Document: MAR-3-33-26

Variation in nitrate assimilation and growth responses of tree seedlings to nitrate loading.

RUDY, M.R.* and H.BASSIRIRAD

University of Illinois at Chicago, Chicago, IL 60607 USA ¹

Abstract:
While tree species in general have a low capacity to utilize nitrate, they differ in their capacity to absorb and assimilate nitrate. Therefore, as nitrate deposition increases, forest communities may respond by a dramatic shift in species composition. The degree to which individual species can respond to nitrate addition may depend on their ability to assimilate nitrate. In a pot experiment, three month old seedlings of Pinus taeda, Acer saccharinum and Fraxinus pennsylvanica were exposed to four nitrogen (N) levels (0.4, 0.6, 1 and 2 mM) at either a 1:1 nitrate to ammonium ratio or ratios of 1:1, 2:1, 4:1 and 9:1 applied in a solution twice a week for 5-8 months. Species differed in growth responses to N addition. Increased N dose significantly increased total biomass in A. saccharinum, but significantly decreased it in P. taeda while it did not affect total biomass in F. pennsylvanica. In Pinus and Fraxinus, the qualitative pattern of growth responses to N addition was similar regardless of the ratio of nitrate to ammonium. However, the magnitude of growth responses depended on relative abundance of nitrate. In Acer, the ratio of nitrate to ammonium did not affect either the magnitude or pattern of growth response to N addition. All three species exhibited a capacity for nitrate reduction as measured by nitrate reductase activity (NRA), however the NRA of these species varied significantly. Furthermore, while in Pinus NRA was higher in the fine roots than the foliage, Acer and Fraxinus had a higher NRA in the foliage. Acer seedlings showed an enhanced capacity to reduce nitrate as nitrate levels increased, whereas NRA of Pinus dramatically decreased at the highest nitrate addition rate. Fraxinus had the lowest NRA among the species and responded little to either N dose or a varying abundance of nitrate. These results show that tree species differ in their response to N addition. Moreover, changes in relative abundance of nitrate to ammonium differentially affect tree species. Some of these differences may be attributed to differential capacities of these species to adjust NRA as nitrate deposition rates increase.

Keywords: nitrate reductase activity, trees, nitrate loading

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.