Anthropogenic N Deposition and the Fate of 15NO3- in a Northern Hardwood Ecosystem

Human activity has substantially increased atmospheric NO3- 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 NO3- 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 NO3- 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 NO3- additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of 15NO3- in plots receiving chronic NO3- 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 NO3--N m-2 year-1). Chronic NO3- deposition significantly increased the N concentration and content (g N/m2) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO3- deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of 15N 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 NO3- over a 1 year duration. Our results indicate that anthropogenic NO3- 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. Short- and 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 they operate.