|Title||Chronic Atmospheric NO3- Deposition Does Not Induce NO3- Use by Acer saccharum Marsh.|
|Publication Type||Journal Article|
|Year of Publication||2008|
|Authors||Eddy WC, Zak DR, Holmes WE, Pregitzer KS|
|Pagination||469 - 477|
The ability of an ecosystem to retain anthropogenic nitrogen (N) deposition is dependent upon plant and soil sinks for N, the strengths of which may be altered by chronic atmospheric N deposition. Sugar maple (Acer saccharum Marsh.), the dominant overstory tree in northern hardwood forests of the Lake States region, has a limited capacity to take up and assimilate NO3−. However, it is uncertain whether long-term exposure to NO3− deposition might induce NO3− uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO3− deposition (30 kg N ha−1 y−1) could induce NO3− uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO3− deposition. Kinetic parameters for NH4+ and NO3− uptake in fine roots, as well as leaf and root NO3− reductase activity, were measured under conditions of ambient and experimental NO3− deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO3− deposition did not alter the V max or K m for NO3− and NH4+ uptake nor did it influence NO3− reductase activity in leaves and fine roots. Moreover, the mean V max for NH4+ uptake (5.15 μmol 15N g−1 h−1) was eight times greater than the V max for NO3− uptake (0.63 μmol 15N g−1 h−1), indicating a much greater physiological capacity for NH4+ uptake in this species. Additionally, NO3− reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO3− assimilation in sugar maple. Our results demonstrate that chronic NO3− deposition has not induced the physiological capacity for NO3− uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO3− deposition.