Bridging atmospheric and terrestrial indicators of nitrogen deposition from a coal-fired power plant

Background/Question/Methods

In the western U.S., current atmospheric reactive nitrogen (N) deposition is nearly 20 times that of pre-industrial levels. In areas with naturally occurring low N levels, any increase in N deposition can facilitate severe ecosystem alterations resulting in biodiversity loss and ecosystem decline. Long-range transport of atmospheric N pollution from point sources such as coal-fired power plants may elicit detectable shifts in nutrient cycling regimes in protected areas across the Southwest such as the Paria Plateau, in northeastern Arizona, USA. The atmospheric point source emission model, CALPUFF, predicts atmospheric N deposition across the Paria Plateau from the nearby coal-fired power plant, the Navajo Generating Station (NGS), to range from < 0.01- > 0.5 kg N ha^-1yr^-1; however, the terrestrial impacts across the predicted N deposition range are not known. This study examines air, soil, and vegetation along a distance gradient from the NGS. Atmospheric NOx, soil N, pinyon pine allometry and δ¹⁵N content of both soils and pinyon pine were collected across the plateau at a distance gradient from the NGS.

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Results/Conclusions

We found distance from the NGS explained over 80% of the variation in measured atmospheric NOx across the Paria Plateau (r ²= 0.86). We also found correlations between soils and vegetation and CALPUFF predicted N deposition.  Both soil and foliar δ¹⁵N signatures were more positive in areas that received higher levels of N deposition (+3.00 ‰ to +10.32 ‰ and -3.1 ‰ to +9.7 ‰, for soil and foliar samples, respectively). The more positive δ¹⁵N signatures correspond to known ranges of δ¹⁵N signatures emitted from coal- fired power plants; this supports the use of isotopic signatures to provide insights into pollution sources and pathways in the environment. Also, pinyon pine allometric variables including branch volume/ needle area (cm) and branch volume/needle mass (cm³/g) showed indications of increased needle production in areas of higher NGS- derived N deposition. Our results suggest a correlation between atmospheric, soil, and vegetation nutrients and CALPUFF predicted N deposition patterns from the NGS. Identifying indicators of N deposition at the ground level bridges atmospheric models with shifts in terrestrial nutrient patterns to then begin to understand the impact of N enrichment in the ecologically sensitive semi-arid Southwest.