural gradients in the Beijing metropolitan region, China, we analyzed the spatial variations of three soil N properties (i.e., total N, alkali-hydrolyzable N, and δ15N) and their potential drivers that are indicative for anthropogenic N sources (trunk road density for traffic N emissions; cropland share for agricultural N emissions), climate conditions (mean annual temperature, MAT; mean annual precipitation, MAP), forest conditions (forest coverage and the number of common tree species) and time for soil N accumulation since park establishment (park age). Soil total N concentration, alkali-hydrolyzable N concentration and δ15N values in the surface (0–10 cm) and subsurface layers (10–20 cm) all showed a decrease up to 40 km (i.e., the urban fertile island phenomenon) from the urban core and then an increase to values in peri-urban natural forests as high as those in urban forests. The spatial variations of the surface and subsurface soil total N concentrations and alkali-hydrolyzable N concentrations were, respectively, mainly and exclusively explained by park age, while trunk road density, MAP and MAT played a less important role in regulating the soil N variation in the surface layer. The spatial variations of the surface and subsurface soil δ15N values were respectively, mainly, and exclusively controlled by cropland share, while park age and MAP played a limited role in regulating the soil δ15N variation in the surface layer but not in the subsurface layer. Our findings reveal the way in which anthropogenic and climatic drivers shape soil N properties across the urban-rural-natural forests. [ABSTRACT FROM AUTHOR]