Nitrogen isotope composition of lichens traces a century of increased air pollution in the Greater Yellowstone Ecosystem from agricultural sources
Atmospheric nitrogen (N) deposition has increased significantly throughout the 20th century with consequences for ecosystems and their sensitive components such as lichens. Lichens have been used as indicators of air pollution for over 150 years, and strong relationships are often observed between amounts of N deposition and lichen N content. We use this knowledge of lichens as air pollution indicators to study how N deposition has changed over the last century in the Greater Yellowstone Ecosystem. Herbaria specimens (n=35) of the lichen Letharia vulpina from Yellowstone National Park and the surrounding area were subsampled and analyzed for elemental and isotope carbon and N composition. Herbaria specimens spanned collection dates from 1901 to 1996.
We found a significant increase in lichen N concentration over time (R2=0.134, p=0.03) corresponding with increased N deposition driven by increased N emissions from agriculture intensification and fossil fuel combustion. Concurrently, increased lichen N concentration corresponded with decreased lichen δ15N over time (R2=0.109, p=0.05). Decreases in lichen δ15N likely reflect the increase in agricultural production upwind of the Greater Yellowstone Ecosystem along the Snake River Plain. Agriculture N emissions have low δ15N values while other N emission sources such as fossil fuel emissions have relatively high δ15N values. Increases in lichen N concentration throughout the 20th Century corresponded to increased N deposition, and most of this increase can be attributed to increased agricultural production upwind, along the Snake River Plain, as indicated by a decrease in lichen δ15N. Even protected areas such as Yellowstone National Park have not been excluded from the anthropogenic influence through N deposition. Reducing any N impacts within the Park and greater ecosystem will require careful consideration of these diffuse agricultural N emission sources located upwind.