Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh.

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO 3- deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO 3- uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO 3- additions equivalent to 30 kg N ha-1 year-1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used 15N to determine the kinetic parameters of NO 3- uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0-3.5 g NO 3- -N m-2 and determined the kinetic parameters of NO 3- and NH 4+ uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO 3- fertilization (range = non-detectable to 33 nmol NO 2- g-1 h-1). Maximal rates of NO 3- uptake in overstory trees also were low, ranging from 0.2 to 1.0 μmol g-1 h-1. In seedlings, the mean V max for NO 3- uptake in fine roots (1 μmol g-1 h-1) was approximately 30 times lower than the V max for NH 4+ uptake (33 μmol g-1 h-1). Our results suggest that A. saccharum satisfies its N demand through rapid NH 4+ uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO 3- .