An assessment of nitrogen saturation in Pinus banksiana plots in the Athabasca Oil Sands Region, Alberta

During the past 15 years, there has been a dramatic increase in the amount of reactive nitrogen (N) in the atmosphere, leading to concerns that chronic elevated N deposition may result in negative effects on natural ecosystems. This study examines the response of jack pine (Pinus banksiana) plots to N air concentrations within the Athabasca Oil Sands Region (AOSR) in northern Alberta, which has experienced elevated N emissions since the 1990s. Air concentrations of nitrogen dioxide (NO2), ammonia, and nitric acid at the study plots are generally low although NO2 is strongly correlated with sulphur dioxide indicating an exposure gradient associated with industrial emissions. Nitrogen concentrations in P. banksiana foliage and two lichen indicator species (Hypogymnia physodes and Evernia mesomorpha) were significantly correlated with annual NO2 exposure. Relationships between NO2 (or N exposure) and other aspects of N cycling were less evident. Nitrogen content and carbon to nitrogen ratio in the forest floor and soil or potential net N mineralization rates were not correlated with N exposure. Nitrification was negligible suggesting efficient ecosystem immobilization of current N deposition. Based on the response of foliage to N exposure, sites closest to industrial activity appear to be in the early stages of N saturation.