Seasonal fluxes and sources apportionment of dissolved inorganic nitrogen wet deposition at different land-use sites in the Three Gorges reservoir area

To identify seasonal fluxes and sources of dissolved inorganic nitrogen (DIN) wet deposition, concentrations and δ¹⁵N signatures of nitrate (NO₃^-) and ammonium (NH₄⁺) in wet precipitation were measured at four typical land-use types in the Three Gorges reservoir (TGR) area of southwest China for a one-year period. Higher DIN fluxes were recorded in spring and summer and their total fluxes (averaged 7.58 kg N ha^-1) were similar to the critical loads in aquatic ecosystems. Significant differences of precipitation δ¹⁵N were observed for NH₄⁺-N between town and wetland sites in spring and between urban and rural sites in summer. For NO₃^--N, significant differences of precipitation δ¹⁵N were observed between town and rural sites in spring and between urban and town sites in autumn, respectively. Quantitative results of NO₃^--N sources showed that both biomass burning and coal combustion had higher fluxes at the urban site especially in winter (0.18 ± 0.09 and 0.19 ± 0.08 kg N ha^-1), which were about three times higher than those at the town site. A similar finding was observed for soil emission and vehicle exhausts in winter. On the whole, DIN wet deposition averaged at 12.13 kg N ha^-1 yr^-1 with the urban site as the hotspot (17.50 kg N ha^-1 yr^-1) and regional NO₃^--N fluxes had a seasonal pattern with minimum values in winter. The contribution to NO₃^--N wet deposition from biomass burning was 26.1 ± 14.1%, which is the second dominant factor lower than coal combustion (26.5 ± 12.6%) in the TGR area during spring and summer. Hence N emission reduction from biomass burning, coal combustion and vehicle exhausts should be strengthened especially in spring and summer to effectively manage DIN pollution for the sustainable development in TGR area.