Wet-only deposition of atmospheric inorganic nitrogen and associated isotopic characteristics in a typical mountain area, southwestern China

Abstract To quantify and compare atmospheric nitrogen (N) deposition and its N isotopic ratio are critical for constraining N sources and effective reduction of reactive N emissions. In this study, a total of 223 rainwater samples were collected by wet-only auto-samplers, and wet-only deposition and isotopic composition (δ 15 N) of reduced (NH 4 + -N) and oxidized (NO 3 − -N) N were measured at three typical mountain sites, including an urban (Wanzhou, WZ), a town (Gaoyang, GY) and a rural (Dade, DD) site in Chongqing, southwestern China in 2016. The wet-only inorganic N deposition (DIN, sum of NO 3 − -N and NH 4 + -N) were 17.50, 8.63 and 12.16 kg N ha − 1  yr − 1 at WZ, GY and DD site, respectively. Annual δ 15 N-NH 4 + values of rainwaters were negative at the urban site (− 3.12 ± 3.21‰, WZ) and positive at both town and rural site (0.65 ± 12.51‰, GY; 2.16 ± 6.11‰, DD). Annual δ 15 N-NO 3 − values, on the contrary, were positive at the urban site (0.33 ± 7.87‰, WZ) and negative at both town and rural site (− 5.59 ± 2.20‰, GY; − 0.39 ± 8.89‰, DD). These results reveal the urban site was wet-only DIN hotspot and had a different N source compared with the town-rural site in the mountain area. Moreover, precipitation DIN had a potentially negative risk on both aquatic and forest ecosystems.