Eleven years of simulated deposition of nitrogen but not sulfur changed species composition and diversity in the herb stratum in a boreal forest in western Canada

Oil sands mining activities in northern Alberta, Canada emit large amounts of nitrogen (N) and sulfur (S) oxides to the atmosphere, increasing N and S deposition. We studied the long-term (2006–2016) effect of elevated concentrations of simulated N and S deposition on soil properties and understory species composition in a mixedwood boreal forest in a two (0 and 30 kg N ha−1 year−1, as ammonium nitrate) × two (0 and 30 kg S ha−1 year−1, as sodium sulfate) factorial experiment. Soil (forest floor and 0–15 cm mineral soil) and understory vegetation samples were collected and the cover of understory vegetation was determined in August 2016. Eleven years of N deposition increased (p = .045) total N concentration and decreased (p < .10 unless otherwise indicated) carbon to N ratio by 11 and 7%, respectively, in the forest floor. Sulfur deposition decreased (p = .045) exchangeable calcium concentration by 36% in the mineral soil. Species evenness (by 7%) and the overall diversity (by 7%) were decreased and community composition was changed (p = .008) in the herb stratum by N but not by S deposition, due to species-specific responses to N deposition. However, elevated concentrations of N and S deposition did not change species diversity and composition in the shrub stratum. Decreased foliar phosphorus and potassium concentrations and increased N to phosphorus ratio in some species indicate a potential risk of nutrient imbalance by N deposition. Reducing N emission to minimize its negative effect on boreal forest ecosystems should be a priority in emissions management in the oil sands.