The fate of 15N-labeled ammonium and nitrate applied on trees canopy in a mature balsam-fir stand, Quebec

Canopy uptake of atmospherically deposited nitrogen (N) in N-limited boreal forests may represent an important source of N for growth. A tracer study was conducted by repeated applications of artificial precipitation with ambient inorganic N concentration (two treatments: 15N-NH4NO3 and NH415N-NO3, 98 at.% with three trees per treatment) misted directly on the crown of individual mature balsam fir trees (Québec, Canada) during two growing seasons. Control trees were used to determine natural 15N abundance values. At the end of the experiment, mature trees' needles and twigs, epiphytic lichens growing on their stem and branches, understory balsam fir seedlings' needles and branches, and litter layer (L), showed significant 15N enrichments for both treatments as compared to control trees, while there were no differences for understory balsam fir seedlings' stem and roots, soil FH and B horizons. Following NH4 and NO3-tracer addition, except for mature trees' needles that showed significantly higher enrichment for 15NH4+ relative to 15NO3-, in all other compartments enrichment was similar for both ions. Because of the way the tracer was added, i.e. as a fine mist on an individual tree basis, a significant amount was lost. When taking into consideration total N biomass of each compartment, total tracer recovery was estimated to be between 25.8 and 53.2% for 15NH4+ and 39.6 and 81.5% for 15NO3-. Calculating the tracer repartition only for the amount recovered showed that most of the added 15NH4+ and 15NO3- was found in aboveground vegetation (64.3 and 70.7%, respectively) as compared to the litter layer (L) and soil organic horizons (FH) (35.7 and 29.4%, respectively). Overall, twigs showed the highest recovery for both ions (49.9 and 63.0%, respectively). Contrary to studies where N is added directly to the soils, these data show that aboveground biomass may capture N in amounts comparable to or higher than the soil when the tracer is applied in situ directly on the canopy in a way that reproduces natural N deposition processes