Assessing nitrogen deposition and chemistry in mountainous regions of Olympic National Park and North Cascades National Park Service Complex, Pacific Northwest, USA.

Anthropogenic nitrogen (N) deposition (N) can disrupt ecosystem dynamics, yet its spatial distribution and chemical composition in mountainous regions remains poorly understood. Accurate measurements of N are needed to determine N critical loads and where deposition levels are expected to cause harm to ecosystem function. Epiphytic lichens can provide direct estimates of N and insight into emission sources for remote mountainous areas where instrumented monitoring sites are not feasible and air quality models have a high degree of uncertainty. We used lichens to assess patterns of N amounts and chemistry at North Cascades National Park Service Complex ('North Cascades') and Olympic National Park ('Olympic'). Lichen %N indicate rates of N below N critical loads; our results contrast with air quality models that predict critical load exceedance in many areas. Lichen δN indicated a dominance of reduced N in N suggesting significant contributions from NH emission sources. Lowland areas adjacent to urban corridors exhibited the highest proportions of reduced N in N. Current policies target oxidized N from fossil fuel combustion, but not reduced N, causing an increase in the reduced:oxidized ratio in N. Species-specific responses to these changes could alter biodiversity, community structure, and ecosystem processes over time. Our study underscores the importance of fine-scale assessments of N and deposition chemistry, particularly in ecologically sensitive mountainous regions.