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_dep) can disrupt ecosystem dynamics, yet its spatial distribution and chemical composition in mountainous regions remains poorly understood. Accurate measurements of N_dep 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_dep 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_dep amounts and chemistry at North Cascades National Park Service Complex ('North Cascades') and Olympic National Park ('Olympic'). Lichen %N indicate rates of N_dep below N critical loads; our results contrast with air quality models that predict critical load exceedance in many areas. Lichen δ¹⁵N indicated a dominance of reduced N in N_dep suggesting significant contributions from NH_x emission sources. Lowland areas adjacent to urban corridors exhibited the highest proportions of reduced N in N_dep. Current policies target oxidized N from fossil fuel combustion, but not reduced N, causing an increase in the reduced:oxidized ratio in N_dep. 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_dep and deposition chemistry, particularly in ecologically sensitive mountainous regions.