PARENT SESSION
Poster Session #27: Nutrient Cycling I.
Tuesday, August 6. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Negative effects of nitrate loading on the physiological responses of a nitrogen-fixing tree, Robinia pseudoacacia.

DYBZINSKI, RAY^*,1, SEHTIYA, HARBANS¹, BASSIRIRAD, HORMOZ¹, ¹ University of Illinois at Chicago, Chicago, IL

ABSTRACT- Chronic atmospheric nitrogen (N) deposition can affect community dynamics and species diversity. In industrial regions, nitrate (NO₃^-) deposition may significantly influence the ecology of nitrogen-fixing plants. We studied the growth and physiological effects of NO₃^- dose on Robinia pseudoacacia (black locust), a nitrogen-fixing tree now ubiquitous in many temperate regions of the USA. For 4 months, seedlings were grown in 4L pots containing sand under four NO₃^- treatments: 0.1, 1.0, 5.0, and 10.0mM. Plants were irrigated with 200ml of nutrient solution containing NO₃^- three times a week. Total biomass increased significantly with NO₃^- loading. However, this coincided with a dramatic decrease in relative nodule mass and numbers. Nitrate addition also significantly reduced in vivo root NO₃^- reducatase activity. Concentration of ammonium and nitrite in the root increased while total amino acids decreased with increased NO₃^- addition. This indicates that black locust cannot fully complete NO₃^- assimilation into amino acids at high NO₃^- exposures. We conclude that the positive growth response to high NO₃^- loading is transient and will not adequately predict the long-term responses of black locust to chronic N deposition. It is clear, however, that nodule formation (and hence N fixation) will be adversely impacted by chronic N deposition. Physiological data indicate that unlike some crop legumes, which can easily switch from fixation to external N supply to meet demand, black locust does not appear to be so plastic.

KEY WORDS: nitrate loading, nitrogen fixation, Robinia pseudoacacia, physiological response