Wet and Dry Nitrogen Depositions in the Pearl River Delta, South China: Observations at Three Typical Sites With an Emphasis on Water-Soluble Organic Nitrogen

Inorganic nitrogen (N) deposition in China is among the highest in the world, yet organic N deposition in the country has not been well constrained. In this study, wet and dry depositions of both organic and inorganic N were observed for 2 years at three contrasting sites (urban-rural-forest) in the Pearl River Delta (PRD) region, South China. Determined annual total dissolved N (TDN) deposition rates were 39.8, 33.8, and 52.0 kg N ha⁻¹ year⁻¹ at the urban, rural, and forest sites, respectively. The contributions of NO₃ ⁻-N, NH₄ ⁺-N, and water-soluble organic N (WSON) to total N deposition were 26.7–37.8%, 34.6–40.9%, and 26.1-32.3%, respectively. Wet N deposition accounted for about 54–68% in total N deposition at the sites. It is worth noting that the deposition rates of WSON in the PRD were among the highest in developed regions in the world. In wet depositions, the concentrations and proportions of WSON were significantly higher during the harvest seasons, especially at the non-urban sites, mainly due to enhanced biomass burning in the period. In dry deposition, the seasonal pattern of WSON was inconsistent with that of NO₃ ⁻-N or NH₄ ⁺-N. Biological/soil organic N might be the important sources of WSON in dry deposition. Our results suggest that WSON contributed significantly to atmospheric N deposition in the PRD and more attentions should be paid to the WSON to get a true picture of N depositions and its impacts on the ecosystem.