With 60% of the total area located in the North China Plain, Beijing suffered with very high atmospheric nitrogen (N) deposition in the south part, which was as high as 100 kg hm^-2 a^-1 including both organic and inorganic nitrogenous species from both wet and dry deposition. To test the responses of foliar N contents of plants, a collection of deciduous broadleaf tree leaves was conducted across Beijing areas. Typical deciduous broadleaf trees, Populus and Salix were selected at 189 sites, which were firstly designed at 5-decimal minute in latitude and longitude grid resolution and then excluded sites without Populus or Salix species. Leaf sampling collection was carried out in the end of August corresponding to annual peak-standing biomass in late summer. Five trees of every species at each site were selected, and five mature leaves of every tree were collected. All the leaf samples were over-dried at 105℃ for 10 minutes and then 65℃ for 48 hours to constant mass. Samples of each species at each site were put together and grounded with a mill to pass 1-mm sieve for N contents analysis. Foliar N was determined following Kjeldehl digestion and δ¹⁵N values were determined by continuous flow isotope-ratio mass spectrometer. The foliar N contents were (24.0±4.0) g/kg and (25.9±4.1) g/kg on average for Populus and Salix, respectively. Spatial variations of both the two genus showed similar trends, with higher foliar N contents in the southeast and lower foliar N contents in the northwest, consisting with the spatial variation of atmospheric N deposition in Beijing area. The foliar δ¹⁵N values were (1.15±2.48)‰ and (2.31±2.60)‰ on average for Populus and Salix, respectively. However, the spatial variation trends of δ¹⁵N values were contrary with the foliar N contents. Higher δ¹⁵N values were found in the city center and northwest, while lower δ¹⁵N values were found in the southeast. With the different δ¹⁵N values of potential sources, the higher δ¹⁵N values in the city center indicated the traffic emission sources, the higher δ¹⁵N values in the northwest indicated the natural N cycling, while the lower δ¹⁵N values indicated both the agricultural and traffic pollution.