Characterization of atmospheric nitrogen wet deposition fluxes and their isotope spectra in typical urban functional zones

With rapid economic development, human activities have led to substantial nitrogen (N) emissions, and the excessive N deposition has attracted widespread attention due to its negative ecological effects on urban areas. This study selected four typical functional areas in Wanzhou, Chongqing Municipality, China—industrial, commercial, residential, and agricultural areas—to provide insights into the flux and source characteristics of atmospheric N wet deposition. One year’s precipitation samples were collected, and then the fluxes of ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃⁻-N), and total dissolved nitrogen (TDN) were analyzed. The highest TDN wet deposition (28.34 kg·ha⁻¹·yr⁻¹) was observed in industrial areas, followed by commercial (19.44 kg·ha⁻¹·yr⁻¹), agricultural (17.67 kg·ha⁻¹·yr⁻¹), and residential areas (16.48 kg·ha⁻¹·yr⁻¹), exhibiting a clear spatial trend. The flux characteristics of NH₄⁺-N wet deposition were similar to those of TDN wet deposition, the highest values was observed in industrial areas (22.76 kg·ha⁻¹·yr⁻¹), followed by commercial areas (13.24 kg·ha⁻¹·yr⁻¹), agricultural areas (11.97 kg·ha⁻¹·yr⁻¹), and residential areas (10.11 kg·ha⁻¹·yr⁻¹), respectively. The NO₃⁻-N wet deposition fluxes were highest in the industrial area (5.86 kg·ha⁻¹·yr⁻¹), followed by commercial (5.46 kg·ha⁻¹·yr⁻¹), residential (4.33 kg·ha⁻¹·yr⁻¹), and agricultural areas (3.98 kg·ha⁻¹·yr⁻¹). The results of isotope modeling source analysis revealed that the highest contributor to NH₄⁺-N in the industrial area was coal combustion (32%), while biomass combustion had the lowest contribution (18%) compared to other sources. In the other three functional zones, NH₄⁺-N contributions were comparable, with the highest contributions originating from animal waste and fertilizers, while biomass combustion contributed the least. The biomass combustion was the primary source for NO₃⁻-N in the four functional areas. This study enriched the δ¹⁵N source spectrum data of typical functional zones and quantified the sources of N wet deposition in different functional zones. This information can provide a reference for N pollution control in specific functional areas.