Microbial activities and gross nitrogen transformation unaffected by ten-year nitrogen and sulfur addition

Oil sands mining in northern Alberta, Canada, emits large amounts of NOx and SO2 to the atmosphere, which will eventually return to the surrounding forest ecosystems. This study was conducted to determine changes in microbial and enzyme activities, and gross nitrogen transformation rates in a boreal forest soil in response to 10 yr (2006-2015) of elevated levels of nitrogen (N) and sulfur (S) addition. The experiment had a two (0 vs. 30 kg N h-1 yr-1, as NH4NO3) × two (0 vs. 30 kg S ha-1 yr-1, as Na2SO4) factorial design with three blocks. A laboratory incubation experiment was conducted using forest floor and the mineral soil (0-15 cm). Ten years of elevated N and S additions did not affect soil chemical (pH, total C, total N and available N concentrations) and microbiological properties (microbial biomass C and N, soil respiration rate, and enzyme activities related to C and N cycling) and gross N transformation rates. Gross N mineralization (0.54-0.62 and 36-49 mg N kg-1 d-1 for mineral soil and forest floor, respectively) and gross NH4 + immobilization (0.39-0.57 and 10-19 mg N kg-1 d-1, respectively) rates were tightly coupled in both soil layers. Gross NO3 - immobilization rates (20-32 mg N kg-1 d-1) were significantly greater than gross nitrification rates (9-20 mg N kg-1 d-1) in the forest floor. Our results suggest that the studied boreal forest soil was resilient or resistant to 10 yr of N and S addition and the studied soils were still N limited. Given the current N and S emission and deposition rates in northern Alberta, the risk for N and S deposition to significantly affect gross N transformation rates is low for the studied forest ecosystem.