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

Long-range atmospheric transport of pollutants threatens the pristine high-elevation environments, highly sensitive to environmental changes as the global increase of nitrogen (N) emissions. Atmospheric depositions represent an important component of N cycling since its role in transferring reactive nitrogen species from the atmosphere to terrestrial and aquatic compartments. To date, monitoring of atmospheric deposition in Alpine tundra are lacking. To reduce this gap, we present the chemical and isotopic composition (d15N, d18O in nitrate) of rain and snow deposition collected in a three-year period (2018-2020) in a LTER site in NW Italian Alps at an altitude of 2901 m a.s.l. Surface meteorological data coupled with air mass back trajectory analysis were used to obtain insights on the influence of hotspot pollution areas (e.g. Po Valley) on the N deposition at the study site. The highest annual volume-weighted mean (VWM) concentrations of oxidized and reduced N (18 and 22 µeq/L, respectively) occurred in the summer. Conversely, the low annual VWM concentration of NO3 and NH4 recorded in the snow season reflected a typical high-elevation background air quality. Mountain-valley breeze, typical of warmer season, promote the vertical mixing in the mountain atmosphere and expose the study site to the N-enriched air masses coming from lowland anthropized areas. The isotopic composition of wet deposition showed seasonal variation with lower d15N-NO3 in summer months reflecting vehicle emissions and likely combined with an increase in emissions from agricultural sources, when the air masses came from the Po Valley. The estimate of N wet deposition load, greatly exceeding the N critical threshold, revealed that the studied high-elevation site is exposed to excessive N input through atmospheric deposition with potential detrimental consequences for the aquatic ecosystems. The reported data suite represents, to date, the first documentation for ecosystems over the tree line in the European scenario.