Air quality issues caused by nitrogen dioxide (NO₂) have become increasingly serious in Chinese cities in recent years. As important urban green infrastructure, urban forests can mitigate gaseous nitrogen pollution by absorbing NO₂ through leaf gas exchange. This study investigated spatiotemporal variations in the NO₂ removal capacity of urban forests in Beijing city from 2014–2019, based on the i-Tree Eco deposition model. The results show that the annual removal capacity of administrative districts within Beijing city ranged from 14,910 to 17,747 tons, and the largest capacity (2684 tons) was found in the Fangshan district. The annual removal rate of NO₂ by urban forests in administrative districts within Beijing was estimated at between 0.50–1.60 g/m², reaching the highest (1.47 g/m²) in the Mengtougou district. The annual average absorption of NO₂ by urban forests can account for 0.14–2.60% of annual total atmospheric NO₂ and potentially reduce the NO₂ concentration by 0.10–0.34 µg/m³ on average. The results of a principal component analysis suggest that the distribution of urban forests in Beijing is not optimized to maximize their NO₂ removal capacity, being higher in suburban areas and lower in urban areas. This study provides insights into botanical NO₂ removal capacity in Beijing city to mitigate atmospheric N pollution, addressing the key role of urban forests in improving human wellbeing.

Previous studies have shown that climatic and anthropogenic factors influence the variations in the stable isotope signature of tree materials in an urban ecosystem. However, the responses of urban trees to various anthropogenic and precipitation interventions are unclear. To fill the research gap, the isotopic composition (δ¹⁵N and δ¹⁸O) and total nitrogen content of the soil-plant system of urban forests were measured, focusing on typical urban tree species (Populus tomentosa and Ficus virens). The research was done under various land uses in China’s Beijing and Shenzhen megacities (with different humidity conditions). The linear mixed-effect model revealed that the δ¹⁵N values in the leaves of roadside Ficus virens were significantly higher than those in urban park sites, away from the road (δ¹⁵N difference = 7.2‰). In contrast, Populus tomentosa leaves exhibited a non-significant difference (δ¹⁵N difference = −1.8‰). Further regression analysis revealed that the variations in δ¹⁵N in tree leaves could be attributed to the proximity to highways, suggesting the influence of vehicle exhaust on δ¹⁵N values in roadside trees. The linear mixed-effect model also revealed δ¹⁸O values significantly higher in Populus tomentosa leaves than in Ficus virens leaves. To assess the influence of humidity conditions on δ¹⁸O values in the leaves, the ratio of the atmospheric and intercellular vapor pressures (e_a/e_i) of tree canopies was estimated. The result revealed that the e_a/e_i of Ficus virens was significantly higher than for Populus tomentosa, ascribed to the variation in the humidity conditions of the two megacities. These results highlight that urban trees can potentially serve as bioindicators for atmospheric pollution and humidity. Our findings highlight the ecophysiological responses of urban forests related to N and O as they vary according to traffic exhaust and relative humidity. Consequently, they are potentially valuable indicators of urban atmospheric contamination, forming a nature-based solution for citizen welfare improvement.

Disturbance of urban and regional nitrogen cycles due to urbanization have resulted in the greenhouse effect, acid rain, eutrophication, and reductions in biodiversity