We hypothesized that differences in microbial and plant N demand in balsam poplar and white spruce stands would control in situ net N transformation and retention following N additions. Throughout the study, N fertilizer (NH4NO3) was added in three increments during the growing season, giving an annual N addition of 100 kg ha-1 yr-1. In balsam poplar, fertilization induced a large (~285%) increase in annual net nitrification but tended to reduce net ammonification. In white spruce, fertilization generally stimulated net N mineralization (via higher net ammonification) while net nitrification increased only slightly or remained unchanged. For 0-20 cm soil cores of both stand types, fertilization rapidly increased extractable DIN pools; however, the absolute amount of this increase was significantly larger in white spruce than in balsam poplar. In both stands, extractable NO3 - in 20-30 cm mineral cores increased within the first year following N additions, indicating that leaching of NO3- -N was fairly rapid. Fertilization did not significantly alter microbial biomass N or C. After four years of fertilizer additions there were slight but insignificant changes in fine-root C:N and % N. The immediate alteration of N transformation rates and extractable DIN pools, notably the higher NO3- -N at the 20-30 cm depth, may indicate that this ecosystem is sensitive to atmospheric N deposition. However, we also theorize that plants and microbes in this ecosystem, in which the extractable DIN pool is dominated by NH4+(NH4+ -N: NO3- -N = 18-30), might be poorly adapted or physiologically unable to assimilate significant quantities of NO3-.