Mitigating traffic-derived NOx in urban areas: Stable isotope evidence of NOx uptake by urban trees

Quantifying the effect of urban trees on the removal of traffic-derived nitrogen oxides (NOx) is essential for mitigating air pollution. This study employed stable nitrogen isotope and oxygen isotope analyses, in combination with a Bayesian isotopic mixing model, to estimate NOx absorption by urban trees in street and park environments in Shenyang, China. Results revealed that broadleaved species exhibited higher nitrogen absorption of both soil and traffic-derived NOx compared with conifers. Sophora japonica showed the strongest absorption of traffic-derived NOx in parks with a value of 1.68 ± 0.48 mg N/g_dw; Salix matsudana and Populus alba demonstrated high traffic-related NOx absorption in street areas with values of 0.88 ± 0.18 and 0.55 ± 0.09 mg N/gdw, respectively. Tree characteristics such as crown width, diameter at breast height, and tree height indirectly influenced traffic-derived NOx absorption by modulating leaf δ¹⁵N. Tree height exerted a statistically significant positive effect on δ¹⁵N with a path coefficient of 0.618, while crown width showed a statistically significant negative effect with a path coefficient of -0.294. The findings of this study enhance our understanding of how urban trees mitigate atmospheric NOx pollution, and offer guidance for urban greening planning regarding air quality improvement.