Abstract: Anthropogenic reactive nitrogen emissions in cities affect air quality and lead to acontinuous increase of atmospheric nitrogen input to surrounding terrestrial ecosystems. However, simultaneous observations of atmospheric reactive nitrogen in terrestrial ecosystems, especially dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON), are still lacking, affecting the comprehensive and accurate assessment of the ecological effects of nitrogen deposition. In this study, we weekly observed the concentrations of bulk ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N) and total dissolved nitrogen (TDN) at Dongling Mountain Forest Ecosystem Orientation Research Station in Beijing from June 2019 to January 2020, calculated the DON concentration and the deposition flux of various forms of nitrogen, and analyzed their intermonthly, dry and wet seasonal differences and the underlying mechanisms. The results showed that the volume-weighted average concentrations of NH₄⁺-N, NO₃^--N, DON, and TDN in atmospheric deposition were 1.45±0.04, 0.70±0.01, 1.81±0.66, and 3.96±0.65 mg N·L^-1, respectively. The annual deposition flux of TDN was 25.00 kg N·hm^-2·a^-1. The annual deposition fluxes NH₄⁺-N, NO₃^--N, DON and their proportions in TDN were 8.76 (35%), 4.38 (18%), and 11.86 (47%), respectively, indicating a high degree of nitrogen pollution at this site. The average values of DIN/DON and NH₄⁺-N/NO₃^--N in atmospheric deposition were 1.1 and 2.0, respectively, indicating that DON was an important form of atmospheric nitrogen input, and NH₄⁺-N was dominant in DIN input. The concentration and percentage in TDN of NO₃^--N decreased with increasing precipitation, indicating that the removal efficiency of NO₃^--N was higher and there was a dilution effect, while the concentration and percentage of NH₄⁺-N and DON had no significant correlation with precipitation. The bulk deposition flux of each form of nitrogen increased with   increasing precipitation, so the average concentration and deposition flux of each form of nitrogen in the wet season (June-August) were generally higher than those in the dry season (September-January), indicating that more precipitation in the future would drive the increase of atmospheric nitrogen input into the forest floor, which is of great significance for plant growth. This study comprehensively reports the bulk deposition fluxes of NH₄⁺-N, NO₃^--N, DON, and TDN in a typical forest of Beijing-Tianjin-Hebei region, clarifies their forms and differences between wet and dry season, and reveals the effects of precipitation on atmospheric nitrogen deposition flux in areas with high nitrogen deposition level. Our results enrich the forest atmospheric nitrogen input data in typical high anthropogenic nitrogen emission areas in China, and provide useful information for evaluating the ecological and environmental effects of anthropogenic nitrogen pollution.

Key words: forest ecosystem, NH₄⁺-N, NO₃^--N, dissolved organic nitrogen, deposition flux.