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

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 (δ15N) of reduced (NH4+-N) and oxidized (NO3--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 NO3--N and NH4+-N) were 17.50, 8.63 and 12.16kgNha-1yr-1 at WZ, GY and DD site, respectively. Annual δ15N-NH4+ 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 δ15N-NO3- 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.