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.