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Kinetics of nitrogen uptake by Populus tremuloides in relation to atmospheric CO₂ and soil nitrogen availability

David E. Rothstein (1, 2), Donald. R. Zak (1, 3), Kurt S. Pregitzer (4) and Peter S. Curtis (5)

1. School of Natural Resources and Environment, University of Michigan, Ann Arbor, MI 48109-1115, USA / 2. Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA / 3. Author to whom correspondence should be addressed (drzak@umich.edu) / 4. School of Forestry and Lake Superior Ecosystems Research Center, Michigan Technological University, Houghton, MI 49931, USA / 5. Department of Plant Biology, Ohio State University, Columbus, OH 43210, USA / Received March 24, 1999

Summary

Sustained increases in plant production in response to elevated atmospheric carbon dioxide (CO₂) concentration may be constrained by the availability of soil nitrogen (N). However, it is possible that plants will respond to N limitation at elevated CO₂ concentration by increasing the specific N uptake capacity of their roots. To explore this possibility, we examined the kinetics of ¹⁵NH₄⁺ and ¹⁵NO₃^– uptake by excised roots of Populus tremuloides Michx. grown in ambient and twice-ambient CO₂ concentrations, and in soils of low- and high-N availability. Elevated CO₂ concentration had no effect on either NH₄⁺ or NO₃^– uptake, whereas high-N availability decreased the capacity of roots to take up both NH₄⁺ and NO₃^–. The maximal rate of NH₄⁺ uptake decreased from 12 to 8 μmol g^–1 h^–1, and K_m increased from 49 to 162 μmol l^–1, from low to high soil N availability.Because NO₃^– uptake exhibited mixedkinetics over the concentration range we used (10–500 μmol l ^–1), it was not possible to calculate V_max and K_m. Instead, we used an uptake rate of 100 μmol g^–1 h^–1 as our metric of NO₃^– uptake capacity, which averaged 0.45 and 0.23 μmol g^–1 h^–1 at low- and high-N availability, respectively. The proximal mechanisms for decreased N uptake capacity at high-N availability appeared to be an increase in fine-root carbohydrate status and a decrease in fine-root N concentration. Both NH₄⁺ and NO₃^– uptake were inversely related to fine-root N concentration, and positively related to fine-root total nonstructural carbohydrate concentration. We conclude that soil N availability, through its effects on fine-root N and carbohydrate status, has a much greater influence on the specific uptake capacity of P. tremuloides fine roots than elevated atmospheric CO₂. In elevated atmospheric CO₂, changes in N acquisition by P. tremuloides appeared to be driven by changes in root architecture and biomass, rather than by changes in the amount or activity of N-uptake enzymes.

Keywords: ammonium, elevated CO₂, global change, nitrate, soil N availability, specific N uptake, total nonstructural carbohydrate, uptake kinetics, uptake regulations.

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ISSN 0829-318X Copyright © 2002–2008 Heron Publishing