It is well established that chronic nitrogen (N) deposition can negatively impact plant-species biodiversity; as such, there is concern that anthropogenic N emissions from the Athabasca Oil Sands Region (AOSR) are impacting surrounding habitats. The objectives of this study were to determine the relative importance of N as a driver of plant species community composition and to identify biodiversity-based thresholds for atmospheric deposition. This assessment was performed on 46 Jack pine dominant forest sites surrounding the AOSR spanning and 35 environmental gradient variables. Soil chemical variables accounted for > 26% of the total explainable variation in the dataset, followed by climate (19%) and deposition variables (5%); the joint-effects between variables also explained a significant portion of the total variation (p < 0.001; redundancy analysis). Total deposited nitrogen (TDN) and sulphur were identified as important variables in gradient forest analysis. A single, definitive threshold across TDN was identified at approximately 5.6 kg N ha^–1yr^–1 (while a TDS threshold was found at 14.4 kg S ha^–1yr^–1). The TDN threshold range was associated primarily with changepoints for several vascular species (Pyrola asarifolia, Pyrola chlorantha, Cornus canadensis, Arctostaphylos uva-ursi), in addition to some bryophyte and lichen species (Pleurozium schreberi, Vulpicida pinastri and Dicranum polysetum), several of which are known indicators for N deposition. These results suggest that across N-sensitive Jack pine forests surrounding the AOSR, the biodiversity-based empirical critical load of nutrient N is 5.6 kg N ha^–1yr^–1.