Nitrogen additions to pristine, high-latitude, forest ecosystems: consequences for soil nitrogen transformations and retention in mid and late succession

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 . 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 NO 3 –N in 20–30 cm mineral cores increased within the first year following N additions, indicating that leaching of NO 3 –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 NO 3 –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 (NH þ 4 –N: NO 3 –N = 18–30), might be poorly adapted or physiologically unable to assimilate significant quantities of NO 3 .