Anthropogenic N deposition and the fate of 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 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 ) and three plots which receive ambient plus experimental N deposition (3.0 g NO3 -N m 2 year ). Chronic NO 3 deposition significantly increased the N concentration and content (g N/m ) 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 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 overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Shortand long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which