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

Abstract

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 represent an important component of N cycling in 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 in a three-year period (2018–2020) in a LTER site (A. Mosso Scientific Institute) in NW Italian Alps at an altitude of 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 the summer, whereas the lowest ones (5.8 and 3.6 μeq L−1, respectively) in 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 potential detrimental consequences for the aquatic ecosystems. The contribution of rain to inorganic N varies between 39 and 68 %, which highlights 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 summer months, reflecting influence from vehicle emissions likely combined with an increase in emissions from agricultural sources, when the air masses came 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 the 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 over the tree line in the European scenario.