Chronic Atmospheric NO 3 − Deposition Does Not Induce NO 3 − Use by Acer saccharum Marsh.

Abstract

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 NO 3 − . However, it is uncertain whether long-term exposure to NO 3 − deposition might induce NO 3 − uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO 3 − deposition (30 kg N ha−1 y−1) could induce NO 3 − uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO 3 − deposition. Kinetic parameters for NH 4 + and NO 3 − uptake in fine roots, as well as leaf and root NO 3 − reductase activity, were measured under conditions of ambient and experimental NO 3 − deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO 3 − deposition did not alter the V max or K m for NO 3 − and NH 4 + uptake nor did it influence NO 3 − reductase activity in leaves and fine roots. Moreover, the mean V max for NH 4 + uptake (5.15 μmol 15N g−1 h−1) was eight times greater than the V max for NO 3 − uptake (0.63 μmol 15N g−1 h−1), indicating a much greater physiological capacity for NH 4 + uptake in this species. Additionally, NO 3 − reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO 3 − assimilation in sugar maple. Our results demonstrate that chronic NO 3 − deposition has not induced the physiological capacity for NO 3 − uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO 3 − deposition.