Urban-rural shifts in elemental composition in leaves and topsoil of street trees in a subtropical city of China

Street trees provide essential ecosystem services and are subject to large inputs of anthropogenic-sourced elements and frequent management operations. However, a systematic understanding of the elemental composition in street tree leaves and topsoil is still lacking. Based on a field survey across urban-rural gradients in a subtropical city (Chengdu) of China, we explored the spatial patterns of ten elements (i.e., nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), aluminum (Al), iron (Fe), zinc (Zn), copper (Cu) and chromium (Cr)) in street tree leaves and in topsoil (0–10 cm) as well as their key drivers. Our results indicate that leaf N, P, K, and Al contents as well as topsoil N, P, and Al contents showed a significant increase toward the central urban area. In contrast, no significant spatial trends were found for other elements across the urban-rural gradients. The first principal component of leaf elemental composition (mainly explained by N, K, Mg, Fe, Cu, and Al contents) and the second principal component of topsoil elemental composition (mainly explained by P, K, Mg, Fe, and Al contents) both increased significantly toward the central urban area. Unexpectedly, leaf and topsoil elemental composition showed no significant correlations with each other. Our findings demonstrate distinctive spatial patterns of the elemental composition of leaf and topsoil for street trees across urban-rural gradients and highlight a decoupling between leaf and topsoil element composition in the street tree systems. These results improve our understanding of how urbanization reshapes the pattern of biogeochemical cycling and provide a baseline for better nutrient management in the street tree systems.