Anthropogenic N deposition and the fate of 15 NO 3??? in a northern hardwood ecosystem

Human activity has substantially increased atmospheric NO 3??? deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic NO 3??? deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5???7 years). Although microbial immobilization is an initial sink (i.e., within 1???2 days) for anthropogenic NO 3??? in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic NO 3??? additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of 15 NO 3??? in plots receiving chronic NO 3??? addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m???2 year???1) and three plots which receive ambient plus experimental N deposition (3.0 g NO 3??? -N m???2 year???1). Chronic NO 3??? deposition significantly increased the N concentration and content (g N/m 2) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO 3??? deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of 15 N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic NO 3??? over a 1 year duration. Our results indicate that anthropogenic NO 3??? initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by ove