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

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