Nitrogen deposition (Ndep) from anthropogenic emissions are increasing Ndep in the Greater Yellowstone Ecosystem (GYE), which can lead to N saturation, altering water quality, biogeochemical cycling and biodiversity. Increased emissions are largely from local and regional intensification of agricultural activity, which primarily releases ammonia (NHx), and transportation and industrial processes, which release primarily nitrogen oxides (NOx). Wildfires and soil emissions likely also contribute 10-15% of NOx and NHx in the GYE. The climate, topography, and sources of N emissions in the region potentially create heterogeneous patterns of Ndep in the GYE. Epiphytic lichens obtain their nutrients from the air and record Ndep. Additionally, δ15N values of lichen biomass may help to discern N sources, because NOx derived δ15N tends to be high (-5 to +20‰ in precipitation) while that of agricultural derived NHx tends to be comparatively low (-8 to -14‰ measured in lichens). The objectives of our research were to improve our understanding of lichens as a monitoring tool for air quality and determine spatial patterns and sources of Ndep across the GYE. We collected 252 lichen samples (Usnea lapponica and Letharia vulpina) from across the GYE and analyzed them for %N and δ15N.
Lichen %N was highest in western areas of the GYE (p=0.04), a region with high agricultural emissions of N. %N in lichens decreased from 1.77 ± 0.43% 0.5km from the western border of protected area in the GYE, where the Caribou-Targhee National Forest meets agricultural land near Driggs, Idaho, to 1.22 ± 0.28% 56km from the western border of the GYE. Additionally, %N increased with elevation (p < 0.01) at two locations, one near agricultural land in the western GYE and one near Jackson, WY, which may be more heavily impacted by vehicles, industrial processes, and oil drilling. These deposition patterns likely occur because Ndep is higher near major sources and Ndep increases with elevation because more precipitation increases wet Ndep. Additionally, the mean δ15N value was -11.8 ± 3.2‰, which suggests an agricultural source of Ndep, but δ15N values increased with higher %N, which indicated sites with high deposition were receiving more N from combustion sources. Lichens are useful to discern areas of higher Ndep and general sources, however, variation in %N and d15N of lichens collected at a single site warrant further investigation on how microhabitat and biological activity influence incorporation of bulk Ndep and 15N signals.