Nitrogen atmospheric deposition in a high-altitude Alpine environment: A chemical and isotopic approach to investigate the influence from anthropized areas

Long-range transport of atmospheric pollutants threatens the pristine high-elevation environments, highly sensitive to environmental changes such as the global increase of nitrogen (N) emissions. Atmospheric depositions is an important component of N cycle transferring reactive nitrogen species from the atmosphere to terrestrial and aquatic compartments. Despite this, monitoring of atmospheric deposition in Alpine tundra regions is lacking. To reduce this gap, we present the chemical and isotopic composition (δ15N, δ18O in nitrate) of rain and snow deposition collected over a 3-year period (2018–2020) in a LTER site (A. Mosso Scientific Institute) in NW Italian Alps at 2901 m a.s.l. The highest annual volume-weighted mean (VWM) concentrations of oxidized and reduced N species (18 and 22 μeq L−1, respectively) occurred in summer, while the lowest (5.8 and 3.6 μeq L−1, respectively) occurred during the snow season. The N wet deposition load (3.9 kg ha−1 y−1.) greatly exceeded the N critical threshold, revealing that the area is exposed to excessive N input through atmospheric deposition with potentially detrimental consequences for aquatic ecosystems. The contribution of rain to inorganic N varied between 39 and 68 %, highlighting the importance of monitoring the composition of the rain component, in addition to the snowpack, even at high-altitude sites. The nitrate isotopic composition of wet deposition showed seasonal variation with lower δ15N–NO3 values (−10.6 ‰ to −2.2 ‰) in the summer months, reflecting the influence of vehicle emissions likely combined with an increase in emissions from agricultural sources, when the air masses originate from the Po Valley. Surface meteorological data coupled to the analysis of stable isotopes and air mass back trajectories showed that local mountain-valley breeze and the elevation of the continuous aerosol layer altitude in the warm season expose the study site to N-air pollutants originating from lowland and more distant anthropized areas (e.g., Po Valley). The reported data suite represents, to date, the first documentation for ecosystems above the tree line in the European scenario. [Display omitted] •The concentration of N species was higher in rain than in snow deposition.•The proportional contribution of rain to the total N deposition load was ∼50%.•N load exceeded the critical threshold for sensitive high-elevation ecosystems.•Thermally driven winds expose the Alps to polluted air masses from the Po Valley.•An