Differential Responses of Lodgepole Pine and White Spruce to Fertilization in Reconstructed Boreal Forest Soils in the Oil Sands Region.

Low nitrogen (N) availability may affect lodgepole pine (Pinus contorta, Pl, planted on soils with tailings sand as a substrate) and white spruce (Picea glauca, Sw, planted on soils with overburden material as a substrate) growth in reconstructed boreal forest soils in the Athabasca oil sands region in Alberta, Canada. The objective of this study was to investigate the effect of fertilization on foliar nutrient concentrations and tree growth. Three sites with poor tree growth were selected for each tree species and five trees in each site received N (N, 200 kg N ha^-1, applied as urea) or complete fertilizer (Complete, 15-30-15-micronutrients, at a rate of 200 kg N, 20 kg P and 20 kg K per hectare, urea was used to compensate the additional N) or no fertilizer (Control) in June 2012 in a single-tree fertilization study. After two growing seasons, both fertilization treatments similarly increased the height, diameter at breast height and aboveground biomass growth of Sw (p = 0.005, 0.009 and 0.015, respectively). In contrast, fertilization did not affect Pl growth. Foliar N concentration and content in current-year needle of Sw were higher in the two fertilization than in the Control treatment in 2012 (p < 0.001 for both), but not in 2013. Foliar δ¹⁵N in current-year and 1-year-old needles increased in the fertilization as compared with the Control treatment for Sw (p < 0.001 for both), corresponding to the increase of δ¹⁵N in soil NO₃^--N. Foliar K concentration and content in current-year needles of Sw were higher in the Complete than in the Control and N treatments in both 2012 (p = 0.007 and < 0.001, respectively) and 2013 (p = 0.026 and 0.032, respectively). No treatment effects on foliar micronutrients (Fe, Mn, Cu, Zn, B and Mo) or soil NH₄⁺-N, NO₃^--N and dissolved organic N were found in both Pl and Sw sites. We conclude that N was the limiting factor for tree growth in Sw but not in Pl sites and N fertilization should be used to improve Sw growth in the reclaimed oil sands sites.