Estimating atmospheric nitrogen deposition within a large river basin using moss nitrogen and isotope signatures

Assessing flux and primary sources of the atmospheric nitrogen (N) deposition with high spatial resolution remained challenging. The epilithic moss is considered a suitable biological monitor to explore N deposition. Our study presented a detailed analysis of flux and major source contributions of ammonium (NH₄⁺) and nitrate (NO₃^-) deposition using N and δ¹⁵N signatures of epilithic moss collected densely from the Yangtze River basin. The results showed a more negative δ¹⁵N and higher N concentration of the moss in cropland and urban area than in forest and grassland of the basin. A gradient of the estimated N deposition (9.6-34.0 kg ha^-1 yr^-1) occurred from the Tibetan Plateau to lower reaches, with amount of NH₄⁺ was approximately three times higher than NO₃^- deposition. The contribution from volatilization to NH₄⁺ deposition (33.28 ± 8.10%) was less than the contribution from combustion (66.72 ± 8.10%), inconsistent with the traditional findings that N fertilizer and livestock waste are the principal sources of NH₃ emissions. Fossil fuel was the dominant sources of NO₃^- deposition, accounted for 70.22 ± 18.67%. From 2006 to 2019, the source contribution of N deposition in forest remained unchanged, while NH₃ volatilization and fossil fuel emitted NO_x in urban areas have increased. Our findings highlighted the importance of combustion sources to N deposition in the Yangtze River basin.