Air Pollution and Climate Regulate the Large-Scale Pattern of Foliar Nitrogen Uptake by Urban Street Trees

Urbanisation has fundamentally altered terrestrial nitrogen (N) cycling and created N-enriched hotspots where street trees serve as essential green infrastructures. These street trees live in a distinctive environment with high-level reactive N emissions and blocked soil N cycling, potentially relying on direct N uptake via stomata as an important N source. However, the spatial pattern and primary drivers governing this non-root N uptake across broad climatic gradients remain elusive. Based on an analysis of leaf-soil difference in stable isotope ratios (∆ δ¹⁵N, i.e., foliar δ¹⁵N-topsoil δ¹⁵N) across 14 large cities in China, we show that urban street trees exhibit a V-shaped latitudinal pattern of isotope-inferred foliar N uptake, being independent of tree health status. Specifically, the strength of isotope-inferred foliar N uptake increases from subtropical to warm temperate zones and then declines toward colder regions. The spatial pattern of foliar N uptake by urban street trees is jointly regulated by various air pollutants and total precipitation from May to August. Specifically, the isotope-inferred foliar N uptake increases with higher road density (a surrogate for traffic NO_X emissions) and air concentrations of NH₃, O₃ and SO₂, as well as lower precipitation. Intriguingly, the isotope-inferred foliar N uptake shows a non-monotonic response to CO₂ concentrations. Our results provide isotopic evidence for a distinctive N acquisition strategy of urban street trees and highlight significant anthropogenic effects on foliar N uptake in urbanised landscapes.