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Biogeochemical responses to multiyear above-canopy applications of nitrogen at a jack pine (Pinus banksiana) forest in northern Alberta, Canada

Elevated atmospheric nitrogen (N) deposition can alter forest biogeochemistry leading to adverse impacts on terrestrial ecosystems. Experimental studies often apply N to the forest understorey at greatly elevated loads, bypassing important canopy interactions. Using a narrow N deposition gradient (0, 5, 10, 15, 20, and 25 kg N·ha−1·year−1), we measured changes in jack pine ( Pinus banksiana Lamb.) forest biogeochemistry in the bituminous sands region of northern Alberta, Canada after 5 years of above-canopy N additions. The canopy intercepted approximately 46% of applied N across all treatments, but during the final year, N interception was reduced to 7% in the highest treatment. Nitrogen concentrations in nonvascular organisms increased with treatment and N was also immobilized in decomposing litter in the highest treatment. Otherwise, vascular plant biomass, foliar chemistry, and soil processes exhibited no relationship with treatment over 5 years. This work suggests that jack pine forests in the region have a high capacity to immobilize N inputs over the short-term (5 years), which restricts other biogeochemical responses traditionally associated with elevated N deposition.