Wet deposition of sulfur and nitrogen in the Jiuzhaigou World Heritage Site, China: Spatial variations, 2010-2022 trends, and implications for karst ecosystem conservation

Excessive sulfur and nitrogen deposition is a threat to natural ecosystems. This study investigated the spatio-temporal changes of wet sulfur and nitrogen deposition in Jiuzhaigou to understand the responses of wet deposition chemistry to local and national air pollutant emission changes and to assess relevant ecological risks. The results show that local emission sources (e.g., tourism, post-earthquake reconstruction, and vegetation damages) largely elevated the concentrations of SO42-, NO3-, NH4+, and Ca2+ at the non-background sites during 2021-2022; However, the gaps of annual sulfur and nitrogen wet deposition fluxes among the background, semi-background, and non-background sites were reduced by annual precipitation. The pH was also higher at the non-background sites due to stronger acid neutralization capacities from crustal dust there, and acid precipitation (pH < 5.6) was not detected at all the sites. During the 2010 to 2022 period, the wet deposition of SO42- and TIN (i.e. total inorganic nitrogen) and the equivalent ratio of SO42-: NO3- generally presented a declining trend at the background and semi-background sites, as a result of regional and national emission reductions. The TIN reduction from 2015 to 2016 to 2021-2022 at the background site was more from wet NH4+ deposition and the site's NO3- concentrations and fluxes were relatively stable. Compared to the critical loads (CLs) of sulfur and nitrogen for acidification and eutrophication, the annual wet and wet+dry deposition fluxes of sulfur and nitrogen were lower in almost the entire watershed, except for the CLmin of nitrogen for acidification in considerable areas. However, all the wet deposition samples were calcite-unsaturated and had annual TIN concentrations higher than the national environmental standard of total nitrogen for surface water. More field investigations are needed to better assess the ecological risks.