Nitrogen deposition sources and patterns in the Greater Yellowstone Ecosystem determined from ion exchange resin collectors, lichens, and isotopes.

Over the past century, atmospheric nitrogen deposition (Ndep) has increased across the western United States due to agricultural and urban development, resulting in degraded ecosystem quality. Regional patterns of Ndep are often estimated by coupling direct measurements from large-scale monitoring networks and atmospheric chemistry models, but such efforts can be problematic in the western US because of complex terrain and sparse sampling. This study aimed not only to understand Ndep patterns in mountainous ecosystems but also to investigate whether isotope values of lichens and throughfall deposition can be used to determine Ndep sources, and serve as an additional tool in ecosystem health assessments. We measured Ndep amounts and δ15N in montane conifer forests of the Greater Yellowstone Ecosystem using canopy throughfall and bulk monitors and lichens. In addition, we examined patterns of C:N ratios in lichens as a possible indicator of lichen physiological condition. The isotopic signature of δ15N of Ndep helps to discern emission sources, because δ15N of NOx from combustion tends to be high (-5 to +25‰) while NHx from agricultural sources tends to be comparatively low (-40 to -10‰). Summertime Ndep increased with elevation and ranged from 0.26 to 1.66 kg ha-1. Ndep was higher than expected in remote areas. The δ15N values of lichens were typically -15.3 to -10‰ suggesting agriculture as a primary emission source of deposition. Lichen %N, δ15N and C:N ratios can provide important information about Ndep sources and patterns over small spatial scales in complex terrain.