Alpine and subalpine ecosystems are nitrogen-limited environments that are highly sensitive to subtle perturbations in reactive nitrogen (Nr) availability. The demonstrated effects of excessive Nr in these environments include shifts in species composition with overall losses in biodiversity, surface water and soil acidification, and additional changes in soil chemistry (e.g., base cation leaching, accumulation of toxic soluble metals). Atmospheric deposition of nitrate (NO3-) and ammonium (NH4+) has been implicated as an important mechanism of Nr transport to remote alpine watersheds, with winter wet and dry deposition accumulation in the snowpack acting as a reservoir of Nr that is subsequently supplied to surface waters upon snowmelt during the growing season. In this study, NO3- stable isotopic data (δ15N, δ18O, ∆17O) will be used to investigate the sources of nitrate in montane lakes, transient and permanent streams, and lower elevation catchments in the Sangre de Cristo Mountains of New Mexico (USA), the southernmost subrange of the Rocky Mountains in North America. In addition to the use of δ15N and δ18O data for partitioning sources of nitrate (e.g., soil NH4+ nitrification, fossil fuel combustion, biomass burning), ∆17O offers unique insight to the post-depositional fate of NO3-. NO3- in the atmosphere primarily derives from the oxidation of nitrogen oxides (i.e., NOx) by ozone (O3), which is enriched in 17O and imparts a largely positive ∆17O value to the resulting NO3- formed via NOx oxidation. The typical ∆17O for NO3- formed in the atmosphere (20-35%) is easily distinguishable from NO3- formed via nitrification (∆17O = 0%) and remains unaltered following deposition and subsequent processing in the environment. NH4+ and dissolved organic nitrogen (DON) stable isotopic data (δ15N) will also be obtained to further constrain the composition of Nr in this system and its origin. Snow, lake and riverine surface water, and soil samples were collected in June and September 2024 to capture the Nr dynamics near the beginning and end of the region's growing season, respectively. NO3-, NH4+, and DON concentrations and isotopic data will be used to infer the processes regulating Nr availability throughout this previously uncharacterized watershed. The results of this study will offer further insight to the extent of nitrogen deposition in alpine and subalpine ecosystems and the fate of atmospheric Nr in these environments, with possible implications for developing protective and mitigative strategies to combat excessive Nr emissions near sensitive montane ecosystems.

1. Department of Physical and Environmental Science, Texas A&M University - Corpus Christi

* Corresponding Author: agirard@islander.tamucc.edu