Anthropogenic N deposition and the fate of 15NO 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 ¹⁵NO 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 NO₃ ⁻-N m⁻² year⁻¹). Chronic NO ₃ ⁻ 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 NO ₃ ⁻ 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 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. 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.