A BIOLOGICAL PROXY FOR VEHICULAR NOX POLLUTION: ELEMENTAL AND ISOTOPIC ANALYSES OF LICHENS NEAR A MAJOR FREEWAY IN WESTERN WASHINGTON

Lichens are defined by the symbiosis between a fungus and a photosynthetic partner and derive the majority of their nutritional requirements from atmospheric sources. Through the unique growth patterns and physiological characteristics granted by their symbiosis, lichens are highly sensitive to changes in nutrient regimes. As such, lichens are excellent bioindicators of ecosystem health. Biogeochemical analyses of lichen tissue can be used to gain insight into the presence and relative magnitude of atmospheric nutrient sources, including anthropogenic pollutants. In this study, we used nitrogen and carbon elemental and isotopic analyses to monitor deposition of NO_x compounds from vehicular emissions on a common lichen (Parmelia sp.) near a major freeway in Tacoma, WA. We aimed to address the following questions: is N from NOx compounds incorporated into lichen tissue, and if so, how does the proportion of NOx-derived N change with proximity to a major freeway? To address these questions we analyzed lichen samples collected from maple tree trunks at different distances (30 - 125 meters) from the freeway. Other biogeochemical studies have investigated the attenuation of vehicular NOx compounds in higher plants moving away from freeways; however, isotopic and elemental analyses of lichens in relation to NO_x pollution remain poorly developed in the literature.

Lichens collected near the freeway contained twice the amount of nitrogen (2.1% by weight) as those collected far away (1.2% by weight). Weight % C/N ratios were significantly different for specimens near (19.2) and far (36.5) from the freeway. This pattern was driven by a significant difference in not only %N of lichen tissue, but also by a significant difference in weight %C between the two sampling groups (near: 40.5%; far: 42.1%). On average, we also found that lichen δ¹⁵N values were higher near the freeway, decreasing from 3.8 permil near the road, to -7.2 permil far from the road. These results suggest a greater contribution of a ¹⁵N enriched nitrogen source like NO_x near the freeway. Highway emissions are the largest source of atmospheric N pollutants in the US today, and the strong elemental and isotopic trends shown here suggest that in situ lichen N values can serve as a proxy for the magnitude of NO_x pollution in environments surrounding major freeways.