Simulated N and S deposition affected soil chemistry and understory plant communities in a boreal forest in western Canada

Aims We conducted a simulated nitrogen(N)and sulfur(S)deposition experi-ment from 2006 to 2012 to answer the following questions:(i)does chronic N and S deposition decrease cation concentrations in the soil and foliage of understory plant species,and(ii)does chronic N and S deposition decrease plant diversity and alter species composition of the understory plant community in a boreal forest in western Canada where intensifying industrial activities are increasing N and S deposition?Methods Our field site was a mixedwood boreal forest stand located~100 km southeast of Fort McMurray,Alberta,Canada.the experiment involved a 2×2 factorial design,with two levels each of N(0 and 30 kg N ha−1 yr−1;applied as NH4NO3)and S addition(0 and 30 kg S ha−1 yr−1;applied as Na2SO4).Four blocks were established in July 2006,each with four plots of 20×20 m randomly assigned to the treatments.Soil and understory vegetation were sampled and cover(%)of individual species of herb(height≤0.5 m)and shrub(height 0.5-1 m)layers was determined in August 2012.Important Findings Seven years after the treatments began,N addition increased dissolved organic carbon and N in the mineral soil(P<0.05),whereas S add-ition decreased exchangeable cations(P<0.05)in the forest floor.In the shrub layer,species evenness,and overall diversity were decreased by N addition(P<0.05)due to increases in abundance of nitrophil-ous species and S addition(P<0.01)due to decreased cation con-centrations in soils.total shrub cover decreased with S addition(P<0.10).Nitrogen and S addition affected neither species richness nor evenness in the herb layer.However,permutational multivariate ana-lysis of variance and non-metric multidimensional scaling analyses(based on plant cover)indicated that the effect of N and S addition on understory plant species composition in the both shrub and herb layers was species-specific.Addition of N decreased foliar phosphorus and potassium concentrations in some species,suggesting potential risk of N-meditated nutrient imbalance in those species.Our results indicate that long-term elevated levels of N and S deposition can negatively impact plant nutrition and decrease the diversity of the understory plant community in boreal forests in northern Alberta,Canada.However,considering that the current N and S deposition rates in northern Alberta are much lower than the rates used in this study,N and S deposition should not negatively affect plant diversity in the near future.