Anthropogenic nitrogen (N) emissions have increased significantly through the 20th century, and the subsequent deposition can have negative effects on terrestrial and aquatic ecosystems. Rates and patterns of N deposition can be highly localized and difficult to predict. Measurement networks are of limited use in many ecological studies or for reconstructing patterns of N deposition because they are fairly recent and sites are often located >100km apart. Here we use naturally occurring lichens as indicators of N deposition. Strong correlations are observed between amounts of N deposition and lichen N content. Furthermore, the N isotope composition of lichens can inform source attribution. We use this background to evaluate how N deposition changed over the last century throughout the western United States.
Four species of herbaria lichen specimens were analyzed for their elemental N content and stable isotope composition (δ15N). Specimens represent collections from throughout the western United States over the last 125 years. We predicted that lichen N content reflects increases in N emissions over time, lichen δ15N declines in areas downwind of agriculture because of N emissions with low δ15N, and lichen δ15N increases with urban development in areas downwind of urban areas where fossil fuel combustion has N emissions with high δ15N.
We observed species-specific responses among the lichens to shifting deposition patterns across the different regions. Three of the four species had increased N content across most regions from the early 20th century until roughly 1990 when the Clean Air Act Amendments (CAAA) required reductions in N emissions. Two common patterns were observed in lichen δ15N across species and regions. One pattern was a consistent decrease in δ15N through the 20th century, and a second was a decrease in δ15N until the 1980s when lichen δ15N began to increase. Decreased lichen δ15N likely relates to the expansion and intensification of agriculture throughout the west. Fertilizer N emissions and N emissions from livestock waste have low δ15N . The increase in δ15N in the second pattern may relate to major regulatory control measures enacted after the CAAA of 1970 and 1977 and the broad implementation of catalytic converters in the 1970s.
Lichens are natural recorders of N deposition because they provide information on sources and rates of N deposition. There is potential for lichens to provide supplemental information on air quality impacts over a broader region and across longer timespans than possible with direct measurement networks.