Indicators of vehicular emission inputs into semi-arid roadside ecosystems

The relative importance of pollution from non-point sources, such as vehicular emissions, is not well understood in sensitive semi-arid protected areas, such as Grand Canyon National Park (GCNP). Roadside N deposition could be substantial in areas with high visitation and traffic volume such as GCNP, although inputs of roadside pollution have not been quantified in GCNP. We used three potential indicators of N enrichment to evaluate impacts of vehicle emissions on roadside ecosystems in GCNP: 1) concentration of nitrogen oxides (NOx) captured by passive air samplers, 2) natural δ¹⁵N abundance in the foliage of mature Piñon pine trees and soil N, and 3) concentrations of available inorganic N (NO₃⁻ and NH₄⁺) in soil. Over an eight-month period, N enrichment was assessed at ten sites located across hypothesized gradients of vehicular N deposition based on distance from primary roadways and traffic levels in GCNP. Ogawa passive air samplers indicated that NOx levels were on average 52% higher at roadside locations compared to 30 m away, and were highest at the most heavily trafficked sites in the park. Natural abundance δ¹⁵N of foliage was significantly higher (F = 16.04, p = 0.007, n = 237) at roadside locations than at 15, and 30 m distances: a pattern that is consistent with local N inputs from vehicle emissions. Available inorganic soil N was significantly higher (F = 3.46, p = 0.013, n = 8) at the South Entrance roadside compared to areas with lower traffic densities. The direct measurement of atmospheric NOx using Ogawa samplers and δ¹⁵N of Piñon pine needle tissues were the best indicators of roadside N deposition. Our results also suggest that vehicular emissions on the South Rim of GCNP were incorporated in the plant nutrient pools near roadside environments. Although N emissions in GCNP comply with current annual air quality standards as specified by the Clean Air Act, it remains unclear whether the standards are low enough to sustain ecological integrity of native ecological communities along roadsides. Coupled with information on N-pollution tipping points and other ecological indicators, the metrics used here may prove to be a cost effective approach for monitoring roadside N-deposition and informing management decisions.