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

ABSTRACT 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 (∆ δ 15 N, i.e., foliar δ 15 N‐topsoil δ 15 N) 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 NH 3 , O 3 and SO 2 , as well as lower precipitation. Intriguingly, the isotope‐inferred foliar N uptake shows a non‐monotonic response to CO 2 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.