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

Abstract

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 (δ¹⁵ N). However, the large-scale pattern of soil δ¹⁵ N and its imprint on plant δ¹⁵ 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 δ¹⁵ N as well as the correlations between tree leaf δ¹⁵ N and topsoil δ¹⁵ N. We further explored the spatial variation of topsoil δ¹⁵ 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 δ¹⁵ N also increased significantly at higher latitudes, being opposite to the latitudinal trend of soil N availability. The latitudinal trend of topsoil δ¹⁵ N was mainly explained by mean annual temperature, mean annual precipitation, and atmospheric deposition of both ammonium and nitrate. Consequently, tree leaf δ¹⁵ N showed significant positive correlations with topsoil δ¹⁵ N across all sampled plant species and functional types. Our findings reveal a distinctive latitudinal trend of δ¹⁵ N in urban forests and highlight an important role of anthropogenic N sources in shaping the large-scale pattern of urban forest ¹⁵ N signature.

Keywords: latitudinal trend; leaf δ15N; nitrogen availability; nitrogen deposition; topsoil C:N ratio; topsoil δ15N.