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 NO3- deposited in temperate forests bec ause plant growth is usually limited by nitrogen (N) availability. Nev ertheless, plants are known to vary widely in their capacity for NO3- uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and green house experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO3- additio ns equivalent to 30 kg N ha(-1) year(-1). We measured leaf and fine-ro ot nitrate reductase activity (NRA) of overstory trees using an in viv o assay and used N-15 to determine the kinetic parameters of NO3- upta ke by excised fine roots. In two greenhouse experiments, we measured l eaf and root NRA in A. saccharum seedlings fertilized with 0-3.5 g NO3 --N m(-2) and determined the kinetic parameters of NO3- and NH4+ uptak e in excised roots of seedlings. In both overstory trees and seedlings , rates of leaf and fine root NRA were substantially lower than previo usly reported rates for most woody plants and showed no response to NO 3- fertilization (range = non-detectable to 33 nmol NO2- g(-1) h(-1)). Maximal rates of NO3- uptake in overstory trees also were low, rangin g from 0.2 to 1.0 mu mol g(-1) h(-1). In seedlings, the mean V-max for NO3- uptake in fine roots (1 mu mol g(-1) h(-1)) was approximately 30 times lower than the V-max for NH4+ uptake (33 mu mol g(-1) h(-1)). O ur results suggest that A. saccharum satisfies its N demand through ra pid NH4+ uptake and may have a limited capacity to serve as a direct s ink for atmospheric additions of NO3-.