A distinctive latitudinal trend of nitrogen isotope signature across urban forests in eastern China

• Published in: Global change biology Vol. 29; no. 19; pp. 5666 - 5676
• Main Authors: Xia, Nan, Du, Enzai, Tang, Yang, Guo, Hongbo
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.10.2023
• Subjects: Ammonium; Ammonium compounds; Annual precipitation; Anthropogenic factors; Availability; Forests; Latitude; Leaves; Nitrogen; Nitrogen isotopes; Plant species; Plants (botany); Soils; Spatial variations; Topsoil; Trends; Urban areas; Urban forests; Urbanization; China; leaf δ15N; nitrogen availability; latitudinal trend; topsoil δ15N; topsoil C:N ratio; nitrogen deposition
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.16899

Rapid urbanization has greatly altered nitrogen (N) cycling from regional to global scales. Compared to natural forests, urban forests receive much more external N inputs with distinctive abundances of stable N isotope (δ N). However, the large-scale pattern of soil δ N and its imprint on plant δ N remain less well understood in urban forests. By collecting topsoil (0-20 cm) and leaf samples from urban forest patches in nine large cities across a north-south transect in eastern China, we analyzed the latitudinal trends of topsoil C:N ratio and δ N as well as the correlations between tree leaf δ N and topsoil δ N. We further explored the spatial variation of topsoil δ N explained by corresponding climatic, edaphic, vegetation-associated, and anthropogenic drivers. Our results showed a significant increase of topsoil C:N ratio towards higher latitudes, suggesting lower N availability at higher latitudes. Topsoil δ N also increased significantly at higher latitudes, being opposite to the latitudinal trend of soil N availability. The latitudinal trend of topsoil δ N was mainly explained by mean annual temperature, mean annual precipitation, and atmospheric deposition of both ammonium and nitrate. Consequently, tree leaf δ N showed significant positive correlations with topsoil δ N across all sampled plant species and functional types. Our findings reveal a distinctive latitudinal trend of δ N in urban forests and highlight an important role of anthropogenic N sources in shaping the large-scale pattern of urban forest N signature.