Anthropogenic N deposition and the fate of 15NO3- in a northern hardwood ecosystem

TitleAnthropogenic N deposition and the fate of 15NO3- in a northern hardwood ecosystem
Publication TypeJournal Article
Year of Publication2004
AuthorsZak DR, Pregitzer KS, Holmes WE, Burton AJames, Zogg GP

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 (5v7 years). Although microbial immobilization is an initial sink (i.e., within 1v2 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 gNm2 year1) and three plots which receive ambient plus experimental N deposition (3.0 gNO3-Nm2 year1). 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.