Anthropogenic and climatic shaping of soil nitrogen properties across urban-rural-natural forests in the Beijing metropolitan region

Abstract

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.