The forest canopy is an important site for nutrient deposition, uptake and transformation, as well as a habitat for microorganisms. Wet and dry deposition of nitrogen and other elements represent a substantial nutrient input into the forest ecosystem. The concentration of ammonium (NH₄) and nitrate (NO₃) in throughfall (TF; precipitation that passes through the forest canopy), has been found to be lower than the concentration of those nutrients in incident precipitation in boreal conifer forests, indicating the uptake of these nutrients in the canopy. The role of canopy microorganisms in this phenomenon remains largely unexplored. The objective of this study was to investigate the relationships between NH₄, NO₃, and dissolved organic nitrogen (DON) in TF and the structure of microbial communities in two black spruce (Picea mariana (Mill) BSP) plantations (aged 15 and 45 years old) in northern Ontario. Preliminary results have shown a seasonal trend in the most probable number (MPN) of bacteria and fungi associated with live tree needles; for example the MPN was found to be higher in August than in July or October in the younger plantation (p<0.05). A seasonal trend was also found in the concentration of NH₄ and NO₃ in TF; in the older plantation, there was a significantly higher concentration of NH₄ in TF collected in August than in October (p<0.05). Forthcoming results will serve to further characterize the microbial community found in these P. mariana canopies and quantify the contribution of canopy microorganisms to the changes in concentrations of NH₄, NO₃, and DON.