[Comparison Between Atmospheric Wet-only and Bulk Nitrogen Depositions at Two Sites in Subtropical China]

Atmospheric emissions of reactive nitrogen (N) species are at high levels and have caused high N deposition in China in recent years. In this study, atmospheric wet-only and bulk N depositions were monitored simultaneously in a two-year study at an agricultural site (HN) and a forest site (XS) in the Jinjing River catchment in Changsha County, Hunan Province in subtropical China. The differences in concentration and deposition of NH₄⁺-N, NO₃^--N, DON, and TN between wet-only and bulk N depositions were compared, and the correlation between wet-only and bulk N depositions was analyzed, with the aim of estimating atmospheric wet N deposition based on bulk N deposition. During the monitoring period, NH₄⁺-N was the dominant species in both wet-only and bulk deposition at the sampling sites. The average values of total N (TN) depositions for wet-only and bulk depositions at HN were 26.2 and 28.9 kg·(hm²·a)^-1, respectively. The proportions of NH₄⁺-N, NO₃^--N, and DON in TN in wet-only deposition were 49.7%, 31.3%, and 19.0%, respectively, while the proportions in the bulk deposition were 48.7%, 31.6%, and 19.7%, respectively. The average values of TN depositions for wet-only and bulk depositions at XS were 23.6 and 27.8 kg·(hm²·a)^-1, respectively. The proportions of NH₄⁺-N, NO₃^--N, and DON in TN in wet-only deposition were 53.9%, 34.78%, and 11.4%, respectively, while they were 49.6%, 31.6%, and 18.9%, respectively, for bulk deposition. The concentrations of N species in wet-only and bulk depositions were significantly and negatively correlated with precipitation, while the amount of N deposition was significantly and positively correlated with precipitation. The concentrations of N species in wet-only deposition had a significant linear correlation with those in the bulk deposition at the two sites (R²>0.82). According to the regression equation for wet-only and bulk N deposition at the monitoring sites, the proportions of NH₄⁺-N, NO₃^--N, and TN in wet-only to bulk deposition were 0.875, 0.774, and 0.852, respectively, at HN and 0.859, 0.783, and 0.819, respectively, at XS. These values were mainly related to the amount of wet-only N deposition and the pollution level of atmospheric particulate N species at the monitoring sites. In the subtropical region of China, atmospheric wet N deposition can be overestimated by 10% to 18% when the atmospheric bulk N deposition is used to replace the wet N deposition. Based on the regression equation between atmospheric bulk N deposition and wet N deposition, the atmospheric wet N deposition can be estimated well using the atmospheric bulk N deposition data.