In order to trace changes in atmospheric nitrogen sources in Guiyang (SW China) from 2006 to 2016–2017, tissue nitrogen content and bulk stable isotope (δ¹⁵N) values of epilithic moss (Haplocladium microphyllum), collected monthly between April 2016 and March 2017 along an urban-rural gradient (urban center, semi-urban, suburban, and rural) in Guiyang, Guizhou (SW China), were determined. These were then compared with previous surveys in 2006. The nitrogen concentrations (1.4 to 3.7%) and δ¹⁵N values (−8.3 to −2.7‰) of the mosses (sampled in 2016–2017) showed an obvious downward trend from the urban center to the rural area, suggesting that ¹⁵N enrichment in urban areas is associated with traffic-related NO_xN. On the contrary, ¹⁵N-depleted mosses collected in the rural area were primarily affected by atmospheric NH_y-N from agricultural activities. Our results showed an opposite trend for δ¹⁵N variation in 2016/2017 compared to that in 2006, this may reflect a historical change in atmospheric nitrogen sources. A Bayesian isotope mixing model was then applied to estimate the contributions of atmospheric nitrogen sources. The results indicated that the dominant atmospheric nitrogen compounds and sources in the urban area had switched from NH_y-N (emissions from coal combustion and untreated waste and sewage) in 2006 to NO_x-N (from traffic emissions) in 2016/2017. These findings are critical for assessing the air quality of cities and for controlling atmospheric nitrogen pollution.