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

Abstract

AbstractRapid 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 (δ15N). However, the large‐scale pattern of soil δ15N and its imprint on plant δ15N 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 δ15N as well as the correlations between tree leaf δ15N and topsoil δ15N. We further explored the spatial variation of topsoil δ15N 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 δ15N also increased significantly at higher latitudes, being opposite to the latitudinal trend of soil N availability. The latitudinal trend of topsoil δ15N was mainly explained by mean annual temperature, mean annual precipitation, and atmospheric deposition of both ammonium and nitrate. Consequently, tree leaf δ15N showed significant positive correlations with topsoil δ15N across all sampled plant species and functional types. Our findings reveal a distinctive latitudinal trend of δ15N in urban forests and highlight an important role of anthropogenic N sources in shaping the large‐scale pattern of urban forest 15N signature.